1

DAILY
CLASS NOTES
Geography

Lecture - 02
Earth and Related Concepts
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Earth and Related Concepts

Earth:
 It is the third nearest planet to the sun.
 It is the fifth largest planet in the solar system.
 It is slightly flattened at the poles, which is described as a Geoid (earth-
like shape), and it is Bulge at the Equator.
 Earth is inclined at 23.5 degrees from vertical.
 The Equator divides the earth into two equal parts or halves which is
Northern Hemisphere & Southern Hemisphere.
 The earth is neither too hot nor too cold , this is called Goldi Lock Zone,
and favourable conditions to support life are found only on the Earth.
 Its two-thirds surface is covered by water thus it appears blue. Therefore, it is also called a blue planet.
 Earth rotates on its axis (called the axis of rotation).

NOTE:
 Centrifugal Force: It is the apparent outward force on a mass when it is rotated.
 Centripetal Force: It is the apparent inward force on a mass when it is rotated.
 Gravitational Force: It is responsible for attraction between two bodies.
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 Earth is flattened at poles and bulges from sides due to the centrifugal force.

 In the solar system, centrifugal and centripetal forces are balancing each other.
 Centripetal force is equal to Centrifugal force.
Latitudes:
 Latitudes are imaginary circles on the surface of the Earth.
 The role of the latitudes is to study various places, and
regions.
 Latitude is the angular distance of a point on the earth’s
surface, measured in degrees from the Centre of the earth.
These are usually measured in degrees.
 It is parallel to the equator; therefore, they are called
Parallels of latitudes.
 These parallels are in the form of circles with unequal
diameters to study every zone of the earth.
 The largest circle is the Equator (0° latitude) while at the poles, they shrink to a point (90° N & S).
 There are 181 latitudes in total.
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Important Parallels of Latitudes:

 Tropic of Cancer (23 1⁄2° N) in the Northern Hemisphere.
 Tropic of Capricorn (23 1⁄2° S) in the Southern
Hemisphere.
 Arctic Circle at 66 1⁄2° in the North of the Equator.
 Antarctic Circle at 66 1⁄2° in the South of the Equator.
 Equator: The Equator is an imaginary line running on
the globe that divides it into two equal parts.
 The northern half of the Earth is the Northern
Hemisphere and the Southern half is the Southern
Hemisphere.
 Parallels of latitude are drawn at an interval of one
degree. If the earth were a perfect sphere, the length of
1̊ of latitude would be a constant value, i.e., 111 km
everywhere on the earth. But to be precise, the degree of latitude changes slightly in length from the equator
to the Poles.
 While at the Equator, it is 110.6 km, at the poles, it is 111.7 km. i.e, the linear distance of a degree of
latitude at the pole is a little longer than at the Equator.
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Heat Zones of the Earth:

 When sunlight falls upon the earth (energy per unit time per
unit area called insolation), at the Equator, it is perpendicular
(overhead).
 Insolation: Incoming Solar Radiation on the Earth surface.
 At the Tropics, it forms an angle of around 45 degrees, and at
Polar latitudes, it is completely oblique.
 At the Equator, there is high insolation.
 At the Tropics, there is moderate insolation.
 At the Poles, there is low insolation. (Therefore, due to
different intensities of sunlight and heat, the vegetation
and weather in different regions are different).
 Torrid Zone: Overhead rays are concentrated in this zone and

thus there is high insolation in a small area.

 The mid-day sun is exactly overhead at least
once a year on all latitudes in between the
Tropic of Cancer and the Tropic of
Capricorn.
 It, therefore, receives the maximum heat.
 The highest temperature on Earth is found
near the Tropical areas.
 Temperate Zones: The mid-day sun never shines
overhead on any latitude beyond the Tropic of
Cancer and the Tropic of Capricorn.
 The angle of the sun’s rays goes on decreasing
towards the poles. They, thus, have moderate temperatures.
 Frigid Zones: Insolation is low and covers a large area. In this zone heat energy is not concentrated.
 Areas lying between the Arctic Circle and the North Pole in the Northern Hemisphere and the
Antarctic Circle and the South Pole in the Southern Hemisphere, are very cold.
 It is because, here, the sun does not rise much above the horizon.
 Coniferous trees are found in these areas.
 Tundra and Taiga type of vegetation is found.
India as a Geographical Unit:
 The Tropic of Cancer passes through eight states in India.
 Gujarat (Jasdan), Rajasthan (Kalinjarh), Madhya Pradesh (Shajapur), Chhattisgarh (Sonhat), Jharkhand
(Lohardaga), West- Bengal (Krishnanagar), Tripura (Udaipur) and Mizoram (Champhai).
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Longitude:
 It's the angular distance east or west of the Prime
Meridian. It is also measured in degrees.
 Longitudes are semicircles running from north pole to
south pole.
 On the globe, longitude is shown as a series of semi-
circles that run from pole to pole passing through the
equator. Such lines are also called Meridians.
 Longitudes to determine the time of region/place.
 Longitude intercepts the equator at 90 degrees.
 The longitudes vary from 0° to 180° eastward and
westward of the Prime Meridian. (Also called
meridian).
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 Prime Meridian: In 1884, it was decided to choose as zero meridian the one which passes through the
Royal Astronomical Observatory at Greenwich, near London as a Prime Meridian.
 Prime Meridian also helped to determine the time zones of the world.
 The distance between longitudes decreases from the Equator
towards the Poles (It is maximum at the equator).
 They have one very important function, they determine local time in
relation to G.M.T. or Greenwich Mean Time, which is sometimes
referred to as World Time.
Facts about Line of Longitude:
 These are also known as meridians that run in a north-south
direction.
 It helps in measuring the distance east or west of the prime
meridian and calculating the local time in different areas.
 These are farthest apart at the Equator and meet at the poles.
 These lines cross the Equator at right angles.
 It lies in planes that pass through the Earth’s axis.
 These are equal in length and halves of great circles.
 The distance between longitudes at the equator is the same as latitude, roughly 69 miles.
 At 45 degrees north or south, the distance between them is about 49 miles (79 km).
 The distance between longitudes reaches zero at the poles as the lines of meridian converge at that point i.e.
the degree of longitude decreases in length from the equator to the pole.
 At the Equator the distance between two longitudes is maximum and at the Poles it is minimum.
 The primary unit in which longitude and latitude are given is degrees (°). There are 360° of longitude (180°
East ↔ 180° West) and 180° of latitude (90° North ↔ 90° South). Each degree can be broken into 60
minutes. Each minute can be divided into 60 seconds.

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 1

DAILY
CLASS NOTES
Geography

Lecture - 03
Time Zones of World
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Time Zones of World

Greenwich Mean Time (GMT):
❖ It is the name for the mean solar time of the longitude (0°) of the Royal Greenwich Observatory in
England. The meridian at this longitude is
called the Prime Meridian or Greenwich
Meridian.
❖ This is the reference line for calculating time.
❖ All time zones or all standard times are
calculated by taking GMT as reference.
❖ Rotation of Earth of 360° in one day or 24
hours. Therefore, 15° in one hour or 1° in 4
minutes.
❖ Earth rotates from West to East, so every 15°
we go eastward, local time is advanced by 1
hour.
❖ If we go westward, local time is retarded by one hour.
➢ East of Greenwich → see the sun earlier → Gain time (East-Gain-Add).
➢ West of Greenwich → see sun later → Lose time (West-Lose-Subtract).
❖ India is 5 hrs 30 min ahead of
GMT.
❖ From Greenwich to 180° E →
Gain 12 hours. Similarly, from
Greenwich to 180° W → Lose
12 hours.
❖ Thus, a difference of 24 hours
between the two sides of the
180° meridian. This is the
International Date Line
(IDL).
❖ Thus, if a traveler is crossing
IDL: East to West → Loses a
day; West to East → gains a
day
❖ IDL passes through the middle of the Pacific Ocean.
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❖ Local time varies from one longitude to another longitude.

❖ In the world, there are 24 time zones.
❖ USSR- 11 Time zone
❖ USA- 4 Time zones (Pacific Time, Mountain Time, Central Time, Eastern Time)

NOTE:
❖ Earth rotates 360 degrees in 24 hours (360 degree rotation equals time for 24 hours).
❖ 360 degree = 24 hours
❖ One hour = 360/24=60/4(One hour = 60 minutes = 15 degrees).
❖ 1 hour = 15 degrees(Earth rotates 15 degrees in one hour).

➢ 15 degree = 60 minutes
➢ 1 degree=60/15 minutes
➢ 1 degree=4 minutes(Earth rotates 1 degree in 4 minutes).

NOTE:
Local Time:
❖ Local Time is the time along any given longitude.
❖ Every longitude has different local time due to movement or rotation of the Earth.
❖ Local time of a given place is calculated by the position of the sun at noon. It is based on the local
meridian passing through that place. When the sun is exactly overhead at that meridian, it is 12 noon.
The places lying North and South on the same meridian will have the same local time.

Standard Time and Time Zones:

❖ If each town were to keep the time of its own meridian,
there would be much difference in local time between one
town and the other.
❖ Travelers going from one end of the country to the other
would have to keep changing their watches if they wanted
to keep their appointments. This is impractical and very
inconvenient.
❖ Most countries adopt their standard time from the central
meridian of their countries.
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❖ In larger countries such as Canada, U.S.A., China, and U.S.S.R, it would be inconvenient to have a
single time zone. So, these countries have multiple time zones.
❖ Both Canada and the U.S.A. have five time zones —the Atlantic, Eastern, Central, Mountain, and
Pacific Time Zones.

❖ The difference between the local time of the Atlantic and Pacific coasts is nearly five hours.
U.S.S.R had eleven time zones before its disintegration.

Indian Standard Time:

❖ Standard Time is time opted by all places of a given country.
❖ The local time of places which are on different meridians is bound to differ.
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❖ Almost 29 longitudes are in India, which means more longitudes and more local times.
❖ In India, for instance, there will be a difference
of about 1 hour and 45 minutes in the local
times of Dwarka in Gujarat and Dibrugarh
in Assam.
❖ In India, the longitude of 82 1⁄2° East (82° 30’
East) is treated as the standard meridian. The
local time at this meridian is taken as the
standard time for the whole country. It is known
as the Indian Standard Time (IST).
❖ States through which IST passes are Uttar
Pradesh, Madhya Pradesh, Chhattisgarh,
Odisha, and Andhra Pradesh.

International Date Line:

❖ International Date Line is an imaginary line where the date changes by exactly one day when it is
crossed.
❖ The International Date Line in the mid-Pacific curves from the normal 180° meridian at the Bering
Strait, Fiji, Tonga, and other islands to prevent confusion of day and date in some of the island
groups that are cut through by the meridian.
❖ For example, if a traveler moves eastward across the Pacific Ocean from Wake Island to the Hawaiian
Islands on June 31, they will jump backward to June 30 as soon as they cross the IDL.
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Why is the International Date Line is Zig Zag?

❖ Some groups of Islands (Polynesia, Melanesia, Micronesia) fall on
either side of the dateline.
❖ If the dateline was straight, then two regions of the same Island Country
or Island group would fall under different date zones.
❖ Thus, to avoid any confusion of date, this line is drawn through where the
sea lies and not land. Hence, the International Date Line is drawn in a zig-zag
manner.

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 1

DAILY
CLASS NOTES
Geography

Lecture – 04
Motion of the Earth &
its Effects
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Motion of the Earth & its Effects

Daylight Saving Time:

 It is the practice of setting the clock forward or backward by generally one hour during the summer
months of the year to extend the amount of daylight in the evenings.
 The idea behind DST is to take advantage of the longer days of summer to save energy, reduce traffic
accidents, and provide more recreational opportunities in the evenings.
 It is observed in many countries around the world.
 Over 150 years ago, the British introduced the Chai-bagan time (CTZ). For Assam's tea estates, collieries,
and oil industries, it was set one hour earlier than Indian Standard Time (IST).
Rotation of Earth:
 Earth rotates along its axis from west to east in an anti-clockwise
direction when looked at from the North Pole.
 It takes approximately 24 hrs to complete one rotation.
 Days and nights occur due to the rotation of the earth.
 The circle that divides the day from night on the globe is called the
circle of illumination.
 Earth rotates on a tilted axis. Earth’s rotational axis makes an angle
of 23.5° with the normal.
 Earth rotates around its axis from west to east.
 This rotational motion has the following effects:
 Cycle of days and nights
 Coriolis Force which acts on all moving objects on the Earth.
 Seasons (because of the tilted axis), when the earth rotates on an inclined axis and revolves also.
 Case I-If the earth is stationary, there will be no winds, no heat distribution, and no life will be
there(No Rotation, No Revolution, No tilt).
 Solar day: Time taken to complete one rotation, i.e. approximately 24 hrs.
 Speed of rotation decreases from the Equator to the poleward.
 The axis of rotation is inclined at an angle of 23.50 resulting in varying lengths of days and nights and
different seasons.
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The Position of the Earth and Sun as Shown in

Image:
 At the southern tropic- sun rays are received
overhead (nearest to the sun). It receives
maximum insolation. People will feel summer
in this region.
 At the Equator- sun rays are oblique.
 At the Northern tropic- sun rays are more
oblique. Receives relatively low insolation.
People will feel winter in this region. At the north pole, there is no sunlight.

The Position of the Earth and Sun as Shown in Image:

 If the sunlight comes from another direction:
 At the southern tropic- sun rays are more oblique.
Receive minimum insolation. People will feel winter.
At the south pole, there is no sunlight.
 At the Equator- sun rays are oblique.
 At the Northern tropic- sun rays are received overhead
(nearest to the sun). Receive maximum insolation.
People will feel summer, the Illuminated zone.

What would happen if the Earth did not rotate?

 The portion of the earth facing the sun would always experience day, thus bringing continuous
warmth to the region.
 The other half would remain in darkness and be freezing cold all the time. Life would not have been
possible in such extreme conditions.
 As the earth is in an inclined position around the sun → a year is usually divided into summer,
winter, spring, and autumn seasons.

Revolution:
 The motion of the earth around the sun in its orbit is called revolution.
 It takes 365¼ days (one year) to revolve around the sun.
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 Six hours saved every year are added to make one

day (24 hours) over a span of four years.
 This surplus day is added to the month of February.
Thus, every fourth year, February is 29 days instead
of 28 days. Such a year with 366 days is called a leap
year.
 Earth revolves around the Sun in an elliptical orbit.
 Time taken is about 365 days and 6 hours to
complete one revolution.
 The extra 6 hours become one extra day in a leap
year after 4 revolutions.
 Earth is closest to the Sun on 3rd January (147
million km), called as Perihelion.
 Earth is farthest from the Sun on 4th July (153
million km), called Aphelion.
 Effects of revolution and rotation on an inclined
axis:
 Seasons- summer, winter, autumn, and spring.
 Length of day and night varies.

 When the Earth is Rotating, But Not

Inclined:
 If the Earth was not inclined, it would have a
much different climate and weather patterns
than it currently does.
 Without tilt, the amount of sunlight received by
each hemisphere would be fairly constant,
resulting in a more uniform climate across the
planet.
 Earth's axial tilt helps to distribute solar
energy more evenly across the planet.
 This is because the angle of the sun's rays hitting
the Earth's surface changes throughout the year as
the planet orbits the sun.
 Without this variation, certain areas of the planet
would receive more solar energy than they could
handle, while others would receive very little.
 Variation of seasons will also not occur in this
case.
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Aphelion and Perihelion:

 When Earth is at Perihelion (Earth closest to
the Sun).
 Southern hemisphere receives the Sun's rays.
So, there is Summer in the Southern
Hemisphere.
 The Northern Hemisphere has Winters.
 When Earth is at Aphelion (Earth farther from
Sun).

 The Northern Hemisphere receives the

Sun's rays. Therefore, there is Summer in
the Northern Hemisphere.
 Southern hemisphere has Winters.
Solstice:
 During Solstice, the Sun shines directly over
one of the tropics.
 When sun rays fall on any of the Tropics.
 Summer Solstice:
 On 21 June → Northern hemisphere
tilted towards the Sun. The rays of the sun fall directly on the Tropic of Cancer → receive more heat.
 The areas near the poles receive less heat as the rays of the sun are slanting.
 The North Pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous
daylight for about 6 months.
 Since a large portion of the Northern Hemisphere is getting light from the sun, it is summer in the regions
north of the Equator.
 The longest day and the shortest night at these places occur on 21st June.
 Areas in the north of the Arctic Circle see 24 hrs sunlight while areas south of the Antarctic Circle
experience 24hrs nights.
 Winter Solstice:
 On 22 December → Tropic of Capricorn
receives direct rays of the sun. A larger
portion of the Southern Hemisphere gets
light.
 Therefore, it is summer in the Southern
Hemisphere with longer days and shorter
nights.
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 The reverse happens in the Northern Hemisphere. This position of the Earth is called the Winter
Solstice.
 Areas south of the Antarctic Circle in 24 hrs. sunlight while areas north of the Arctic Circle in 24 hrs
darkness.
Equinox:
 When the sun shines directly over the Equator –
Equal lengths of day and night at all points on Earth.
 Vernal equinox (spring equinox), 21st March: Spring
in the northern hemisphere.
 Autumn equinox: 23rd September: Autumn in the
northern hemisphere.
 On 21st March and September 23rd, direct rays of the
sun fall on the equator.
 At this position, neither of the poles is tilted towards the sun, so the whole Earth experiences equal days and
nights. This is called Equinox.

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 1

DAILY
CLASS NOTES
Geography

Lecture 05
Theories of Formation of
Universe
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Theories of Formation of Universe

Theories of the Formation of Universe
❖ Theories of Earth Formation: A large number of hypotheses were put forth by different philosophers and
scientists regarding the origin of the earth.
These theories can be broadly categorized into two sections:
❖ Religious: In 1664, with a literal interpretation of the Bible, Archbishop Usher (1664) in his book “Annals
of the World" computed the time of Earth's creation as 9:00 AM, Oct. 26, 4004 B.C.
❖ Scientific: Scientific school believes in logic(Few theories were based on Scientific concept).
➢ There are two types of schools:
➢ Hot Origin(School): Solar system & Earth formed from the matter which was initially hot. Initially, the
temperature was hot and slowly it became habitable after cooling.
➢ Cold Origin(School): Solar system & Earth formed from matter which was initially cold. After
formation, Earth might have heated up due to radioactive elements or due to intense pressure created
due to a load of outermost layers.
Classification of Concepts of Origin:
❖ Monoistic Concept: (One Star Hypothesis): According to this hypothesis, the Solar system originated from
one star (only one heavenly body) due to the gradual evolutionary process. The hypothesis of Kant, Laplace,
Roche, and Locklear comes under this category.
❖ Dualistic Concept: According to the dualistic concept, the Solar system originated from two stars (two
heavenly bodies). The hypothesis of James Jeans, Chamberlain, Molten, Weitzacker’s, and Russell comes
under this category.
❖ Binary Star Concept:This concept involves more than two heavenly bodies.
❖ Modern Concept: The Nova Hypothesis of Hoyle and Littleton, The Big Bang Hypothesis, the Cepheid
Hypothesis of A.C. Banerjee, Rossgunn’s rotational and tidal hypothesis, Kuiper’s hypothesis (1949),
Fosenkov’s globule concept, Voitkevich’s protoplanetary, Chondrules concept (1971), Jupiter Sun binary
system hypothesis of E.M. Drobyshvski are some of the examples of the Modern Concept.
Earlier Theories:
❖ One of the earlier and more popular arguments was by German philosopher Immanuel Kant. Mathematician
Laplace revised it in 1796. It is known as Nebular Hypothesis. Initially it was Nebula.
➢ The hypothesis considered that the planets were formed out of a cloud of material associated with a
youthful sun, which was slowly rotating.
❖ In 1950, Otto Schmidt in Russia and Carl Weizascar in Germany somewhat revised the ‘Nebular
Hypothesis’, though differing in details.
➢ They considered that the sun was surrounded by a solar nebula containing mostly hydrogen and helium
along with dust.
➢ The friction and collision of particles led to the formation of a disk-shaped cloud and the planets were
formed through the process of accretion.
 3

➢ However, scientists in later periods took up the problems of the origin of the universe rather than that of
just the earth or the planets.
Gaseous Hypothesis of Kant:
❖ The German philosopher, Kant, put forward his hypothesis in 1755
claiming that his hypothesis was based on sound principles of
Newton’s first law of gravitation and rotatory motion.
❖ In this model the stars rotated about the centre of the Galaxy just as the
planets revolved around the centre of the sun. Kant even suggested that
the faint nebulae seen may be composed of individual stars.
❖ Although Kant considered that the sun's distance from the centre may
be great, he did not estimate this distance and did not estimate the size
of this universe.
❖ Kant's thoughts were not taken as scientific proof but could be
considered the start of the controversies at the heart of the Curtis -
Shapley debate.
❖ Assumptions:
➢ There was a slowly rotating cloud of Gas and Matter (Nebula).
➢ It was very cold (0 Kelvin) and consisted of solid & motionless particles.
➢ Particles after some time began to collide with each other due to Gravitational Attraction.
Mechanism of Origin:
❖ Mutual Attraction & Collision: Random motion of particles
within Nebula produced heat which helped in the rising
temperature of primordial matter.
❖ Random Motion of particles: It induces rotary motion. Earlier it
was a motionless cloud. After a random collision, the vast Nebula
started spinning around its own axis.
❖ Rise in temperature: It produced more random motion and
spinning. In this phase, matter changed from solid to gas, which led
to the expansion in the size of the Nebula.
❖ As the heat increased, it resulted in an increase in size as well. The
increase in angular velocity gave rise to Centrifugal Force.
❖ Centrifugal force dominates Centripetal force. This caused the
separation of rings, and nine rings were separated & thrown out.
❖ There were some irregularities in the rings. One portion of the ring known as the core (knot) started
contracting first, and ultimately grew as a planet.
❖ The process repeated for each ring, which helped in the formation of satellite planets.
Criticism:
❖ This theory failed to explain the source of the origin of the Primordial Matter.
 4

❖ It failed to explain the source of energy for the random motion of particles which were motionless. This is a
violation of Newton’s first law of motion.
❖ Why only nine planets, why not eighteen planets?
❖ Collision among the particles of primordial matter can never generate rotary motion. It requires torque.
❖ It explained that the increase in the rotary speed of Nebula increased with size. This contradicts the Law of
Angular Momentum.
Nebular Hypothesis of Laplace:
❖ By French Mathematician-Laplace (1796):
➢ Elaborate the concepts in the book- “Exposition of World
Systems”.
➢ Assumption:
✓ There was a huge & hot gaseous Nebula in space. Thus,
he solved the heat-related criticism of Kant.
✓ From the beginning, huge & hot Nebula spinning on its
axis.
✓ Gradually, the nebula continued to cool (from radiation).
Thus, after losing heat, it contracted, which induced a
reduction in size.
Mechanism of Origin:
❖ Initially, huge and hot gaseous rotating Nebula in space.
❖ Gradually, it was losing heat from the outer surface due to
rotation.
❖ Heat loss resulted in cooling and contraction. It also decreased in size
and volume.
❖ Reduced size increased the velocity (Conservation of Angular
Momentum). It increased the Centrifugal Force. Thus, centrifugal
force dominates over centripetal force.
❖ Outer layer was condensed due to cooling. It couldn’t cope up with
the adjacent bottom. This resulted in separation as rings.
❖ Initially one ring separated from Nebula. Thereafter, nine other
rings got separated from the original ring.
❖ All the materials of each ring got condensed at a Knot, known as a "hot gaseous agglomeration", which
cooled down to form a Planet.
❖ As a result, nine planets were created from nine rings, and the sun was created from the nebula's remaining
center nucleus.
❖ Repeated the aforementioned processes and mechanisms, which led to satellites being generated from
planets.
 5

Criticism:
❖ Laplace made the assumption that a hot, whirling nebula existed at first, but he did not specify where it came
from. Also, he made no attempt to explain where the heat and movement in that nebula originate.
❖ He did not clarify the cause of the irregular ring's creation. Also, no explanation was given on why the
irregular ring that broke off from the nebula only produced nine rings.
❖ If the sun was the residual nucleus, it must have bulged around its middle part (equator of the sun).
However, the sun's central portion is devoid of this bulge.
❖ If the planets were formed from the nebula, they must be in the liquid phase, but liquid can't rotate
like solids.
❖ Nebular theory states that all satellites should rotate in the same way as their parent planets, but Saturn's
and Jupiter's satellites rotate in the opposite direction.
❖ Great British physicists James Clerk Maxwell and Sir James Jeans proved that the mass in the rings was
insufficient to generate gravitational pull for condensation to form separate planets about a century after the
Laplace hypothesis was first proposed.
❖ It violated the Law of Angular Momentum. The Sun possesses 99.9 % of the total mass and planets have
0.1%, but more than 99% of the total Angular Momentum is concentrated in Planets.
Planetesimal Hypothesis by Chamberlain & Moulton:
❖ T.C. Chamberlin, a geologist, in collaboration with Forest Ray Moulton an astronomer, postulated a
hypothesis known as the ‘Planetesimal Hypothesis’.
❖ According to this hypothesis, the planets originated from Planetesimals.
❖ The idea of Dualistic Concept(two heavenly bodies) gained currency which tried to explain the origin of
Earth, Continents, and Oceans.
❖ Initially, there was a:
➢ Proto Sun – Cold, solid, and formed of gaseous & very small particles.
➢ Companion Star - Destined to pass very close to the Sun.
❖ When Companion Star came close to the Proto Sun, an infinite number of small and big particles detached
(separated) from the Sun due to the gravitational pull, which were termed as Planetesimals.
❖ These Planetesimals were attracted by the proto-Sun and started revolving around the Sun.
❖ These detached particles (planetesimals) were non-uniform and different in size (small and big).
➢ The larger particles served as the nucleus and attracted the smaller Planetesimals towards them
(accretion).
➢ Gradually, the bigger Planetesimals became bigger and formed the present planets.
❖ Satellites around the planets formed due to repetition of the same process.
 6

❖ Forces Responsible:
➢ The main force responsible for the ejection of the small jets & planetesimals was Tidal Force (Tidal
Force is a gravitational effect that stretches a body along the line towards the center of mass of another
body). It acted by the approaching star upon the surface of the sun.
Stages of Evolution of Earth:
1st Stage - Period of Planetesimal Accession:
There are two primary sources from which the earth's atmosphere developed:
1. External Evolution:
➢ The effectiveness of the earth's ability to capture free air molecules increased as it grew in size. (due
to Gravitational force)
➢ As the earth had already absorbed the majority of the molecules, the supply of air molecules, which had
previously been greater, gradually diminished over time.
2. Internal Evolution:
➢ Carbon dioxide, water vapour, and nitrogen gases were produced internally. By erupting from
volcanoes and fissures, these gases particles left the earth's interior and are now an integral part of the
atmosphere.
➢ The volcanic outburst therefore produced oxygen.
❖ Origin of Heat: There were various sources of heat such as –
➢ Mutual Collision of planetesimals during the accretion produced heat.
➢ Intensive Pressure due to increased aggregation of planetesimals also produced heat.
➢ Chemical reaction was responsible for heat production as well.
nd
2 Stage - Period of Dominant Volcanism:
❖ During the early stages of the earth's evolution, gradual internal heat accumulation led to the selective
melting (called Magma) of rocks in the planet's outer regions, which sparked extensive volcanic activity.
❖ Between the planetesimals, there were enormous fissures. Additionally, the pieces of the earth's surface were
not solidly consolidated.
❖ Violent volcanic explosions were caused by the volatile materials (Magma) in the earth's interior escaping,
leaving "crater-like hollows" on the surface.
❖ Evolution of continents and Oceans initiated. Primitive oceans were formed. Water vapour collected in
these Craters and received from numerous lakes got connected and formed Oceans.
3rd Stage - Actual Geological Period:
❖ The ancient surface features of the earth's surface were created during the final stage of the earth's history
as a result of dominating tectonic events like dominant vulcanicity, folding and faulting, submergence and
emergence, and dominant tectonic events like dominant vulcanicity.
Criticism:
❖ The large amount of heat produced during the ejection of planetesimals from Proto Sun would have been
volatilized (from solid to gaseous phase). It is impossible to condense in the form of planets.
 7

❖ Origin of Star-Where it comes from, and Where did it go?

❖ Why do big planets have more satellites like Jupiter, and why small planets do not have satellites like
Mercury?
➢ No explanation was given for the increase & decrease in the size of planets.
➢ No explanation was given for the fixed number of planets.
➢ It doesn’t offer any explanation for the Higher Angular momentum in planets.
➢ In infinite space in the Universe, close encounters are impossible.


 1

DAILY
CLASS NOTES
Geography

Lecture - 06
Universe (Part-02)
 2

Universe (Part-02)
Tidal Hypothesis:
❖ Tidal Hypothesis was given by Sir James Jeans (1919).
❖ It is one of the modern hypotheses.
❖ Assumptions:
➢ Solar system – It formed from the sun and an intruding star.
➢ Sun – The sun was first a large incandescent gaseous mass of the substance.
The sun in the beginning was a stationary body that rotated on its axis.
➢ Intruding Star – It was much bigger in size than the primitive sun. It was
moving and destined to come nearer to the primitive sun.
➢ The intruding star's tidal force (Gravitational Pull) had a significant effect
on the primitive sun's surface.
➢ Gravitational force is inversely proportional to the
distance between two bodies.
❖ Mechanism of Origin - Evolution of Filament:
➢ The intruding star was continuously moving towards the
Primitive Sun and exerted Gravitational pull.
➢ When it came nearer, Gravitational pull increased
considerably. A cigar-shaped tide was created on the
outer surface of the primitive sun.
➢ A huge mass of matter in a Cigar shape was ejected out
of the Sun filament.
➢ Nine Planets were formed due to the cooling and
condensation of the gaseous matter.
➢ The filament began to cool down. It contracted and
resulted in breaking into several parts. Each piece was
condensed to form a separate planet.
➢ Filament allowed the bigger planets to
form in the Middle (Jupiter & Saturn) &
smaller ones towards their tapering
ends.
➢ The remaining part became the Sun.
➢ Gravitational pull by the Sun on
newly formed planets helped in the
formation of satellites.
➢ The bigger planets cooled slowly and
ejected a large amount of matter.
➢ This continued and formed a large number of satellites.
 3

➢ Small Planets were cooled fast. No further ejection of material resulted in the formation of satellites.
Example-Mercury, Venus, and Pluto.

Evidence in Support:
❖ Shape and Planetary Order - The primitive sun's filament had a cigar-like form. It was narrow at the ends
and thick in the middle.
❖ Ordering and Arrangement of Satellites - According to this theory, just as the planets were produced by the
condensation of incandescent gaseous matter emitted from the sun, the satellites of the planets were created by
the same process. Thus, the arrangement of satellites around the planets is essentially identical to the
arrangement of planets around the sun in our solar system, with smaller satellites at the end and larger satellites
in the middle.
❖ Number and Size of Satellites - Bigger planets showed slow cooling and a greater number of satellites, such
as Jupiter & Saturn are having the highest number of satellites. While planets such as Mercury, Venus &
Pluto do not have satellites.
❖ Rotation, Revolution & Inclination of Planets Axis – The Filament rotated around the sun and was slightly
inclined. Except for Venus and Uranus, all other planets rotate in the same direction.

Criticism:
❖ According to B. Levin, the universe is infinite in both space and time and because the stars are so far apart, it
is highly unlikely that they will ever come close to one another.
❖ James Jeans did not provide any information regarding the location or ultimate fate of the intruding star
that triggered a massive tidal eruption on the surface of the early sun.
❖ As per the calculations (Parisky, Hoyle, Russel), the planets should have been very close to the Sun, but they
are far away (300-500 times the Sun's Diameter).
❖ It failed to elaborate the process and mechanism of the condensation of the matter ejected from the primitive
sun.
❖ The planets in our solar system are primarily composed of heavy elements, but the sun's building blocks,
hydrogen, and helium, are lighter in weight and are thought to have originated the planets. The tidal hypothesis
is unconvincing in its attempt to explain such an abnormal scenario.

Supernova Hypothesis:
❖ In 1946, F. Hoyle, a mathematician from Cambridge University (U.K.), proposed his speculative theory, also
known as the "supernova hypothesis." In his essay titled "Nature of the Universe," he explained his
hypothesis, which was founded on the "nuclear physics" principles.
 4

❖ As per the theory, there were two stars:

➢ Primitive Sun.
➢ Companion Star.
❖ The Companion Star was initially a big star that
later became a supernova as a result of a
nuclear reaction.

Mechanism of Formation:
❖ Energy Emitted by any star in light form is
generated by Nuclear Fusion.
❖ Stars contain Hydrogen which combines to
form Helium (Heavier) & Energy. The same
Nuclear Fusion was happening in Primitive Sun
& Companion Star.
❖ As time passes, all Hydrogen consumed in
Nuclear Reaction & the star collapses &
violently explodes.
❖ Enormous Dust formed & started revolution
around the sun.
❖ Gaseous matter coming out of the explosion
changed into circular disc-matter which
became building material for future planets.
❖ Explosion caused high heat of temperature 5 *
(10)9 deg C which is sufficient to start nuclear
fusion.
❖ Intense Heat & Nuclear Fusion became responsible for the formation of Heavy Elements (presently on
Earth.)
❖ Planets were formed due to condensation of the heavy matter of the disc formed out of SuperNova.

Evaluation:
❖ The "supernova hypothesis" of F. Hoyle aids us in resolving three fundamental issues about the formation
of the planet and solar system that have been brought up by critics ever since James Jeans proposed the "Tidal
Hypothesis":
➢ The issue of the planets and the sun being quite far apart.
➢ Problems with the planets' angular momentum.
➢ The issue with planets' compositions containing heavier elements than the sun.
In the following ways:
➢ Shattered matter out of Supernova was thrown away at great distances.
➢ A large amount of Angular momentum was given by Violent Explosion.
➢ Materials became heavier due to intense heat & Pressure.
 5

Big Bang Theory:

❖ It is also called Expanding Universe Hypothesis.
❖ This theory was propounded by Georges Lemaitre in the late 1920s.
❖ Edwin Hubble in 1920 provided evidence that the universe is expanding.
❖ The expansion of the universe means an increase in space between the galaxies.

❖ As time passes, galaxies move further and further apart. The Big
Bang Theory considers the following stages in the development
of the universe:
➢ "Tiny ball” (singular atom) which is an unimaginably small
volume, infinite temperature, and infinite density.
➢ At the Big Bang the "tiny ball" exploded violently creating
a huge expansion.
➢ It happened 13.7 billion years before the present. The
expansion continues even to the present day.
➢ Within 300,000 years from the Big Bang, the temperature
dropped to 4,500K (Kelvin) and gave rise to atomic matter.
The universe became transparent.

Evidence of Big Bang Theory:

❖ Hubble observed the distance between stars in the universe
through a telescope and established a relationship between the
Galactic distances and speed of movement.
❖ As per Hubble Galaxies were moving away, and Increasing
distance.
❖ Wilson and Penzias gave the Cosmic Microwave Background
Radiation hypothesis; they observed the leftover big-bang
radiation and calculated its temperature. These radiations were
13.7 billion years old with temperatures around 2.7 kelvin.
 6

Note:
❖ Suppose if there is gas, it is compressed(Increases the pressure) then it will be high temperature due to friction
between the molecules.
❖ When size will increase then Intermolecular will increase, then friction will decrease and there will be low
heat generation then temperature will be lesser, then it will be cooling.
Universe:
❖ The Universe is all existing matter & space. It is incomprehensibly large. It consists of both physical
(subatomic particles like electrons, protons to galactic superclusters) and non-physical (light, gravitation,
space etc.) components.
❖ In the galaxies there are solar systems, and in the Solar system there are planets, dwarf planets, Asteroids,
Meteors, Stars.
Dark Matter:
❖ Dark matter is a hypothetical form of matter that is believed to exist in the universe but is invisible and does
not interact with light.
Black Holes:
❖ Black holes are regions of space-time where gravity is so strong that nothing, not even light, can escape from
them.


 1

DAILY
CLASS NOTES
Geography

Lecture - 07
Universe (Part 03)
 2

Universe (Part 03)

What is the Universe?
 The age of the Universe is about 13.79 billion years old. It comprises the Galactic Mega clusters to the
tiniest subatomic particles.
 It is the cluster in which there are a number of galaxies present. In those galaxies there are a number of
bodies known as planets, asteroids, comets, etc.
 The Big Bang Theory is the most acceptable for the origin of the Universe. Optical and radio telescope
studies indicate the existence of about 100 billion galaxies in the visible universe.
 It is proved by the evidence that the galaxies are moving apart, and the universe is still expanding.
 Inside the universe we can find dark matter.
 The Big Bang is having its effect till now, which is seen in the evidence given by the Hubble telescope and
Cosmic Microwave Background Radiation.
 Due to this evidence, the Big Bang theory has been proven as the most reliable and acceptable.
 The Wilson experiment in the United States shows evidence of residual radiation from the Big Bang which
occurred 13.7 billion years ago.
 3

Galaxy:
 It is a collection of millions or billions of stars and planets that are held together by gravitational pull.
 One galaxy is interacting with other galaxies.
 According to the Big Bang theory, the distance between the galaxies
has been increasing. It means that galaxies are moving apart.
 The Milky Way is one such galaxy in which the Earth lies.
 It is called the Milky Way because it looks like a river of milky light
flowing from one corner to another of the sky. It is spiral in shape. It is
also known as Akash Ganga.
 The nearest galaxy to the Milky Way is Andromeda. It is a spiral galaxy
and approximately 2.5 million light-years from the Earth.
 Inside the galaxies we can find out Black holes also.
 Inside the galaxy there can be a number of Solar systems, and also
there can be supernovas, neutron stars, Cepheids.
Star Formation:
 The distribution of matter and energy was not even in the early universe. These initial density differences in
gravitational forces caused the matter to get drawn together.
 These formed the bases for the development of galaxies. A galaxy contains a large number of stars.
 A galaxy starts to form by the accumulation of hydrogen gas in the form of a very large cloud called a
Nebula.
 Eventually, the growing nebula develops localized clumps of gas.
 These clumps continue to grow into even denser gaseous bodies,
giving rise to the formation of stars.
Stars:
 Stars are heavenly bodies that have their own light and other
radiant energy. Whereas the planets do not have light of their own.
 The stars are made of extremely hot burning gasses.
 Stars have a fusion reaction taking place due to which energy is
being released in the form of light energy (that light is emitted in the
form of photons).
 The distribution of matter and energy was not even in the early
universe.
 These initial density differences in gravitational forces caused the
matter to get drawn together.
 These formed the bases for the development of galaxies. A galaxy
contains a large number of stars.
 A galaxy starts to form by accumulation of hydrogen gas in the form
of a very large cloud called a nebula.
 4

 Eventually, growing nebula develop localised clumps of gas. These clumps continue to grow even denser
gaseous bodies, giving rise to formation of stars.
What is the life cycle of a Star?
 The Stars start its life as clouds of dust and gas known as Nebula. It is the hot gaseous matter comprising
particles and gasses (hot in nature).
 It starts losing heat, and because of the heat loss, it cools down. When a system starts cooling down,
contraction happens, and its size reduces.
 Because of this condensation and reduction in size, the formation of stars occurs and its very first stage is
known as Proto-star.
 In other words, a gaseous matter of Nebula further contracts to make a dense region named Proto-Star.
 Proto-Star further condenses to a critical stage of mass where nuclear fusion begins, and the star finally
comes into existence.
 Inside a star, the hydrogen gas converts into helium. When a small star runs out of hydrogen (in the core)
then it starts fusing hydrogen into helium just outside the core, releasing energy & light and expanding the
star which is known as the Red Giant.

NOTE:
 Planetary Nebula is a type of Emission Nebula. Emission Nebula is nothing but ionized gasses. These
ionized gasses emit light of different wavelengths. The Planetary Nebula consists of ionized gasses that
are ejected from red giant stars.

 When there is no hydrogen left, the process stops, and the star collapses (because of its own weight and
gravity). Because of the collapse of the star, the dust and the gasses come out of the star, and the star gets
converted into a White dwarf.
 A white dwarf star becomes dark balls of matter on cooling to make Black Dwarf Star.
 White Dwarf Star is a dead star because of the end of fusion reaction and energy production.
 When a white dwarf star stops emitting light it will convert into a Black dwarf and this is the life cycle of
small stars.
 Red giant (is a small star) and Red supergiant (in case of a large star).
 A supernova is the explosive death of a star and often results in the star obtaining the brightness of 100
million suns for a short time. A great proportion of primary cosmic rays comes from supernovae.
 Neutron stars are composed mainly of neutrons and are produced after a supernova, forcing the protons and
electrons to combine to produce a neutron star.
 5

What happens with the large star?

 Large stars contain more amount of hydrogen which converts into helium, so a lot of energy is there.
 When it starts consuming its fuel - Hydrogen and Helium (H & He) in the core, it leads to the expansion of
the outer layer of the star outward resulting in an increase in its size which is known as the Red Supergiant.
 It becomes heavy and explodes because of its own weight which is known as Supernova. In other words, it
is defined as the explosive death of a star.
 The protons and electrons left in the Supernova are forced to combine to produce a very dense Neutron star.
 If the mass of a neutron star is significantly high, it will sink further (due to high gravitational force) to
produce a Black hole.
 6

Black Hole:
 Physicist Archibald Wheeler coined the Term 'Black Hole'.
 Black holes are the points/regions in space, they are so deeper that they can create a sink of gravity. The
density of matter in a black hole cannot be measured (infinite!). The gravitational pull is so great that
nothing can escape from it, not even
light.
 This is not really black and not exactly in
the hole.
 Black holes distort the space around them
and can suck neighbouring matter into
them including stars.
 Star mass can be five to twenty times the
mass of the sun. Star mass is 1 million
times the mass of the sun, that is called
super massive black hole.
 Inside the Black hole you have that Event
Horizon(Pulls every matter towards it-
Matter accelerates then it gets heated up).
 Gravitational waves are created when two
black holes orbit each other and merge.
 In 20221 there was a triple merging event
occured(when three black holes merged
then it will increase the size).
 The evolved Laser Interferometer Space
Antenna (eLISA) is a mission led by the
European Space Agency. The purpose of
the eLISA project is to detect and
accurately measure gravitational waves.


 1

DAILY
CLASS NOTES
GEOGRAPHY

Lecture – 08
Universe and Its Constituents
 2

Universe and Its Constituents

Sun:
 It is a star experiencing fusion reactions at a very large scale (hydrogen getting converted into helium).
 It is made up of extremely hot gasses, particularly
Hydrogen (70%), Helium (26.5%), and others
(3.5%) gasses.
 It is 109 times bigger than the Earth and weighs
2*10 (27) tonnes.
 It is 150 million km away from Earth. The sunlight
takes 8 minutes to reach the earth’s surface.
 It continuously gives off energy in the form of
visible light, infrared, ultraviolet, X-rays, gamma
rays, radio waves, and plasma gas.
 Due to infrared radiation, the heat of the sun is felt.
 The Sun is a giant ball of plasma (electrified gas
field).
What is Plasma?
 Plasma is the fourth state of matter.
 It is an electrically conducting medium in which there are roughly equal numbers of positively and
negatively charged particles, produced when the atoms in a gas become ionized.
Heat Transfer:
 It is the energy that flows between a hot body and a cold body.
 This heat transfer mechanism happens in three modes:
 Conduction: By the direct contact between two bodies.
 Convection: Heat is transferred between a solid body and fluid.
 Radiation: Radiating heat without direct contact or medium.
Interior of the Sun:
 Solar Flares: The sudden flash of brightness observed near the sun’s surface which is a collection of magnetic
energy including electrons, protons, and neutrons are called solar flares. They are concise particles and are
harmful for satellite communication.
 Core: The core of the sun (15 Million Degrees Celsius) consists of hydrogen atoms which fuse together due
to compression and creates helium. This is called nuclear fusion.
 Radiative Zone: Heat transfer zone of the sun.
 Convection Zone: It is next to the core of the sun. Here, the temperature drops to 2 million degrees Celsius.
 Photosphere: Its temperature is 6,000 degrees Celsius.
 3

 The atmosphere of the sun consists of the chromosphere and corona.

 Corona: It is seen in a form of spectral lines emitted by iron, calcium, and Nickel ions. The ionization of these
elements increases the temperature of the corona. The high temperature of the corona is due to the magnetic
field of the sun.
 Solar Flares (wind) is a stream of charged particles released from the upper atmosphere of the sun.
 These charged particles when trapped by the earth’s magnetic field while entering the upper atmosphere
result in the auroral (light) display. At the north pole, these lights are known as Aurora Borealis, and at
the south pole, these are known as Aurora Australis.
 Solar flares removed the gasses, debris, particles around them, and it did not remove any debris.
Sunspots:
 The dark-appearing areas present in the photosphere
from where solar flares originate are called the
sunspots.
 They are relatively a region cooler than its surrounding
and have strong magnetic fields.
 It appears and disappears after every 11 years. This
period is called the Sunspot Cycle.
 The greatest number of Sunspots in any given solar
cycle is designated as the “solar maximum” and the
lowest number is the “solar minimum”.
 It has harmful effects on the Earth's atmosphere.
Solar Storm:
 A solar storm is a disturbance on the Sun, which can
emanate outward across the heliosphere, affecting the
entire Solar System, including Earth and its
magnetosphere.
 A Geomagnetic Storm is a temporary disturbance of
the Earth’s magnetosphere caused by a solar wind
shockwave and/or cloud of a magnetic field that
interacts with the Earth’s magnetic field.
 The geomagnetic storms affect communication, satellites, navigation, magnetic lines of force, electrical
system, electronic system, magneto perception of birds and animals, etc.
Magnetic Storms:
 A period of rapid magnetic field variation is known as magnetic storms.
 They occur when strong gusts of solar winds collide with the magnetosphere of the earth.
 This results in the generation of electric currents in near-earth space. These are known as Ring currents and
they are mostly concentrated over the equator.
 4

 These storms and currents can harm our artificial

satellites and long-range radio communication.
 Global positioning systems dependent on satellites and
radio communication are also impacted.
Aurora:
 Electrons and ions sometimes descend from the
magnetosphere into the upper atmosphere and excite
the molecules of nitrogen and oxygen in the
atmosphere. These excited (ionized) molecules
produce light seen as Auroras.
 Auroras are mostly seen around the poles because of the highest intensity of the geomagnetic field.
 The auroras in Earth's Northern Hemisphere are called the Aurora Borealis.
Our Solar System:
 Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids
and comets, and a huge quantity of dust grains and gasses.
 The Sun is the central star of our solar system.
 There are eight planets in our solar system Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and
Neptune (My Very Efficient Mother Just Served Us Nuts).
 Mercury is the smallest & nearest planet to the Sun.
 These planets are divided into:
 Inner/Terrestrial Planets
 Outer/Jovian Planets
 5

1. Inner or Terrestrial Planets:

 These are the planets which have higher
densities and a solid rocky surface.
 These are Mercury, Venus, Earth, and
Mars.
 The terrestrial planets were formed in the
close vicinity of the Sun where it was too
warm for gasses on the surface to condense
into solid particles.
 The intense solar wind near the sun blew
off lots of these gasses and dust from the
surface of the terrestrial planets. That's
why the terrestrial planets have a rocky
surface.
2. Outer or Jovian Planets:
 These are the planets which have lower densities.
 These are Jupiter, Saturn, Uranus, and Neptune.
 The solar winds were not all that intense to cause similar removal of gasses from the Jovian planets, that’s why
the Jovian planets have gaseous surfaces.
Parameters Terrestrial Planets Jovian Planets
Location  Formed in the closed vicinity of the parent  At a distant location.
star (sun), here too warm for gases to
condense to solid particles.
Effect of  It blew off lots of gas and dust from the  The solar winds were not all that intense
Solar Winds terrestrial planets. Thus, have a rocky to cause similar removal of gases from
surface. the Jovian planets.
Size  The size of these planets is smaller as the  The size of these planets is bigger and
low gravity could not hold escaping gas have thick atmospheres
Density  Higher  Lower
Evolution of the Earth’s Atmosphere:
 Loss of Primordial Atmosphere: Initially (approximately 4.5 billion years ago) when the Earth was formed
it was hot and had a rough surface. The early atmosphere was mostly made up of hydrogen and helium. These
gases are very light, and they were able to escape the Earth's gravity relatively easily. The escape of these gases
was accelerated by the high temperatures of the Earth's surface.
 6

 Cooling of Earth's Interior: As the

Earth formed, it was initially a hot,
molten mass. Over time, the planet's
interior began to cool down. As the
Earth's interior cooled, volcanic
activity became more prevalent.
Volcanoes release gases, such as
water vapor (H2O), carbon dioxide
(CO2), nitrogen (N2), and smaller
amounts of other gases, into the
atmosphere (known as degassing).
 Modification of Earth: After 2-3
million years, when the earth cools
down, the atmosphere forms with
carbon dioxide and water vapor.
Because of the water vapor, clouds
formed, and rainfall started on the
earth’s surface. After which water
accumulated on the surface of the
earth and formed oceans. Almost
1500 to 2000 million years ago,
there was the modification of
Earth.
 After the formation of oceans life evolved in the form of algae.
 The algae started the process of photosynthesis by utilizing the carbon dioxide from the atmosphere.
During photosynthesis, Oxygen is generated in the atmosphere.

 1

DAILY
CLASS NOTES
Geography

Lecture - 09
Our Solar System
 2

Our Solar System

Our Solar System:
 Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like
asteroids and comets, and a huge quantity of dust grains and gases.
 The Sun is the central star of our solar system.
 There are eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and
Neptune (My Very Efficient Mother Just Served Us Nuts!!).
 These planets are divided into:
 Inner/Terrestrial Planets
 Outer/Jovian Planets

1. Inner or Terrestrial Planets:

 These are the planets which have
higher densities and a solid rocky
surface.
 These are Mercury, Venus, Earth,
and Mars.
 The terrestrial planets were formed in
the close vicinity of the Sun where it
was too warm for gasses on the
surface to condense into solid
particles.
 The intense solar wind near the sun
blew off lots of these gases and dust
from the surface of the terrestrial
planets That’s why the terrestrial planets have a rocky surface.
 3

2. Outer or Jovian Planets:

 These are the planets which have lower densities.
 These are Jupiter, Saturn, Uranus, and Neptune.
 The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.
That's why the Jovian planets have gaseous surfaces.

Parameters Terrestrial Planets Jovian Planets

Location Formed in the closed vicinity of the parent star (sun), At a distant location.
here too warm for gases to condense to solid particles.

Effect of Solar It blew off lots of gas and dust from the terrestrial The solar winds were not all that
Winds planets. Thus, have a rocky surface. intense to cause similar removal of
gases from the Jovian planets.

Size The size of these planets is smaller as the low gravity The size of these planets is bigger
could not hold escaping gas and have thick atmospheres

Density Higher Lower

Kuiper Belt
 It is also known as the Edgeworth-Kuiper belt.
 It is a region of the Solar System that exists beyond the
eight major planets.
 It extends from the orbit of Neptune (at 30 AU) to
approximately 50 AU from the Sun.
 It is similar to the asteroid belt, in that it contains many
small bodies, all remnants from the Solar System’s
formation.
 Unlike the Asteroid Belt, it is much larger –20 times as
wide and 20 to 200 times as massive.
The Moon:
 The moon is the only natural satellite of the Earth.
 Its diameter is only one-quarter that of the Earth and
it is about 3,84,400 km away from the Earth.
 The moon is tidally locked to the earth, meaning that
the moon revolves around the earth in about 27 days
which is the same time it takes to complete one rotation.
 As a result of tidal locking, only one side of the moon
is visible to us on Earth.
 Generally, the formation of the moon as a satellite of
the earth is the outcome of a ‘giant impact’ or what is
described as “The Big Splat”.
 4

 A body of the size of one to three times that of Mars collided into the
Earth sometimes shortly after the Earth was formed.
 It blasted a large part of the Earth into space. This portion of blasted
material then continued to orbit the Earth and eventually formed into
the present moon about 4.44 billion years ago.
Earth:
 Distance from Sun is around 149 million Km
 147.5 million km at Perihelion - closest to the Sun
 152.2 million km at Aphelion farthest from the Sun.
 Shape: Geoid (an oblate spheroid) slightly flattened at the Poles
and bulging at the Equator.
 Ratio of Land to Water: 3:7; Northern Hemisphere 2:3:
Southern Hemisphere 1:4 (southern hemisphere has a larger
surface area under water).
 Speed of rotation around the axis is maximum at the equator
and decreases poleward.
 Earth lies in the Goldilocks Zone where water can exist in a
liquid state.
 Goldilocks zone is a zone of balance (neither too hot nor
cold), also called the habitable zone.
 It is the densest planet in the solar system.
Evolution of the Earth:
 It is not possible to know exactly how the Earth was formed about 4,500 million years ago.
 Around 4.5 billion years ago, the Earth was a hot body, fluidic and molten. At that time a Big Splat Event
occurred, in which a larger body known as Theia (similar to the mass of Mars) hit upon the Earth and its
core started merging with Earth, but its atmosphere came out (exploded) and mixed with the atmosphere of
the Earth.
 Then a large portion was cut out from the Earth and
became round, which is presently known as the moon.
 Evidence regarding the earth’s structure, volcanic
eruptions, earthquake waves, etc. are doubtful.
 Earth has concentric layers of crust, mantle, and core.
 Crust- first part/ layer
 Mantle- the middle layer of the earth
 Core- innermost layer
 The reasons for the generation of heat are the
following:
1. Radioactive Material: The presence of radioactive
materials inside the earth also aided in the increase in the temperature. The decay or disintegration of the
radioactive materials leads to the generation of energy in the form of heat.
2. Big Splat Event: Due to the Giant Impact the earth was further heated up. This gave rise to the process
of differentiation, meaning that the earth-forming material got separated into different layers (depending
on densities).
 5

3. Compression and Pressure: The

compression and pressure due to
the pressure of several layers in the
interior of the earth increased the
temperature in the core.
 Due to heating and mixing, several
materials like Iron, Nickel, silicates,
etc. redistributed into several layers
depending on their densities.
 Starting from the surface to the central
parts, the earth has layers like the crust,
mantle, outer core, and inner core.
 From the crust to the core, the density
of the material increases.
 Heavy metals like Iron and Nickel
moved towards the Core of the Earth.
 Lighter materials (Silicates) moved
upwards and formed Mantle and Crust.
 Around 95% of the Earth's material
includes Iron, Magnesium, Silicate,
and Oxygen.
 Earth is the fifth largest planet in the
Solar System.
 It is also called a Blue Planet because,
from outer space, the Earth appears blue because its two-thirds surface is covered by water.
Evolution of the Earth’s Atmosphere:
 Initially (approximately 4.5 billion years ago) when the Earth was formed it was hot and had a rough surface.
Gases (carbon dioxide, water vapor, oxides of nitrogen, etc.) were coming out from the surface of the earth
due to the volcanism inside
the earth’s surface (known
as degassing).
 After 2-3 million years,
when the earth cools down,
the atmosphere forms with
carbon dioxide and water
vapor.
 Because of the water vapor,
clouds formed, and rainfall
started on the earth’s
surface.
 After which water
accumulated on the surface
of the earth and formed
oceans.
 6

 After the formation of oceans life evolved in the form of algae.

 The algae started the process of photosynthesis by utilizing the carbon dioxide from the atmosphere. During
photosynthesis, Oxygen is generated in the atmosphere.
Asteroid Belt:
 The asteroid belt is a torus-shaped region in the Solar
System, located roughly between the orbits of the planets
Jupiter and Mars.
 It contains a great many solid, irregularly shaped bodies,
of many sizes, but much smaller than planets, called
asteroids or minor planets.
 NASA’s OSIRIS-Rex Mission was launched in 2016 to
study the asteroid Bennu.
Meteorites:
 Meteorites are extraterrestrial objects that survive their journey through Earth's atmosphere and land on the
planet's surface, providing valuable insights into the formation and evolution of our solar system.
 Three stages:
 Meteoroide: Moving (in the universe) far away from Earth’s atmosphere.
 Meteors: When it enters the atmosphere of the earth.
 Meteorites: When it hits upon the earth surface.
Comets:
 These are icy objects which have icy fluids
which can be water or gas (nitrogen etc.).
When these bodies reach near the sun, their ice
melts.
 Comets are celestial bodies composed of ice,
rock, dust, and organic compounds.
 They originate from the outer regions of the solar
system and, when heated by the Sun, develop a
glowing coma and tail.
 Comets are considered time capsules, offering
clues about the early stages of our solar system
and its composition.
 It has no moons and no rings.
 It has no atmosphere but only a tail of gases and dust.
About Some Planets:
Mercury:
 It is the smallest and the nearest planet to the Sun.
 On Mercury, one day is equal to 59 Earth days.
 It is a rocky planet and its atmosphere contains Oxygen, Sodium,
Hydrogen, Helium, etc.
 Scientific Missions for exploration are Mariner & Messenger.
 The temperature on this planet is around -430 (Day) & -180 (Night).
 7

Venus:
 It is considered as 'Earth's-twin' because its size and shape are very
much similar to that of the Earth.
 Second closest planet to the sun at a distance of about 108 million km.
 It is probably the hottest planet (high temperatures of almost 480° Celsius
(900° Fahrenheit) because its atmosphere contains 90-95% of carbon
dioxide with clouds of Sulphuric acid.
 Venus spins backward (retrograde rotation) when compared to the other
planets.
 One day on Venus lasts as long as 243 Earth days.
Mars:
 On Mars one day is equal to 24 hours.
 It is a rocky planet and its Atmosphere contains Carbon Dioxide,
Nitrogen & Argon.
 It is also called the Red Planet because it has Iron as a mineral in the soil.
(Oxidation)
 It has two Moons-Phobos & Deimos.
 Scientific Missions for exploration are Mangalyaan & Perseverance.

Jupiter:
 It is the largest planet in the solar system.
 Its atmosphere contains hydrogen, helium, methane, and ammonia.
 Jupiter is a gas giant planet and therefore does not have a solid surface.
 Atmosphere is made up mostly of hydrogen (H2) and Helium (He).
 Jupiter has around 80 and 95 moons.
 It has a faint ring system that was discovered in 1979 by the Voyager 1
mission.
Saturn:
 It is the second largest planet in the solar system.
 It has around 83 (latest record as per NASA) known moons.
 It contains a spectacular Rings System- Nine Rings.
 Scientific Missions for exploration are Cassini, Hyugens, and
Voyager.
 Its Atmosphere contains Hydrogen & Helium.
 It is a Gas Giant and has no solid surface.
 One day on the planet lasts as long as 10.7 hours.
 Its distance from the sun is around 1.4 Bn Kms.
Uranus:
 One day on the planet lasts around 17 hours approx.
 It is an Ice Giant that contains around 80% Icy fluid of
Ammonia, water & methane.
 8

 It has around 27 Moons.

 Scientific Mission for exploration is Voyager.
 On this planet there is no evidence of Life is present.
Neptune:
 Its distance from the sun is around 4.5 Bn kms.
 One day on the planet lasts as long as 16 hrs.
 It is also called the Sister giant of Uranus.
 Its Atmosphere contains Hydrogen, Helium & Methane.
 It has around six Rings.
 The Scientific Mission for exploration is Voyager-2 (The only
spacecraft to have gone to Neptune).
Pluto:
 It was discovered in the year 1930.
 The International Astronomical Union (I.A.U) demoted its
status to Dwarf Planet in 2003.
 Pluto is 0.07 times the mass of other objects.
 It is the second-largest dwarf planet in the Solar System.
 It has five moons.
 Its largest moon is named Charon.

Conditions for the Planet (as per IAU):

1. Must be in orbit around the Sun.
2. Must be massive to be a sphere due to Gravitational Force.
3. Must have cleared the neighborhood.


 1

DAILY
CLASS NOTES
Geography

Lecture - 10
Interior of Earth
 2

Interior of Earth
Sources of Information about the Interior of the Earth:
❖ There are two types of sources about the Interior of the Earth:
○ Direct Sources
○ Indirect Sources
❖ Direct Sources: Rocks obtained from mining areas and materials obtained from volcanic eruptions are
the direct sources of information about the interior of the earth.
❖ Indirect Sources: Indirect sources of Earth's interior include seismic waves, gravitational fields, magnetic
fields, falling meteorites. Indirect sources help to understand the pressure profile, density profile, and pressure
analysis also.
 3

❖ Mponeng gold mine (deepest mine in the world) and TauTona gold mine (second
deepest mine in the world) in South Africa are deepest mines reaching to a depth of
Direct only 3.9 km.
Sources ❖ Volcanic eruption forms another source of obtaining direct information.
❖ Integrated ocean drilling project
❖ Deep ocean drilling project
❖ Increase in pressure and temperature with depth
❖ Seismic waves
Indirect
❖ Meteorites
Sources
❖ Gravitation
❖ Magnetic field

Direct sources:
❖ Mines: Mponeng Mines
❖ Located in South Africa
❖ Depth: 2.4 miles (3.9 km)
❖ Deepest gold mine
❖ We can study the direct effects like
temperature, pressure, soil, etc.
Volcanic Eruptions:
❖ It can lead to the formation of rocks and soils.
❖ We can study physical composition (shape, size, etc)
and chemical composition (elements present in it).
Integrated Ocean Drilling:
❖ We send mechanized ships into the deep ocean, to
study pressure, temperature, different layers, etc.
❖ There are a number of drill stations present which collect ocean resources
and study the interior of the earth.
Minerals on Earth:
❖ Silicate: Quartz, Feldspar, Ferromagnesian
❖ Carbonate: Calcite, Limestone, Marbles
❖ Sulphate: Pyrite, Iron, Sulphate
❖ Metallic: Iron, Aluminum

Indirect Sources:
1. Temperature and pressure: It changes with the depth. As we go deep into
the earth's crust both the temperature and pressure increase.
2. Gravity: After studying the gravitational force we get to know about the solid
mass inside the earth.
 4

3. Meteorites:
➢ When they fall on earth, their outer layer is burnt during their fall due to
extreme friction, and the inner core is exposed.
➢ The heavy material composition of their cores confirms the similar
composition of the inner core of the earth, as both evolved from the same
star system in the remote past.
4. Magnetic Field: It tells us the material inside the earth, which is rotating and
experiencing high currents.
➢ Because of the presence of the magnetic field earth is protected
from solar storms.
➢ The overall surrounding is known as the magnetosphere.
➢ As per Lenz law- whenever the current will move in a ring,
magnetic fields emerge. Because of the molten form of the deepest
part of the earth, there is a magnetic field (called the dynamo
effect).
5. Seismic Waves: Seismic Waves are forms of energy which are
generated within the Earth. These are the waves that cause releasing
energy. The study of seismic waves provides a complete picture
of the layered interior.
➢ When energy is released seismic waves are formed.
➢ These are of two types- Body waves (P and S waves) and
surface waves.
➢ By studying the properties of different types of waves like
speed in different materials geographers know about the
interior of the earth (layers, density, pressure, and
temperature of earth).
➢ Seismographs are instruments used to study seismic waves.
➢ The point where the earthquake starts is called Focus.
➢ The point above the focus on the surface is called the epicenter.

Types of Seismic Waves:

❖ Earthquake waves are basically of two types- body waves and surface
waves.
❖ Body waves are generated due to the release of energy at the focus and
move in all directions travelling through the body of the Earth. Hence, the
name body waves.
❖ The body waves interact with the surface rocks and generate a new set
of waves called surface waves.
❖ Surface waves move along the surface. The velocity of waves changes as
they travel through materials with different densities.
 5

Body Waves:
❖ Body waves flow inside a complete volume and they are two types of body waves: P and S-waves.
❖ P waves can travel through liquid and solids and gasses, while S waves only travel through solids. Scientists
use this information to help them determine the structure of Earth.

Body waves Primary (P) waves: ❖ First to arrive at surface.

Body waves are ❖ They are longitudinal waves, so can pass through
generated due to solids, liquids and gases.
the release of ❖ They create density differences in the material leading
energy at the to stretching and squeezing of the material. The higher
focus and move the density of medium the higher their velocity.
in all directions Secondary (S) waves: ❖ They are transverse waves, so can't pass through
travelling liquids.
through the body ❖ As they vibrate perpendicularly, these waves create
of the earth. crests and troughs.
Surface Waves:
❖ The surface waves are the last to report on seismographs.
❖ These waves are more destructive.
❖ They cause displacement of rocks, and hence, the collapse of structures occurs.
❖ These are also called long-period waves.
❖ They are low frequency, long wavelength, and transverse vibration.
❖ Generally affect the surface of the Earth only and die out at smaller depths.
Surface Waves: ❖ R waves are analogous to water waves
❖ The body waves interact with Raleigh (R) waves: i.e. movement of particles takes place in
the surface rocks and generate the vertical plane.
new set of waves called surface
waves. ❖ In L waves movement of particles
❖ These waves move along the takesplace in the horizontal plane only but
surface. at 90 deg to the direction of propagation of
❖ They are the last to report on the wave
seismograph. ❖ L waves are most destructive. The surface
❖ These waves are more Love (L) waves: waves get significantly amplified when they
destructive. pass through a soft ground like alluvial
❖ They cause displacement of deposits.
rocks, and hence, the collapse ❖ They are surface waves and don't go deeper
of structures occurs. into the earth. L waves are faster than R
waves. So the sequence of arrival is PSLR.

Features of Seismic Waves:

❖ Velocity of waves changes with density and elasticity (shear strength). If we go down inside the Earth, the
velocity of waves increases due to an increase in density.
❖ The denser the material, the higher the velocity.
 6

❖ Their direction also changes as they reflect or refract when coming across materials with different densities.
❖ It has helped scientists to understand the structure of the interior of the Earth like composition, physical
properties (solid, liquid and gas) etc.
❖ The waves refract while traveling in different mediums. This gives us information about the material inside
the Earth.

Upper Part of the Earth:

❖ Jaffery’s in his study of earthquakes in Kulpa Valley (Croatia), observed the seismic waves and
measured the velocity of different waves. (Pg-Sg waves with average velocity 4.5km/s, and average
density 27 gms/cm3 )
Middle Part of the Earth:
❖ Cornard in his study of earthquakes in 1923 measured the average velocity and density of the seismic
waves. (Average velocity 5.5km/s and the average density is 3gm/cm3 )
Lower Part of the Earth:
❖ As per the studies P and S waves were found in the lower layer.
❖ Average velocity of waves was 7-8 km/s. (Fastest speed/velocity)
❖ Average density was greater than 6gm/cm3.
❖ This showed the increase in the density of material and velocity of waves with the increase in the
depth of the earth.

Interior of the Earth:

❖ The structure of the earth's interior is made up of several
concentric layers.
❖ These different layers formed due to the redistribution by the
Big Splat Event (origin of the earth).
❖ Temperature and pressure increase as we go deeper towards the
center of the Earth because of the presence of radioactive
materials.
❖ Edward Suess in his study observed that the upper-most layer
(Crust) of the earth is made up of sedimentary rocks and
contains minerals like Mica and Feldspar.
❖ He identified three layers in the interior of the earth on the basis
of different compositions of the elements.
❖ Broadly three layers can be identified:
➢ Crust- first part/ layer (SIAL)
➢ Mantle- the middle layer of the earth (SIMA)
➢ Core- the innermost layer (NIFE)
❖ The term SIAL refers to the composition of the upper layer of Earth's
crust, namely rocks rich in aluminum silicate minerals. The depth
of the SIAL zone is 50 to 300 kms. SIAL is floating on the SIMA.
❖ SIMA: The SIMA, the next lower layer in Earth, which is rich in
silica and magnesium and is often exposed in the ocean basins. SIMA is a source of magma.
 7

❖ NIFE: Iron and nickel make up the majority of the earth's innermost core(that is why high densities are there),
known as the NIFE. It has metallic properties.
❖ Presently we can practically (actual data) analyze only 8-10 km deep layers of the earth and beyond that, our
analysis is totally based on the assumptions and studies.
❖ Modern Views: After analysis and calculation of seismic waves there are three layers of the Earth that are
Crust, Mantle, Core.
❖ Crust is divided into two parts i.e., Continental crust and oceanic crust.
❖ Continental crust (SIAL) and Oceanic crust (SIMA) interact with each other which leads to the formation
of mountains, plateaus, islands, etc.
❖ Oceanic crust is thinner in size, and continental crust is thicker.
❖ Continental crust floats on oceanic crust.
❖ Mantle is divided into two parts i.e, upper mantle and lower mantle.
❖ Core is also divided into two parts i.e., outer core and inner core.
❖ Old theories are less relevant in the present time and as per recent studies
scientists gave details about the interior of the earth.
❖ Because of the divisions into various layers there are various
discontinuities. It means the change in the profile, composition, and shape.


 1

DAILY
CLASS NOTES
Geography

Lecture - 11
Interior of Earth (Part 02)
 2

Interior of Earth (Part 02)

Structure of Earth:
❖ The structure of the earth’s interior is made up of several concentric layers. Temperature, pressure increase as
we go deeper towards the centre of the earth because of the presence of radioactive materials.
❖ Broadly three layers can be identified:
➢ Crust
➢ Mantle
➢ Core

Crust:
❖ The crust is the uppermost solid part
and a thin layer of the earth.
❖ The thickness is variable:
➢ Oceanic and Continental areas.
❖ Oceanic crust is thinner (30 km thick) as
compared to the continental crust (50-70
km thick).
❖ Oceanic crust density is more (3 gm/cc)
as compared to the continental crust (2.7
gm/cc).
❖ The total crust occupies 0.5-1% volume
of the whole earth.
❖ The total crust occupies 1% mass of the
whole earth.
❖ The crust is 70 -100 km thick in the
Himalayan region.
❖ The temperature of the crust increases
with depth, around 200 °C to 400 °C near the mantle-crust boundary.
 3

❖ The crust’s outer layer is made up of sedimentary material, and beneath it are acidic crystalline, igneous, and
metamorphic rocks.
❖ Lighter (felsic) sodium, potassium, aluminum silicates rocks, such as granite, make up the continental crust.
❖ On the other hand, the oceanic crust is made up of thick (mafic) iron, magnesium, silicate, and igneous rocks
like basalt.
❖ Continental Crust:
➢ It is thicker: it mean thickness is 30 to 70 km & density is 2.7g/cm3
➢ Rock Type: Granite
➢ Minerals: Silica + Aluminum = SIAL
❖ Oceanic Crust:
➢ It is thinner layer, it mean thickness is 5 to 30 km & density is 3g/cm3
➢ Rock type: Basalt
➢ Minerals: Silica + Iron + Magnesium = SIMA
❖ Because of the divisions, there are various discontinuities. It means the change in the profile, composition, and
shape.
 4

Mantle:
❖ It is a layer below the crust.
❖ The mantle extends from Moho’s discontinuity (35 km) to a depth of 2,900 km.
❖ It occupies roughly 83 percent of the Earth’s volume and 67 per cent of the Earth's mass.
❖ The crust and the uppermost part of the mantle are called the lithosphere.
❖ Outermost solid part:
❖ Crust and uppermost layer of the mantle are combinedly called the lithosphere (10 to 200 km).
❖ The upper portion of the mantle is called the asthenosphere(~400KM). It is in a semi-liquid state.
❖ Lithosphere floats on the asthenosphere.
❖ Density:
➢ Upper Mantle- 2.9-3.3 g/cm3
➢ Lower Mantle- 3.3-5.7 g/cm3.
❖ Upper mantle has low density while the lower mantle has higher density.
❖ Density of the mantle increases with the depth increases because seismic waves velocity increases with the
depth increases.
❖ A convective material circulation occurs in the mantle as a result of the temperature differential between
upper core and mantle.(although solid, the high temperatures within the mantle cause the silicate material to be
sufficiently ductile).
❖ So, heat is transferred from the upper core to the mantle, and the mantle gets heated up. But the crust will be
on the cooler side and the lower mantle is very hot, so the upper mantle will be cooler in comparison to the
lower mantle.
❖ Heat gives energy to the molecules in the hot mantle, so molecules run away means expansion of the hot
mantle takes place(Size increases), volume increases.
❖ Density is inversely proportional to volume, so density of hot mantle decreases.
❖ Heated molecules expanded and started to move towards the cold mantle.
 5

❖ Molecules in the cold mantle closed pack, so the cold mantle shrinks and volume decreases and density
increases (Mass is constant).
❖ Therefore the cold mantle will become heavier than the hot mantle, so the cold mantle moves towards the hot
mantle. It creates a complete circle here.
❖ This phenomenon is called Convection currents due to differences in temperature.
❖ The motions of tectonic plates represent the mantle’s convection at the surface.
❖ The convection currents become the root cause of the formation of volcanoes, islands, and mountains.
❖ Seismicity in the mantle should be inhibited by high-pressure circumstances. However, earthquakes have been
detected in subduction zones as far as 670 kilometers below the surface (420 mi).
❖ The mantle extends from Moho’s discontinuity to a depth of 2,900 km.
❖ The material in the upper mantle portion is called magma.
❖ Once it starts moving toward the crust or it reaches the surface, it is referred to as lava.
Core:
❖ Core-mantle boundary is located at a depth of 2,900 km.
❖ It is made of NIFE layer (nickel and iron).
❖ Core lies between 2900 km and 6400 km below the mantle of earth.
❖ Density of the core is higher than the mantle and varies from 5.5 to 13.6 g/cm3 of earth.
➢ Highest densities are obtained in core, because of heavy elements present here with high pressure of
superincumbent load.
❖ Outer layer of core is in molten state and inner layer Solid State (5000 Celsius).
❖ Volume and mass of the core are 16% and 32% of the total volume and mass.
❖ The total pressure exerted by the mantle and crust of the earth trying to compress the core.
Temperature Analysis:
❖ As we go down the temperature increases.
❖ At every 100 meters the temperature increases by 2-3 degrees Celsius.
❖ At the depth of 40 km the temperature is around 1000 degree Celsius. Due to molten rocks and volcanic
activity.
❖ If it has the same rate of rise, the temperature of the core will be 25000 degrees Celsius.
➢ But, the rate of temperature rise is reduced from crust to core because the maximum amount of radioactive
elements is present in the upper layers.
❖ The temperature of the core is around 4000-5000 degrees Celsius. It's very high due to:
➢ The disintegration of the radioactive materials.
➢ Because of the increasing load, the entire gravitational energy which is occurring due to the increasing
load is converted into thermal energy.
❖ Rocks are bad conductors of heat that's why the heat of the core is not transferred to the surface.
 6

Rate of increment of temperature is unequal in Core:

Depth (in KM) Temperature (Degree Celsius)

1000 km 1100

400 km 1500

700 km 1900

2900 km 3700

5100 km 4500
Pressure Analysis:
❖ As we go down from crust to core the mass of rocks increases and pressure also increases.
❖ Even in this high temperature, because of high pressure in the inner core, it does not melt as pressure increases
the melting point or pressure does not allow it to melt, therefore the inner core is in a solid state.
❖ As we move upward toward the outer core there is a decrease in the pressure and temperature, thus it is in a
liquid state.
Density Analysis:
❖ In the crust velocity was around 1-2 km/s. (Minimum density)
❖ The mantle velocity was around 3-4 km/s. (Moderate density)
❖ The core velocity was 5-8 km/s. (High density)
❖ The density is rising because of:
➢ Closed pack system of earth interior (Minimum Volume)
➢ Heavy elements are increases with depth
➢ Rising temperature and pressure.
❖ The evidence for this density change is seismic waves (P and S waves).
❖ As we went down the
velocity of waves was
rising.
❖ According to the geographers
the velocity rises with the
rising density.
The Structure of the Earth:
❖ The continental crust,
oceanic crust, and upper
mantle are known as the
lithosphere. It floats upon the
asthenosphere due to the buoyancy force.
 7

❖ The lower mantle is known as the Asthenosphere.

Chemical Composition of Earth:

❖ The Earth is composed of various elements and compounds that form its overall chemical composition. Here
are the main components of the Earth's composition:


 1

DAILY
CLASS NOTES
Geography

Lecture - 12
Geomagnetism
 2

Geomagnetism
Earth’s Geomagnetic Field:
 Earth has three layers- Crust, Mantle, and Core.
 Geomagnetism is the study of the dynamics of the Earth's magnetic field, which is produced in the outer
core.
 The earth acts as a dipole magnet where the
geomagnetic south pole is near the earth's geographic
north and vice versa.
 The geomagnetic field is a dynamic field and it
changes with location and time.
 Magnetic lines of force created by the bar magnet
(Hypothetical) causing Magnetism to align around the
earth called the Geomagnetic field of Earth.
 Earth’s axial tilt (Geographical) is about 23.5 degrees.
 Approximately, it is the field of a magnetic dipole
presently tilted at an angle of about 11 degrees with
respect to Earth's rotational axis, as if there were a
bar magnet placed at that angle at the center of the
Earth.
 The position of the magnetic pole is different from the earth’s geographic poles (difference of alignment).
 Since opposite poles attract, the North Magnetic Pole of the Earth is the south pole of its magnetic field. And
vice-a-versa
 Earth's magnetic field moves from the north to south pole of the Earth's magnetic bar.
Theories of formation of magnetic field:
 Gilbert's theory: It was the first theory. He said that there is an existence of a real bar magnet inside the
earth and because of it there is a magnetic field.
 Criticism: It is impossible to have a solid bar inside the earth (liquid core) because Geographers said
that inside the earth the very high temperature is nearly 5000 degrees Celsius and it will melt the bar
magnet.
 Rock magnetism: There are many magnetic rocks (randomly placed) that exist inside the Earth, they cause
magnetism of earth around the Earth.
 Criticism: Randomly distributed rocks can’t result in this large-scale magnetism and rocks will melt
inside the deep earth because of the high temperature in the core of the earth.
 Theory of Dynamo effect: It was propounded by Walter. The outer core of the earth is in a molten state
and the inner core is solid. The Magnetic Field is generated by the motion of molten iron alloys in the outer
core of the Earth.
 Differences in pressure, temperature, and composition within the core of the earth cause convection
currents in the molten metal.
 3

 The flow of liquid iron generates electric currents and

produces magnetic fields. This effect is known as
Dynamo Effect.
 A molten outer core inside the earth is rotating and
traveling deep inside the earth, a rotating molten mix
contains positive and negative ions in the molten
layers.
 The Magnetic Field of the Earth is generated by the
motion of molten ions in a clockwise direction in the
Earth’s outer core. This flow of liquid iron generates
electric currents (electrons movement), which in turn
produce magnetic fields.
 This effect is known as Dynamo Effect (As per Right Hand Thumb Rule).
 Right-Hand Thumb Rule explains that the clockwise rotation of current leads to the magnetic
field moving in the direction of the thumb (downward).
 Rotation of cores is due to the difference between the density of cores.
 Radioactive atoms disintegrate in the core of the earth and explode in small localized explosions
which exert force in the core and start the movement. Heavy elements come down and light
elements go up in the core layers in this process.

NOTE:
 Ionization: It is the process in which an atom/molecule acquires a positive/negative charge by losing or
gaining electrons. The resulting species that carry charges are known as ions. The positively charged
species are known as cations while the negatively charged species are known as anions.

Geomagnetic Poles:
 The Geomagnetic poles (dipole poles) are the intersections of the Earth's surface and the axis of a bar magnet
hypothetically placed at the centre of the Earth.
 There is such a pole in each hemisphere, and the poles are called "the
geomagnetic north pole" and "the geomagnetic south pole",
respectively.
 The magnetic poles (the magnetic north pole and the magnetic south
pole) are the points at which magnetic needles become vertical.
 The difference in the position of magnetic poles and geomagnetic
poles is due to the uneven and complex distribution of the earth's
magnetic field.
 Magnetic lines of force come out from the earth at Magnetic North
and all Magnetic lines of force are converged into the Magnetic
South.
 Why are the Magnetic poles and Geographic Poles separate?
 4

 Because magnetic orientation keeps on changing. So, the North Magnetic Pole of the Earth is the
south pole of its magnetic field. And vice-a-versa.
 Earth's magnetic field lines keep on changing with time;
it is not constant.
 Magnetic north pole is not in line with the magnetic south;
the magnetic pole axis is a little bit tilted. This misaligned
axis is because of the molten magnetic field movement
continuously so poles change regularly.
 The geomagnetic poles are antipodal points where the axis
of a best-fitting dipole intersects the surface of Earth.
 However the actual magnetic poles of the Earth are not
antipodal; that is, the line on which they lie does not pass
through Earth's center. Because the core of the earth is in
the molten state and the surface is uneven, that is why the
axis will be uneven (not in a straight line).
 If the Earth's magnetic field were a perfect dipole, then the
field lines would be vertical to the surface at the Geomagnetic
Poles and would coincide with the North and South magnetic
poles.
 However, the approximation is imperfect, and so the Magnetic
and Geomagnetic Poles lie some distance apart.
 North of the Compass will be pointing towards the south of the
magnet, because the north and south attract each other.
Geomagnetic Reversal:
 A geomagnetic reversal is a change in an Earth's magnetic
field or magnetic lines of force changing its direction, such
that magnetic north and magnetic south interchange their
positions. This happens in 10,000 years to 2.5 lakh years.
 The intensity of the geomagnetic field is greatest
near the poles and weaker near the Equator.
 The earth's magnetic pole in the northern
hemisphere is located on a peninsula in
northern Canada.
 Earth's magnetic equator passes through
Thumba in South India.
 At present, earth's magnetic poles are:
 Magnetic South is located in Ellesmere
Island, Canada, and Magnetic North is
located in Antarctica.
 5

Causes of the Reversal:

 Sun spot theory: Solar storms of a high magnitude completely cover the earth and which led to a change in
its polarity. But this theory is not a valid reason.
 Comet hit theory: A huge comet came from the universe and hit Earth due to which the core of the Earth
shook and the Earth's magnetic field changed.
 Local dipole and heat transfer: A small magnetic field is generated due to localized rotation. It is the most
valid theory.
 The localized magnetic fields in small dipoles and heat of the core get transferred to the mantle, which
leads to the change in density of the dipoles locally inside the core.
 Density change leads to the movement of parts of the core of the earth and due to which there is a
reverse in the rotation. Some of the rotations are in opposite directions, due to which after 10,000
years to 2.5 lakh years the overall magnetic field gets reversed.
 Local small dipoles rotate in the reverse direction because they start to lose the heat and get cold. They
become heavy and fall down on one another dipoles and by hitting each other leads to changing their
direction. So the direction of the core rotation also changed.
Significance of Geomagnetic Field:
 The Geomagnetic field acts as a shield and blocks the harmful solar winds emerging from the sun. The
solar winds consist of charged particles that can severely damage life on the Earth.
 However, some particles manage to enter our planet and are directed by the magnetic field towards
the poles, which produce amazing lights known as Polar Lights or Auroras.
 It also helps in navigation by use of a compass.
 Magneto-Perception: Some animals can use this magnetic field to navigate while migrating over long
distances. This phenomenon is called Magneto-Perception. Example: olive ridley turtles come to Odisha
coast every year etc.
 The study of paleo-magnetism provides
information about the past record of
geomagnetism and the age of rocks on
the surface of the planet.
 Paleomagnetic studies have also helped
in developing the theories of Seafloor
spreading and Plate Tectonics.
 Geomagnetic field leads to the
formation of the magnetosphere
around the Earth.
 It can also cause various natural disasters
like forest fires (due to heating),
tsunamis, earthquakes, etc.
 6

Additional Information:
Magnetosphere
 It is a region of space surrounding the Earth (or any other
planet or star) in which charged particles are affected by
the geomagnetic field or magnetic field of that body.
 It traps charged particles from the solar winds and funnels them
into a plasma.
 It extends up to 60,000 km on the side facing the Sun and to a
greater extent on the opposite side.
 Its boundary is known as Magnetopause, outside which is a
turbulent magnetic region known as magneto-sheath.
 It contains the Van Allen radiation belts containing high-
energy charged particles.
 The lower belt contains electrons and protons extending from 1000 to 5000 km above the Earth’s
equator.
 The upper belt has mainly electrons extending from
15000 to 25000 km above the equator.
Magnetic Storms:
 A period of rapid magnetic field variation is known as
magnetic storms.
 They occur when strong gusts of solar winds collide
with the magnetosphere of the earth.
 This results in the generation of electric currents in
near-earth space. These are known as Ring currents
and they are mostly concentrated over the equator.
 These storms and currents can harm our artificial satellites and long-range radio communication.
 Global positioning systems dependent on satellites and radio communication are also impacted.
Auroras:
 Electrons and ions sometimes descend from the magnetosphere into the upper atmosphere and excite the
molecules of nitrogen and oxygen in the atmosphere. These excited molecules produce light seen as
Auroras.
 Auroras are mostly seen around the poles because of the highest intensity of the geomagnetic field there.
 The auroras in Earth’s Northern Hemisphere are called the Aurora borealis.
 Their southern counterparts, which light up the Antarctic skies in the Southern Hemisphere, are known as
Aurora Australis.


 1

DAILY
CLASS NOTES
Geography

Lecture - 13
Geomorphology
 2

Geomagnetism (Part-02)
❖ Carbon dioxide existed before the oxygen in the earth's atmosphere. In the process of photosynthesis, oxygen
is released into the atmosphere.
❖ Lava in the process of volcanism exists in the mantle layer.
❖ Highest density and highest velocity of seismic waves in the core layer of the earth.
❖ Oceanic crust has a higher density than the Continental crust.
❖ At the time of the summer solstice in India, the continuous night is present in the south pole.

Geomorphism:
❖ The term ‘Geo’ means earth and ‘morphism’ means change/formation.
❖ The formation of different forms of structures on the earth is called Geomorphism.
➢ For example coasts, mountains, valleys, plateaus, etc.
❖ It studies the movement of the earth’s crust layer movements.

Geomorphic processes:
❖ The endogenic and exogenic forces that cause physical stresses and chemical actions on the earth's surface
or materials and change the configuration of the surface of the earth are called geomorphic processes.
❖ These different relief features can be mountains, valleys, plateaus, etc.
❖ These processes are caused by geomorphic agents, such as magma, water, wind, sunlight, bacteria,
earthworm, etc.
❖ The geomorphic forces can be both internal (example: Magma) or external (example: Water, waves, currents).

Forces Responsible for Geomorphism:

❖ Heat: Heat is generated because of the decay or disintegration of radioactive elements.
➢ Magma present inside the Earth is forced itself to come out from the Earth in the form of lava and it
forms a plateau. This process deforms the earth's crust.
❖ Temperature and pressure differential: Temperature and pressure differential cause changes in density in
the interior of the earth. Because of density changes heat flow happens. It starts Convection currents inside the
earth.
➢ Convection currents induce magma which comes out on the surface of the earth as lava from
volcanoes.
➢ Magma contains molten metals, molten rocks, many gases, water vapor, etc,
❖ Rotation of Earth: The earth rotates in the direction of west to east and tries to rotate water and air in the same
direction. This force is called the Coriolis force. It affects winds, rainfall, cyclones, mountains, etc. which
help in changing the topography of hills, and deserts on the surface of the earth.

Types of Earth Movements/Geomorphic process:

❖ The surface of the earth is subjected to external forces originating within the Earth’s atmosphere and by internal
forces from within the Earth.
❖ There are two types of forces:
 3

➢ Endogenetic/ Endogenic Forces

➢ Exogenetic/ Exogenic Forces
❖ ‘Endo’ means inside and ‘genesis’ means formation.
❖ ‘Exo’ means outside and ‘genesis’ means formation.

❖ There are two types of forces that are responsible for the earth's movement. The external forces are known as
exogenic forces and the internal forces are known as endogenic forces.
❖ Endogenic forces:
➢ These forces are responsible for the movements inside the earth.
➢ They cause the building or construction on the earth.
➢ It is responsible for the formation of Islands, continents, volcanic mountains, plateaus, etc.
➢ Types of Endogenic forces:
✓ Diastrophic movements: These are very slow, it takes thousands of years. It is present deep within
the earth. It is constructive on the earth and helps in the creation of mountains, plateaus, valleys, etc.
✓ Sudden Movements: They occur very quickly and take less time. These are destructive in nature.
For example: earthquakes, volcanoes, etc.
❖ Exogenic forces:
➢ These forces are responsible for the movements outside the earth.
➢ They cause the wearing down (degradation) of relief/elevations and filling up (aggradation) of
basins/ depressions on the earth.
➢ Weathering, mass wasting, erosion, and deposition are slow exogenic geomorphic processes.
➢ It is responsible for the formation of dry hills from greenery hills.

Diastrophism:
❖ It includes all the processes that move, elevate or build up portions of the earth’s crust.
❖ It includes (i) Orogenic processes (ii) Epeirogenic processes:
 4

❖ Epeirogenic processes:
➢ Epeiro means continent and genic means genesis.
➢ Involves uplift or warping of large parts of the earth’s
crust.
➢ It leads to continent-forming processes.
➢ Forces acting radially (force is along the radius), and
leads to the formation of continents.
➢ These involve vertical movements, it can be both
upward or downward.
➢ Upward movement is called as upliftment
(Example: Kathiawar coast, Tirunelveli coast,
etc.), and downward movement (Example: Rann of
Kachchh, Andaman and Nicobar, western coast, etc.) is the subsidence or down thrusting of
landforms.
➢ Due to epeirogeny, there may be simple deformation.
❖ Orogenic processes:
➢ Oro means mountain and genic means genesis.
➢ It involves mountain formation through folding and affecting long and
narrow belts of the earth’s crust.
➢ In this process the crust is severely deformed into folds.
➢ Types of Orogenic processes:
✓ Tangential (tensile) forces are horizontal forces. It is also known as
tangential forces. It works in two opposite directions. It leads to the formation
of rift valleys, faults, and cracks (fissures). Example: Block mountains-
Black forest etc.
✓ Compressional processes: It is pushing force. It will lead to the formation
of folds. It can lead to up-warping and down-warping. Examples of fold
mountains: the Rockies (North America), Andes(South America),
Himalayas, etc.
 5

Additional Information:
❖ Shear processes: It is the force acting in a direction
that's parallel to (over the top of) a surface or cross-
section of a body. It leads to the formation of faults
(Example: San Andreas fault).
❖ Fault: It is a fracture or discontinuity in a volume of
rock across which there has been significant
displacement as a result of rock-mass movements.
❖ Folding: A fold is an undulating or wave-like structure that forms when rocks or a part of the earth’s
crust is folded or deformed by bending under compressional stress. The folds are made up of multiple strata
or rock layers.

Sudden Movements:
❖ These are quick or sudden earth movements.
❖ There can be divided into two parts:
➢ Volcanism and Earthquake
❖ Volcanism: It is the process of the movement of molten rock (magma) toward the earth’s surface. It leads to
the formation of many intrusive and extrusive volcanic forms due to the pressure generated by the Magma.
Examples: Volcanic mountain (Mount Vesuvius, Italy), Volcanic Plateau ( Deccan plateau).
❖ Earthquake: It is a sudden release of energy in the form of waves due to plate interaction, rock slippage, and
human activities (explosion, construction of the reservoir, etc.). These waves' energy shakes the earth's surface.
It is destructive in nature and can lead to the formation of rifts, schisms (cracks/splits) and valley formation.

NOTE:
Significance of studying Endogenic and Exogenic movements:
❖ Formation and destruction of various features: like formation of Himalayas (example Mt Everest
height is rising), Aravalis(undergoing erosion), Islands, and destruction like Joshimath is sinking.


 1

DAILY
CLASS NOTES
Geography

Lecture – 14
Geomorphology
(Part - 02)
 2

Geomorphology (Part - 02)

Exogenic Forces:
 The forces that take place outside the Earth's surface are called Exogenic Processes.
 Forces responsible for Exogenic movements:
 The insolation from the sun is responsible for heat. This heating and cooling cause contraction and
expansion of the rocks and lead to the formation of cracks and loose material comes out. It will lead to
breakage of rock and change the face, size, and configuration of the rock.
 This heat leads to wind movement, rainfall, chemical reactions, biological phenomena, and physical
weathering.
Factors affecting the Exogenic processes:
 Climate: Climate is the sum total of weather conditions over a large area for a long period of time which is
more than thirty years. It includes
various agents like seasons,
insolation, winds, temperature, and
rainfall, which help in the process of
denudation.
 Rocks type: Different types of rocks
offer varying resistances to various
geomorphic processes. The faults and
cracks in the rocks result in easy
breakage of rocks into small particles. The softness, hardness, inclination, and orientation of rocks (rock
slides) of rocks decide the rate of disintegration of rocks.
Denudation:
 All the exogenic geomorphic processes are
covered under the term denudation.
 The term ‘denude’ means to strip off or to
uncover. Hence denudation means removing
the top layer of the earth's surface or stripping
off the material cover.
 It includes weathering, mass wasting/
movements, erosion, and transportation. These
all remove the top layer/cover and change the face and configuration of the earth.
 Weathering is the result of the action of elements of weather and climate over the earth's materials.
 Winds, rainfall, flood water, cyclones, and hail storms can be the reasons for denudation.
 3

Types of Denudation Processes:

 Erosion: Erosion is the process of removal of topsoil from any field by external forces such as rain, wind,
etc. It involves the acquisition and transportation of rock debris.
 A massive rock breaks into smaller fragments or particles through weathering and other agents like,
running water, groundwater, glaciers, wind, and
waves remove and transport it to other places depending
upon the dynamics.
 Mass Movements: Mass movements is the movement of the
rocks, boulders, sand, and particles down the slopes.
 It transfers the mass of rock debris down the slopes
under the influence of gravitational force. Example:
Landslides, avalanches, debris flow, etc.
 Weathering:
 It is the mechanical disintegration and chemical
decomposition of rocks through the actions of various agents like weather and climate.
 It is a phenomenon of mechanical disintegration (breaking into small parts) and chemical
decomposition due to physical factors (pressure, temperature) and chemical processes (oxidation,
reduction, hydration) of the earth's relief features and materials .
 In this process there is very little or no motion of materials takes place, therefore it is an in-situ or on-
site process.
 It is conditioned by several complex geological, climatic, topographic, and vegetative factors.
 There are three major types of weathering processes : (i) chemical weathering (chemical reaction) (ii)
physical or mechanical weathering (Temperature and pressure) (iii) biological weathering
processes (due to biological organisms- plants, animals, etc).
 Physical weathering happens due to the action of temperature and pressure stress (internal
resisting forces). For example, it causes cracks in the rocks.
 Chemical weathering happens due to the action of chemical reactions such as the action of water
and carbon dioxide on the rocks. It may be in the forms of hydration, solution weathering,
carbonation, oxidation, or reduction.
 Biological weathering happens due to the action of termites, bacteria, fungi, plants, animals, etc.
Factors Influencing the Weathering:
 Geological Structure: The type of rock (hardness/softness), endogenic forces, etc., is a factor that influences
weathering. For example, softer rocks are more susceptible to weathering than harder rocks. Endogenic
forces, such as earthquakes and volcanoes, can also break down rocks and contribute to weathering.
 4

 Heat by the sun/Climatic weather phenomenas: Temperature, precipitation, different pressures, dry or
arid, etc., are factors that influence weathering. For example, higher temperatures can speed up
chemical weathering, while more precipitation can lead to physical weathering. Dry or arid conditions
can also contribute to weathering by dehydrating rocks and making them more susceptible to breaking
down.
 Topography: Land that is plain or hill, etc., is a factor that influences weathering. For example, rocks that
are exposed to more sunlight and precipitation are more likely to weather than rocks that are sheltered. Hills
and mountains can also contribute to weathering by exposing rocks to more wind and water.
 Natural Vegetation: Trees, plants, etc., are factors that influence weathering. For example, trees and plants
can help to protect rocks from weathering by providing shade and trapping moisture. However, they can also
contribute to weathering by releasing acids that break down rocks.
Significance Of Weathering:
 It is responsible for the formation of soils and erosion and deposition.
 The biodiversity of a region is basically depending on the depth of weathering.
 Weathering also aids mass wasting, erosion, and reduction of relief and changes in landforms.
 The weathering of rocks and deposition helps in the enrichment and concentrations of valuable ores of
iron manganese, and aluminum copper.
Difference between Physical, Chemical, and Biological Weathering:

Physical Biological Chemical

Thermal changes: Repeated Burrowing and wedging Dissolution; dissolving action of

temperature change, stress develops, by organisms like water and various organic acids.
and rocks crack and split. (onion earthworks termites
peeling/ exfoliation)

Frost action: sharp angular fragments Rodents help in exposing Hydrolysis; reaction with
of rocks are formed the new surfaces to hydrogen in air or water
chemical attack and assist
in the penetration of
moisture and air

Physical Biological Chemical

Pressure release -- Oxidation; Reaction of oxygen in

air or water with minerals in the
rock
 5

Hydraulic action; repeated wetting -- Solution

and drying stress

Hyaloclastite; mechanical weathering -- Hydration

-- -- Carbonation

Types of Physical Weathering:

Exfoliation: It is the process in which large flat or curved upper sheets of rock get fractured and detached from
layers of rocks.
 Exfoliation due to Pressure Release:
 The intrusive rocks formed deep beneath the Earth’s surface
are under tremendous pressure due to the overlying load of
the above rock.
 The removal of the overlying rock load/material due to
erosion causes vertical pressure release and freeing up of the
bottom rocks (which expands due to pressure release).
 The upper layers of the rock continue to expand and result in the
disintegration of rock masses.
 The fractures in the rocks will develop nearly parallel to the ground surface.
 In the areas the ground surface is curved or arched tends to produce massive sheets or exfoliation slabs
of rock.
 The exfoliation sheets resulting from expansion and pressure release may expand up to hundreds or
even thousands of meters horizontally.
 The large, smooth, and rounded domes formed due to exfoliation are called exfoliation domes.
 The loose materials of the rocks will flow due to wind and water.
 Exfoliation due to Thermal Effect:
 The rocks are subjected to heat in the daytime.
 Various minerals in rocks have their limits of expansion
and contraction and with the rise in temperature, every
mineral expands.
 Because of insolation the top layer of rock heats up and
expands. It exerts pressure on its surrounding rocks.
 As the temperature falls the rocks cool down and contract
during night.
 6

 The differential heating and expansion & contraction of surface layers of the rocks lead to the
formation of thermal stress. It results in heaving and fracturing of the rocks.
 In this process the top layer of the rocks breaks down and with time due to winds and rainfall, loose
material of the rocks is removed in the form of mud.
 Due to exfoliation in some rocks such as granites , the surface becomes smooth. The rounded (small
or big) boulders are called tors.
 This process is most effective in dry climates and high elevations where diurnal temperature changes
are very high.
 Exfoliation due to Granular Disintegration:
 This process occurs in the rocks which are composed of different types of coarse-grained minerals.
 The rocks are composed of different types of dark and light-colored minerals.
 Due to heat and temperature, light color minerals rocks absorb less heat and expand less
compared to dark color grains (dark-colored grains as they expand more). This leads to tension and
results in the development of cracks and breakage of rock.
 After some time, the dark-colored minerals rocks separate from the rock. The cracks of the rocks
expanded and lead to the separation of the loose material in the form of sand and small rocks, which are
called granules. This process is called as exfoliation due to the granular disintegration.
 Because of differential disintegration the weathering process takes place.
 This type of weathering takes place in cold regions and locations situated at high altitudes.
 Frost weathering:
 It occurs due to the formation of ice within pores and cracks of rocks
during repeated cycles of freezing during the night and melting during
the daytime.
 The volume of ice is more than the volume of water, so ice expands
and results in the disintegration of the rock.
 This force of ice expansion is very powerful, its resulting expansion
continuously, and with time the small intergranular fractures become
wider and ultimately break apart the rock.
 This process is most effective at high elevations in mid-latitudes where freezing and melting are
common. The glacial areas are subject to frost wedging.
 In this process, the rate of freezing is important, rapid freezing of water causes sudden expansion.
 Freeze-Thaw Cycle: It is the continuous expansion (freezing) and contraction (converting into water) of
water inside the rocks is called the freeze-thaw cycle.
 7

 The repeated freeze -thaw cycles weaken the rocks which,

over time leads to breakdown of rocks into small pieces.
 The process of the splitting of rocks along the joints into blocks
is called block disintegration.
 Freeze wedging is caused by the repeated freeze-thaw cycle due
to which the cracks filled with water and subsequent freezing
and thawing results in disintegration.
 Rock shattering: Continuous severe frost can disintegrate rocks along weak zones to produce many small
angular pieces with sharp corners and edges through the
process of shattering.
 It breakdown the rocks completely. The shattered piles
up of rock fragments are called scree.
 These are generally found at the foot of mountain areas or
along slopes.
 Salt Weathering:
 Salts in the sea rocks have some water in it, because of
insolation water evaporates and salt gets heated up and expands (due to thermal action, hydration,
and crystallization).
 Several salts like calcium, sodium,
magnesium, potassium, barium, etc. have a
property to expand.
 The rate of expansion of salts depends on
temperature and their thermal properties.
 The expansion of salt leads to disintegration
and breaks the rocks.
 Salts in rocks expand due to the thermal
expansion causing the splitting of individual grains within rocks and eventually breaking the rock.
 It is a slow process and occurs in areas with frequent wetting and drying conditions which favors
the salt crystal growth.
 The high temperature ranges (between 30 and 50 degrees Celsius) in desert areas favor salt expansion.
The materials like sodium chloride and gypsum in desert areas heave up overlying layers of materials
and result in the development of polygonal cracks all over the heaved surface.
 Due to the crystallization of the salt, chalk breaks down most readily, followed by limestone,
sandstone, shale, gneiss, and granite, etc.
 8

Chemical Weathering Processes:

 It is the chemical decomposition of rocks due to the action of chemical reactions.
 Carbonation, hydration (action of water), oxidation, and reduction leads to chemical weathering
processes through chemical reactions by oxygen,
surface, and/or soil water and other acids.
 Natural dissolution:
 Dissolution: It is a process where a solute in
gaseous, liquid, or solid phase dissolves in a
solvent to form a solution.
 Some minerals due to their natural solubility
(like nitrates, sulphates, and potassium), oxidation
potential (non rich minerals), will weather through
dissolution naturally (rains).
 Oxidation:
 Oxidation means a reaction of a mineral with oxygen to form oxides or hydroxides.
 This process occurs in rocks where there is ready access to the atmosphere and oxygenated waters.
 Red soil appears in red color because of the presence of iron oxide.
 The minerals most commonly involved in this process are iron, manganese, sulphur, Aluminium, etc.
 Iron rods and aluminum rods disintegrated and weakened because of the oxidation process. The red
colour of iron upon oxidation turns to brown or yellow.
 Sulphur oxide leads to acid rain which leads to a reaction with material and its disintegration.
Example: Taj Mahal etc.
 In the process of oxidation rock breakdown due to the disturbance caused by the addition of oxygen.
 If the oxidised minerals are placed in an environment where oxygen is absent, the process of reduction
takes place. Such conditions exist generally in areas where the water table is low (in areas of stagnant
or waterlogged ground).
 Due to the reduction the red colour of iron turns to greenish or bluish-grey.
 These weathering processes are interrelated, hydration, carbonation, and oxidation go hand in hand and
hasten the weathering process.
 Carbonation:
 Carbonation is the reaction of carbonate and bicarbonate with minerals.
 In this process there is a formation of carbonates and bicarbonates and carbonic acids. It is a
common process helping the breaking down of feldspars and carbonate minerals
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 Carbon dioxide from the polluted atmosphere is mixed with the water, forming carbonic acid (acts as
a weak acid).
 Carbonic acid mixed with the rain leads to acid rain which when falls on the limestone (calcium
carbonate) leads to the formation of sinkholes.
 Calcium carbonates and magnesium carbonates are dissolved in carbonic acid (that forms Calcium
bicarbonate) and are removed in a solution without leaving any residue resulting in cave formation.
(Example: Karst topography).
 The carbonation process speeds up with a decrease in temperature. The colder water holds more
dissolved carbon dioxide gas. Therefore, this is a very common feature of glacial weathering.
 Hydration: In this process chemical addition of water takes place.
 Several minerals take up water and expand and cause an increase in the volume of the material or rock.
 The process of hydration (the chemical addition
of water) involves the rigid attachment of H+
and OH- ions to the atoms and molecules of a
mineral.
 When water gets on the surface of rock it reacts
with rock or material and forms hydroxides.
 Calcium sulphate on the reaction with the water
turns into gypsum, which is more unstable
than calcium sulphate.
 This process is reversible and its long,
continued repetition causes fatigue in the rocks which may lead to disintegration of the rocks.
Solution Weathering:
 This type of weathering occurs when the solvent is an
acidic solution rather than simple water.
 The plants during the phase of growth expand their
roots inside the landforms which leads to the
formation of acid.
 The natural reaction of roots with moisture,
temperature, and pressure forms acid.
 This acid reacts and dissolves rocks, minerals, and
sand around the roots.
Biological Weathering:
 It is the process of the contribution or removal of minerals and ions from the rocks and its physical changes
due to the growth or movement of organisms.
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 Lining organisms such as rodents (rats, lizards, gecko), worms, termites, and bacteria remove the material
on the roots of the trees, which help in the exposure of new surfaces to chemical attack and assists in the
penetration of rainwater and air.
 Several organisms create favorable conditions for weathering.
 Mosses and lichens create a humid environment, which is favorable for fasting the chemical reaction
(decomposition) and promotes bacterial action.
 Roots of the plants create pressure on the soil and move the soil aside, creating places for water and air
(physical weathering).
 In the absence of oxygen, organisms in groundwater decompose and start the process of reduction. The
decaying plant and animal matter helps in the production of humic, carbonic, and other acids that enhance
the solubility of some elements.
 Human beings also help by disturbing vegetation, ploughing, and cultivating soils . It helps in mixing
and creating new contacts between air, water, and minerals in the earth's materials.

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 1

DAILY
CLASS NOTES
Geography

Lecture - 15
Rocks
 2

Rocks
❖ Lithosphere layer is also called the Rockosphere because litho means rock.
❖ Many elements of the earth get together to form compounds, these compounds are called minerals.
❖ The aggregates of minerals and non-minerals are called rocks. These are formed due to heat, temperature,
and pressure.
❖ Rocks are the consolidated form of minerals of a wide variety that are formed due to heat, temperature, and
pressure.
❖ These rocks form a major portion of the lithosphere.
❖ Rocks are home to minerals and many minerals such as gold, silver, iron, calcium, platinum, copper, silica,
silicates, nickel, etc.

Important Minerals:

Minerals Properties

Silicates It has Quartz, Feldspar, and Ferromanganese. It is used in the glass industry.

Sulphites It has Iron sulphide, Pirate (Iron ore). It has wide applications in the Iron and steel industries,
infrastructure development, etc.

Carbonates They are sensitive to or highly affected by chemical weathering. Example: Calcium
Carbonate used in building industries as marble, Limestone caves, etc.

Metallic Metallic minerals are divided into Ferrous, Non Ferrous, and Precious Minerals. It has Iron,
Elements Manganese, Aluminium, and Copper. It has wide applications in the automobile, aircraft,
aluminum copper, refrigeration industries, etc.
 3

Importance of Rocks:
❖ Nature of Erosion: Erosion of soil depends upon the
structure and composition of the rock. If the parent rock
is hard then it will slowly erode. If the parent rocks are soft
rock then it will erode rapidly.
❖ Type of Landscape: According to W.M. Davis the
landscape of the land is decided by the dominant rocks in
that region.
❖ Dating age of the Earth: If the rocks are the book of
history then the fossils are its pages. The study of the rocks
gives information about the earth such as History,
Geology, Life evolution, Environment, etc.
❖ Constitute Manuscript of History of Earth: According
to Morgan and Woolridge, the rocks give knowledge
about the history of the earth.

Classification of Rocks:
❖ The study and analysis of rocks is called
Petrology.
❖ Classification of rocks is done on the basis of the
formation of rocks.
❖ Rocks are mainly divided into three types:
➢ Igneous Rocks
➢ Sedimentary Rocks
➢ Metamorphic Rocks
 4

Igneous Rocks:
❖ These are formed due to cooling,
solidification, and crystallization of magma
or lava.
❖ They are known as primary rocks because
these were the first to originate during the
crust formation of Earth. Subsequently, other
rocks such as sedimentary and metamorphic
rocks were formed.
❖ Also they are known as ‘Parent rocks’ and
crystalline rocks.
❖ Granite, gabbro, pegmatite, basalt, etc. are
some examples of igneous rocks.

Sedimentary Rocks:
❖ These are formed due to lithification, compaction & cementation of the Sediments. For example: Sandstone,
Limestone, Shale etc.

Metamorphic Rock:
❖ These are formed due to application of heat and pressure causing complete change in form. For example:
Gneiss, Marble etc.

Characteristics of Igneous Rocks:

❖ These rocks are very hard and rough and do not allow water to percolate (diffuse inside). Igneous Rocks
elements are strongly bonded together. But the Basalt type of igneous rocks can allow water percolation
because it is soft rock.
❖ Igneous rocks can be divided into Plutonic and Volcanic rocks, based on the place and time taken in cooling
of magma.
❖ These are granular and crystalline in nature and comprise different sizes and textures of grains. If the molten
material of Magma is cooled suddenly at the surface, it results in small and smooth grains (small grains).
Very slow cooling of magma results in the formation of big-size grains.
❖ There are no layers in these rocks because these are formed due to cooling, which does not give layering (
crystallization only takes place).
❖ Because of no percolation of water these rocks are least affected by weathering. There is no chemical reaction
and weathering takes place. But if the grain size (coarse grains) is big it will be affected by the chemical
weathering (Solution weathering, hydration, and oxidation) because there is a small space between the small
grains of the big grains.
❖ These rocks do not have any fossils. Because fossils got destroyed due to burning by hot magma.

Classification of Igneous Rocks:

❖ Igneous rocks classification on the basis of grain size:
➢ Fine Grain: It is formed due to fast cooling. Example: Basalt.
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➢ Coarse Grain: It is formed due to slow cooling. Example: Granite.

❖ Igneous rocks classification on the basis of silica content:
➢ Acidic Rocks: It has more than 80% of silicon dioxide.
➢ Basic Rocks: It has 45% to 65% of silicon dioxide.
➢ Ultra Basic Rocks: It has less than 40% to 45% of silicon
dioxide.
❖ Igneous rocks classification on the basis of the cooling zone:
Based on the place and time taken in cooling of the molten matter.
➢ Plutonic Rocks: The cooling of Magma takes place below the
crust at the depth, known as intrusive
rocks. Intrusive igneous rocks cooled
slowly at great depth within the crust or
mantle are called Plutonic rocks.
➢ Volcanic Rocks: The cooling of Lava takes
place outside the crust, known as extrusive
rocks. These rocks are dark and dense, seen
in the USA (Snake Plateau, Columbia) and
the Deccan region of India.

NOTE:
Economic Importance of the Rocks:
❖ Building Materials: For example Silica is used in Iron and Steel Industries.
❖ Important ores: Source of important metal ores like Bauxite, Magma etc.
❖ Important Minerals: Like Iron, Gold, Silver, Copper used in various types of industries like pharma, Auto
industries.

Magma As Source Of Igneous Rocks:

❖ The mixture of the molten Rocks, volatiles (gas), and other solids (originating from partial melting of the
lower crust and upper mantle) is called Magma. It leads to the formation of the Igneous rocks.
❖ When the molten magma goes down deep within the earth and gets solidified, it is called Plutonism. The
molten magma can also come out on the surface of the earth through volcanic eruptions.

Plutonic Rocks or Intrusive Rocks:

❖ The molten matter which does not reach the surface and cools down very slowly at great
depths.
❖ This slow cooling will result in the formation of big-sized rocks (large grains).
❖ Granite is the main example of intrusive rocks..
❖ These rocks appear on the surface only after being uplifted and denuded.
 6

Lava or Volcanic Rocks or Extrusive rocks:

❖ These are formed by rapid cooling of the lava erupted during volcanic activities.
❖ Rapid cooling prevents crystallization, as a result, such rocks are fine-grained.
❖ Basalt is a typical example of this type of rock.
❖ The Deccan traps in the peninsular region have a basaltic origin.
Plutonic Vs Volcanic Rocks

Plutonic Volcanic rocks

Intrusive rocks Extrusive rocks
Granite Basalt
Slow cooling allows big-sized crystals (large Rapid cooling prevents crystallization, as a result,
grains) such rocks are fine-grained
Less dense and are lighter in color than basic rocks Denser and Darker in color

Acid Rocks:
❖ All the highest mountains are Acid rock.
❖ These are characterized by a high content of silica of more than 80 percent, while the rest is divided among
aluminum, alkalis, magnesium, iron oxide, lime, etc.
❖ These rocks have a low percentage of heavy minerals like iron and magnesium and normally contain quartz
and feldspar.
❖ These rocks are hard, compact, robust, highly massive, and highly resistant to weathering.
❖ These rocks constitute the SIAL portion of the crust of the earth.
❖ Acidic magma cools fast and it does not flow and spread far away.

Basic Rocks:
❖ These rocks have low silica content (about 45% to 65%) and magnesia content is up to 40 percent. Other
minerals like iron oxide, lime, aluminum, alkalis, potassium, etc. are also present in these rocks.
❖ These rocks have low silica content and cool slowly. Thus it flows and spreads far away to form the source of
the eruption.
❖ The presence of heavy elements imparts a dark color to these rocks.
❖ Basalt, gabbro, and dolerite are examples of these rocks.
❖ These rocks are not very hard and weathered relatively easily.
 7

Acid Vs Basic Rocks:

Acidic rocks Basic rocks
High content of silica (up to 80 percent) and Poor silica content. Magnesia content (40 percent)
Due to the excess of silicon, acidic magma cools Due to low silica content, the parent material of such rocks
fast cools slowly
High Volcanic mountains are formed of this type Forms plateaus. Deccan Traps
of rock. Mt Fuji, Japan
Lesser content of heavier minerals like iron and Presence of heavy elements imparts a dark color to these
magnesium and normally contain quartz and rocks.
feldspar. Hence they are lighter in color
Granite, quartz, feldspar, etc. Basalt, gabbro, dolerite etc..
Add rocks are hard, compact, massive, and Not being very hard, these rocks are weathered relatively
resistant to weathering. easily.

Economic Importance of Igneous Rocks

❖ The important minerals of great economic value are found in igneous rocks such as magnetic iron, silicon,
nickel, copper, lead, zinc, lead, chromite, manganese, gold, diamond, platinum, etc. These are utilized for
infrastructure, manufacturing, electronics, jewelry, and medicines.
❖ These rocks are used as building materials.
➢ Silica is used in iron and steel plants, furnaces, cement industry, etc.
➢ Granite is used as a building material as they come in beautiful shades.
❖ When Magma cools down it becomes lava. It is the chief source of metal ores, many of which are associated
with igneous rocks.

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 1

DAILY
CLASS NOTES
GEOGRAPHY

Lecture – 16
Rocks (Part 02)
 2

Rocks (Part 02)

Sedimentary Rock:
 These rocks are formed due to aggregation (get together), lithification (force application means pressure)
and cementation of sediments.
 Sediments are formed due to weathering and erosion of pre-existing rocks.
The rocks are formed by the cementation of sediments. These are also called
detrital rocks.
 These rocks are stratified in varying thicknesses. Examples: sandstone,
shale, etc.
 Pre-existing (igneous/metamorphic/sedimentary) rocks are subjected to winds,
insolation, and rainfall that lead to physical and chemical weathering. It also
leads to the disintegration and decomposition of rocks.
 These disintegrated loose particles flow into the water bodies and form layers
of loose materials. These layers are pressurized due to the heavy weight of rocks and water which causes
lithification and cementation.
 These deposits through compaction turn into rocks. This phenomenon of compaction due to pressure is
known as Lithification.
Chief Characteristics of Sedimentary Rocks:
 These rocks are generally
porous in nature (water can
percolate through them easily).
 Sedimentary rocks are also
subject to weathering.
 These rocks consist of a number
of layers or strata.
 These rocks are generally porous
and allow water to percolate
through them.
 These rocks cover around 75
percent of the total surface
area of the globe and 5 percent
of the earth’s crust.
 These rocks are gradually formed
with time formed due to the
layering of weathered material.
 Fossils are present in these rocks due to the presence of remains of plants and animals.
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NOTE:
Formation Process:
 Erosion and Weathering: Disintegration of existing rock, then form the sediments.
 Deposition of sediments: In water bodies formation of the first layer.
 Another layer of sediments: there is a process of compaction(Application of pressure).
 Lithification: Hard layers of rocks are formed due to pressure.
 Cementation: Bonding among particles(Submergence of plants, trees and organisms to form fossils).
 The plants and animal remains get pressurized
under the layers of sedimentary rocks. Due to
the heat of the earth, these plants and animals
get converted into fossils.
 These rocks are not massive in comparison to
igneous rocks.
 These are secondary rocks and are
characterized by marks left behind by the
erosional work of water currents, waves, etc.
 Classification of Sedimentary Rocks on the
Basis of Nature of Formation:
 4

Mechanically Formed Sedimentary Rocks:

 Sedimentary rocks formed by mechanical agents pressure like
running water, wind, ocean currents, ice, etc.
 Examples: Loess, Shale, Conglomerate, etc.
1. Arenaceous rocks:
 This type of sedimentary rocks are formed outside the water
bodies.
 They are hard, coarse-grained, and porous in nature and have
more sand. Example: sandstone.
2. Argillaceous rocks:
 These types of sedimentary rocks are formed in water bodies.
 They are fine particles, softer, impermeable, and non-porous
rocks.
 They are easily weathered and eroded.
 Example: shale.
Chemically Formed Sedimentary Rocks:
 Sedimentary rocks are formed by the action of a
chemical reaction.
 Example: Formation of Rock Salt and Gypsum.
 Stalactites and Stalagmites are other examples of these
types of sedimentary rocks.
 Hydration, reaction with oxygen, and reaction with
acid lead to the formation of sedimentary rocks.
 Water containing minerals evaporates at the mouth of
springs or salt lakes and gives rise to Stalactites and
stalagmites.
 Deposits of limestones left over by the lime-mixed water
as it evaporates in the underground caves, which results
in the formation of stalagmites.

 Reaction with water: Hydration. (dissolves the rocks, deposition of salts, etc.)
 Reaction with oxygen: Oxidation (Example: rusting, alumina, etc.).
 Reaction with acid: Acidification (dilute the rocks).
 Reactions with organisms: Biological weathering. (Example: fungus etc.)
 5

Organically Formed Sedimentary Rocks:

 The remains of plants and animals in the sediments due to
heat and pressure from overlying layers lead to the formation
of organically formed sedimentary rocks.
 Coal, Peat, Dolomite, and limestone are well-known
examples of these rocks.
 Plant remains to give rise to coals of different grades
depending upon the proportion of carbon and the degree of
overlying pressure.
 Sedimentary rocks may be calcareous (limestone, chalk,
dolomite) or carbonaceous (coal) depending on the
predominance of calcium content or carbon content.
 The Peat and Lignite (brown coal) coal have below 45 percent of carbon. Bituminous variety of coal contains
60 percent carbon.
 Limestone is composed of shells and skeletons of dead marine animals that once lived in shallow, warm, and
clear waters of a sea or lake.
Spread of Sedimentary Rocks in India:
 Alluvial deposits in the Indo-Gangetic plain and coastal plains are of sedimentary accumulation. These
deposits contain loam and clay.
 Different varieties of sandstone are spread over Madhya Pradesh, eastern Rajasthan, parts of the
Himalayas, Andhra Pradesh, Bihar, and Orissa.
 The great Vindhyan highland in central India consists of sandstones, shales, and limestones.
 Coal deposits occur in river basins of the Damodar, Mahanadi, and Godavari in the Gondwana sedimentary
deposits.
Economic Significance of Sedimentary Rocks:
 These rocks are utilized for building material. For example: Limestones, Sandstone, and Conglomerates.
 These are used for Agriculture purposes. Gypsum is utilized in the soil to reduce salinity in the agricultural
lands.
 Sedimentary rocks are loose material deposited at a place, these places are highly fertile in nature.
 These rocks are a source of energy due to fossils. Examples: Gas, Coal, Crude oil (The decay of tiny marine
organisms yields petroleum).
 Sedimentary rocks are not as rich in minerals of economic value, but important minerals such as hematite
iron ore, phosphates, building stones, coals, petroleum, and material used in the cement industry are
found.
 Important minerals like bauxite, manganese, and tin are also derived from other rocks but are found in gravels
and sands which are carried by water.
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Metamorphic Rocks:
 The word metamorphic is derived from metamorphosis, which means ‘Change of form’.
 These are formed from pre-existing igneous rocks or sedimentary rocks under the action of Pressure,
Volume and Temperature (PVT) changes.
 These rocks are formed by complete alteration of the appearance of pre-existing rocks due to changes in the
composition of minerals and texture of the rocks by means of pressure and temperature (Heat).
Metamorphism:
 Metamorphism changes the complete form of rock.
 In the process of metamorphism (in some rocks) grains or minerals
get arranged in layers or lines. Such an arrangement is called
Foliation or Lineation.
 Pre-existing crystalline rocks may re-crystallise and
reorganization of materials within the original rocks. Many new
minerals are added up by this process.
 Metamorphism is a process in which pre-existing materials in
consolidated rocks undergo recrystallization and reorganization
of materials.
 This process occurs when rocks are forced down to lower levels by
tectonic processes or when molten magma rising through the crust
comes in contact with the crustal rocks.
 Sometimes minerals or materials of different groups are arranged
into alternating thin to thick layers. This structure is called Banding.
 Some examples of Metamorphic Rocks are Gneissoid, slate, schist, marble, quartzite, etc.
There are Two Basic Types of Metamorphic Rocks:
 Foliated metamorphic rocks: such as gneiss, phyllite, schist, and slate which have a layered or banded
appearance that is produced by exposure to heat and directed pressure.
 Non-foliated metamorphic rocks: such as marble and quartzite which do not have a layered or banded
appearance
Agents of Metamorphism:
 Intense Heat: Intense heat changes the mineral composition entirely due to the contact of a rock with volcanic
magma.
 Compression: Compressive forces of folding, warping, crumpling, and high temperatures of the earth
change the composition of minerals. These processes change the initial form and appearance of existing rocks.
 Lava Inflow: Hot gasses from magma inside the earth’s crust and water will change the chemical composition
of rocks under the influence of intense temperature pressure and cause changes in them.
 Orogenic Movements: when two crust interacts with each other metamorphic rocks form.
 Geodynamic Forces: The geodynamic forces such as Plate Tectonics also play an important role in
Metamorphism.
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Types of Metamorphism on the basis of Agents:

Thermal Metamorphism:
 The change of form or re-crystallisation of minerals of
sedimentary and igneous rocks under the influence of intense heat
of magma is known as Thermal Metamorphism.
 Sources of high temperatures may be hot magma, hot gasses,
vapors and liquids, geothermal heat, etc.
Dynamic Metamorphism:
 This refers to the formation of metamorphic rocks under the stress
of intense pressure.
 This pressure leads to compression, which changes the form of the rock.
Hydro Metamorphism:
 Pressure of the water exerted on the rock and chemical reaction in the crust
layer of the earth changes the rock structure.
Hydro-Thermal Metamorphism:
 Sometimes high pressure is accompanied by high temperatures and the action of chemically charged water
changes the rock structure.
Types of Metamorphism on the basis of Place/Area:
 Contact Metamorphism: It takes place when rocks come in contact with
magma. Temperature is a dominant factor due to the increase in depth. For
example, Limestone changes into Marble.
 Regional Metamorphism: It is also known as dynamic metamorphism. When
rocks undergo metamorphism at a large scale or extensive area is called Regional
Metamorphism. Pressure plays a very important role here. Examples:
Mountain areas folding (Himalayas).
 Dynamo Thermal Metamorphism: The combination of directed pressure and
heat is very powerful in producing metamorphism because it leads to more or less complete re-crystallisation
of rocks and the production of new structures called dynamo thermal metamorphism. Under high pressure,
granite is converted into gneiss, clay, and shale are transformed into schist.
 8

Examples of Metamorphosis:
Igneous or Sedimentary rock Influence Metamorphosed rock
Granite Pressure Gneiss
Clay, Shale Pressure Schist
Sandstone Heat Quartzite
Clay, Shale Heat Slate - Phyllite
Coal Heat Anthracite - Graphite
Limestone Heat Marble
Some Examples of Metamorphosis:

Marbel Gneiss State Quartzite

Marble is used for Gneiss is used in flooring, Slate is used for flooring Quartzite is used for
countertops, statues, and grave stones, and building and roofing. Slate is black building materials,
decorative stone, Marble materials. Gneiss is red, and is very durable. roofing, flooring,
is white, gray, or black. black, brown, or gray. decorative rocks, and tiles
Marble has a pearly feel
to it.

Gneiss Slate Schist Phyllite

Gneiss is a rock that has a Slate is a rock that is gray, Schist is a rock that is a Phyllite is fine-grained
banded/foliated structure. green, or a type of blue, fine-grained rock that with a very well-
It is usually fine-grained that is very easily split consists of many different developed structure. It is
and is mostly made of into smooth and flat layers of minerals that very intermediate
feldspar, quartz, and mica. pieces. could be split into very between slate and schist in
many irregular sheets. a level of metamorphism.
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Metamorphic Rocks in India:

 The Gneisses and Schists are commonly found in the Himalayas, Assam, West Bengal, Bihar, Orissa,
Madhya Pradesh and Rajasthan.
 Quartzite is a hard rock found over Rajasthan, Bihar, Madhya Pradesh, Tamil Nadu and areas
surrounding Delhi.
 Marble occurs near Alwar, Ajmer, Jaipur, and Jodhpur in Rajasthan and parts of Narmada Valley in
Madhya Pradesh.
 Slate, which is used as a roofing material and for writing in schools, is found over Rewari (Haryana), Kangra
(Himachal Pradesh) and parts of Bihar.
 Graphite is found in Odisha and Andhra Pradesh.
Rock Cycle:
 Rock cycle (Transformation cycle)
is a continuous process in which
old rocks are transformed into
new ones.
 Igneous rocks are primary rocks
and other rocks form from these
rocks.
 The fragments derived out of
igneous and metamorphic rocks
form into sedimentary rocks.
 Sedimentary and Igneous Rocks
themselves can turn into
metamorphic rocks.
 The crustal rocks (igneous,
metamorphic and sedimentary)
may be carried down into the
mantle (interior of the earth)
through the subduction process,
and the same melt down and turn
into molten magma, the original source for igneous rocks.
Some Rock-Forming Minerals
 Feldspar: Half the crust is composed of feldspar. It has a light color and its main constituents are silicon,
oxygen, sodium, potassium, calcium, and aluminum.
 10

 Quartz: It has two elements, silicon

and oxygen. It has a hexagonal
crystalline structure. It is
unleavened, white or colorless. It
cracks like glass and is present in
sand and granite. It is used in the
manufacture of radio and radar.
 Bauxite: A hydrous oxide of
aluminum, it is the Ore of
aluminum. It is non-crystalline and
occurs in small pellets.
 Cinnabar: It is mercury sulfide and
mercury is derived from it. It has a
brownish color.
 Dolomite: A double carbonate of
calcium and magnesium. It is used in the cement and iron and steel industries. It is white in color.
 Gypsum: It is hydrous calcium sulfate and is used in cement, fertilizer and chemical industries.
 Haematite: It is a red ore of iron.
 Magnetite: It is the black ore (or iron oxide) of iron.
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 1

DAILY
CLASS NOTES
GEOGRAPHY

Lecture – 17
Volcanism
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Volcanism
Volcano:
 A vent or opening in the Earth, which is circular or non-circular in form, through which molten lava, gasses,
bombs, pyroclastic debris, water vapour and many heated materials are ejected out from the heated interior to
the surface of earth crust is known as Volcano.
 The material that reaches the ground includes lava flows,
pyroclastic debris (fragments of the rocks), volcanic bombs,
ash and dust, and gasses such as nitrogen compounds,
sulphur compounds, and minor amounts of chlorine,
hydrogen, and argon.
 The phenomena associated with the movement of hot molten
magma from the interior to the surface of the earth is known as
Volcanism.
 As per A. Holmes a volcano is essentially a fissure or vent,
communicating with the interior, from which flows of lava,
fountains of incandescent spray, or explosive gasses & ashes
are erupted.
Vulcanicity
 Vulcanicity includes all processes and mechanisms related to the origin of Magma. Its “Ascent (rise) in
upward direction” and its appearance over the earth's surface are explained under volcanicity.
 Vulcanicity has different mechanisms:
 Endogenic Mechanism: This type of volcanicity includes the formation or origin of hot liquid magma
& gasses in the mantle and crust. It includes the expansion, upward ascent, intrusion, cooling, and
solidification of magma in the form of volcanic landforms (Sill, Dyke, Batholith) below the crustal
surface.
 Exogenic Mechanism: It is the process of the appearance of lava, volcanic dust, ash, fragmented
materials, debris, mud smoke, etc in different forms, fissure volcanoes, lava floods, violent explosions,
hot springs, geysers, fumaroles, etc.
 A volcano is called an active volcano if the materials are being released or have been released in recent times.
 The layer below the solid crust is the mantle and has a higher density than that of the crust. The mantle contains
a weaker zone called the asthenosphere. It is from this that the molten rock materials find their way to the
surface.
 The lava that cools within the crustal portions assumes different forms. These forms are called intrusive
forms.
 The lava that cools on the surface portions assumes different forms. These forms are called extrusive forms.
 There are more than 1500 active Volcanoes in the world.
 Italy’s Stromboli Volcano has been erupting for more than 2500 years.
 3

 The 1883 eruption of Indonesia’s Krakota volcano was so loud that blasts were heard 3000 miles away.
 Mauna Kea in Hawaii is the tallest Volcano on Earth. The meaning of its name is ‘White Mountain’ as it is
snow-capped. Its height is 4205 meters from Sea Level; however, if it is measured from its oceanic base, it is
higher than Mount Everest (over 10000 meters).
Causes of Volcanoes:
 The differential radioactivity inside the earth's mantle leads to the generation of extreme levels of energy
with high temperatures with different layers of temperatures which leads to the melting of the rocks.
 The molten material starts to move upward and the upper cold layer of the crust pushes this material
downward creating convection cycles.
 Light magma moves upward and hot and molten magma come down in these cycles.
 These convection cycles lead to the accumulation of light hot magma near the crust. Because of this
tensile forces are acted on the crust, it leads to thinning of the crust by the high force of the hot magma
on crust.
 It tears off the earth's crust and comes out from the earth's crust in the form of lava.
 Volcanoes can be caused along convergent, divergent,
and some continental plate boundaries.
 Earth is divided into many different size plates
(lithospheric plates).
 Convection cells lead to the movement of these
tectonic plates (even in between the oceanic plate and
continental plate).
 Subduction of one plate under another in the case of
convergent plate boundary results in the
melting of rocks due to high temperature and
pressure which leads to the flow of magma
along the fissures of rocks, gasses, and water
vapour. Examples: Mount Sabang volcano
and Semeru volcano (Indonesia), Mount
Etna, Mount Vesuvius, Mount Stromboli
(Italy), etc.
 In the case of diverging boundaries, thinning
of the upper crust leads to a reduction in the
overlying pressure of rocks causing a decrease
in rock melting point and formation of magma which rises and erupts as lava from fissure volcanoes.
 Some continental volcanoes are located away from plate boundaries due to stressing of plates and the
creation of faults.
 4

Mechanism:
 Volcanic eruptions on the earth are associated with the weaker zones of the earth surfaces represented by
mountain building at:
 Convergent plate margins
 Fracture zones represented by divergent plate boundaries, and
 Zone of transform faults represented by conservative plate boundaries
 Mechanism of Volcanism is associated with several interconnected processes:
 Gradual increase in temperature with the rise in
depth at 1 degree per 32 meters due to heat
generated from the disintegration of the
radioactive elements deep within the earth.
 Origin of Magma due to lowering of the
melting points caused by reduction in the
presence of the overlying superincumbent load
due to fracture caused by the splitting of the
plates and their movements in the opposite
directions.
 Origin of gasses and vapour due to heating of water which reaches underground through percolation of
rainwater and meltwater.
 The ascent of the magma is forced by the enormous volume of gasses and vapour from the thinner layer
of the crust.
 At last, volcanic eruptions occur.
Benefits of Volcanoes:
 Volcanic rocks upon weathering and decomposition can yield very fertile soils.
 The ash and dust are found very fertile for fields and orchards.
 They have a great deal of scenic beauty in the form of geysers, and springs of hot water.
 These geysers and water springs have the potential to be developed as geothermal electricity.
 They add extensive plateaus and volcanic mountains.
 Volcanic activity produces valuable minerals and gasses.
Magma:
 Magma is a mixture of molten rocks or semi-molten rock, suspended crystals, molten metallic and
nonmetallic elements, gasses (NOx, SOx, Hydrogen, Chlorine, CO2, Argon and Silicon dioxide, etc.) and
volatiles.
 Magma exits from the earth in the form of lava flows, which also include glasses, water vapour, debris,
bombs, tephra, lapilli, ashes, dust etc.
 5

 The abundant elements in earth’s crust and mantle are oxygen and silicon which combine to make Silica i.e.
SiO2. Increasing Silica content in the earth implies a lower temperature, an explosive eruption behaviour and
an increased viscosity of Magma.
 Hot basaltic magma is an extremely hot liquid and semi-liquid rock located under Earth’s surface.
 This magma can be pushed through holes or cracks in the crust, causing a volcanic eruption. When magma
flows or erupts onto Earth’s surface, it is called lava.
 The high temperatures and pressure of Earth’s crust keep magma in its fluid state.
 There are three basic types of magma: Basaltic, Andesitic, and Rhyolitic, each of which has a different
mineral composition.
 All types of magma have a significant percentage of silicon dioxide.
 Basaltic magma is high in iron, magnesium, and calcium but low in potassium and sodium, depending upon
the silica, magma type also changes.
 It ranges in temperature from about 1000C to 1200C (1832F to 2192F).
 Andesitic magma has moderate amounts of all the minerals, with a temperature range from about 800 degrees
Celcius to 1000 degrees Celsius (1472 degrees Fahrenheit to 1832 degrees Fahrenheit).
 Rhyolitic magma is high in potassium and sodium but low in iron, magnesium, and calcium. It occurs in the
temperature range of about 650 degrees Celsius to 800 degrees Celsius (1202 degrees Fahrenheit to 1472
degrees Fahrenheit).
 Both the temperature and mineral content of magma affect how easily it flows.
 The viscosity (thickness) of the magma that erupts from a volcano affects the shape of the volcano.
 Volcanoes with steep slopes tend to form from very viscous magma, while flatter volcanoes form from magma
that flows easily.
 The composition of magma may change as we move deeper into the earth, due to changes in temperature,
pressure, etc. so it can be different in the crust and mantle.
 It can be crustal magma (Siliceous magma- high silica due to abundance in the crust) and mantle magma
(Mafic magma/ Basaltic magma).
 Magma often collects in magma chambers that may feed a volcano or turn into a pluton.
 The term Tephra is given to all the pieces of volcanic material such as Ash, Plumes, Volcanic Bombs,
Volcanic Blocks, lapilli, etc.
Volcanic Terms:
 Tephra: Fragments of material ejected out during volcanoes.
 Ashes: The term ashes are gray/black fine particles.
 Lapilli: Gravel of fine particles mixed in molten lava.
 Blocks: Big size boulders ejected out during volcanism.
 Tufts: Layer of ashes getting deposited.
 Bombs: Lumps of lava thrown by volcanoes.
 6

How Magma is formed?

 The initial composition of the magma depends upon the
composition of the source rock and the degree of partial
melting.
 Melting of a mantle source (garnet peridotite) results in
mafic/basaltic magmas while melting of crustal sources yields
more siliceous magmas.
 Then, the transportation toward the surface or during storage
in the crust can alter the chemical composition of the magma.
This is called magmatic differentiation and includes some
processes such as assimilation, mixing, and fractional crystallization.
Classification of Volcanoes:
 Volcanoes are classified on the basis of the nature of eruption and the form developed at the surface. Major
types of volcanoes are as follows:
Classification of Volcanoes (on the Basis of Composition):

1. Acidic/Andesitic Volcanoes (Composite or Stratovolcano lava):

 These are light-coloured, of low density, and have a high amount of silica, which cools down fast.
 These lavas are highly explosive, and highly viscous with a high melting point.
 There is a fast solidification, it can’t flow much (low fluidity) and seldom travels far before solidifying, forming
the resultant cone as a steep slope.
 7

 These can be found on destructive boundaries like ocean-ocean convergent boundaries, ocean-continent
convergent boundaries, etc.
 When two plates converge, forming these
volcanoes.
 Because of the fast solidification of lava, there is
lump formation. Lumps block the way in which the
vent obstructs (lumps-small stones) the flow of the
out-pouring lava, resulting in loud explosions, and
throwing out many volcanic bombs or pyroclasts.
 Sometimes the lavas are so viscous that they form a
spine at the crater like that of Mt. Pelee in
Martinique.
 Some of these spines are very resistant and while
most of the material of very old volcanoes is removed by erosion the spine may remain, for example, Puy de
Dome. France.
2. Basic/Basaltic Volcanoes (Shield lava):
 These are the hottest lavas, about 1,000°Celsius. (1,830°Fahrenheit) and are highly fluidic lava.
 They are dark-coloured like basalt, rich in iron and
magnesium (high density) but very low in silica content, so
there is slow cooling of lava.
 So solidification will be slow and lava will be highly fluidic
for more time.
 High fluidity leads to a low deposition rate and forms gentle
sloping.
 Due to their high fluidity, they flow readily at the speed of
10 to 30 miles per hour.
 There is no lump in the way of the lava flow, no blockade.
 They flow out of volcanic vents quietly and are not very explosive.
 These are quiet volcanoes and can be found on divergent boundaries.
 They affect extensive areas, spreading out as thin sheets over great distances before they solidify (Example:
Deccan Traps were formed).
 The resultant volcano is gently sloping with a wide diameter and forms a flattened shield or dome. Example:
Laki Fissure.
Classification of Volcanoes (on the Basis of Mode of Eruption):
 Central Eruption Type: These eruptions occur through a central pipe small opening by breaking and
blowing off the crustal surface due to violent and explosive gasses accumulated deep within the earth. The
eruption is so rapid that volcanic materials are ejected thousands of meters into the sky. They are of the
following types:
 8

 Hawaiian Type:
 It erupts quietly due to less viscous lavas.
 Rounded blisters of hot glowing mass of lavas
when caught by strong winds glide in the air like
red hairs (Peles hairs Hawaiin goddess).
 These are volcanic islands.
 When lava flows in residential areas, people are
forced to flee from that place.
 Example: Kilauea Volcano of South Hawai.
 Strombolian Type:
 These erupt with moderate intensity.
 Fluidic in nature volcanoes.
 With lava, other volcanic materials like stones
(pumice, scoria), and bombs are also ejected up
to greater heights in these volcanoes.
 There is no lump inside the volcano, so a quiet
eruption with a gentle slope.
 Eruptions are rhythmic. Example: Stromboli
volcano of Lipari Island (known as the
lighthouse of the Mediterranean)
 Vulcanian Type:
 These erupt with great force intensity (highly
explosive).
 There are some obstructions present, so explosions will be
in great intensity.
 Here, Lavas are highly viscous(thick) pasty in that they are
quickly solidified and hardened between two eruptions and
act as a plug.
 These plugs obstruct the escape of violent gases in the
next eruption.
 Consequently, violent gases break to shatter the lava
plugs into angular fragments and appear in the sky as
ash-laden volcanic clouds.
 Example: Vulcano of Lipari Island in the
Mediterranean Sea.
 Peleean Type:
 These are the most violent and most explosive.
 9

 These ejected lavas are viscous and pasty.

 Lava domes are formed over the conduits of volcanoes.
 Every successive eruption has
to blow off these domes and it
occurs with greater force
intensity and roaring noise.
 Example: Mt Pelee on
Martinique island, Antilles.
 Vesuvius Type:
 These are similar to the
strombolian and Vulcanian
types.
 The difference lies only in the
intensity of the expulsion of
lavas and gasses.
 Volcanic materials are thrown up to greater heights in the sky.
Classification of volcanoes by Periodicity of Eruption:
 These volcanoes are classified as active, dormant, or extinct.
Active Volcanoes:
 Active Volcanoes erupt frequently and constantly eject lavas,
ashes, gasses, and fragmental materials.
 These are mostly located around the Ring of Fire.
 Mount Stromboli is an active volcano and it produces so many gas
clouds that it is called the Lighthouse of the Mediterranean.
 Mount St Helens is located in Washington USA.
 Mount Etna is located in Sicily (Italy).
Dormant Volcanoes:
 Dormant Volcanoes are those that are not extinct but have not
erupted in recent history but may erupt at a future time.
 There are no indications for future eruptions but suddenly they
erupt very violently.
 Example: Vesuvius volcano 79 AD
Extinct Volcanoes:
 Extinct or inactive volcanoes have not erupted in distant geological
pasts (thousand to lakh years ago).
 They are called extinct when there are no indications of future eruptions.
 10

 In most cases the crater of the Volcano is filled with water making it a lake Mount Thielsen 2799 meters or
9182 feet is an extinct volcano north of Crater Lake, Oregon that last erupted about 300,000 years ago.

 1

DAILY
CLASS NOTES
Geography

Lecture - 18
Volcanism (Part 02)
 2

Volcanism (Part 02)

World Distribution of Volcanoes:

❖ Around 2/3 rd of volcanoes are in the Pacific Ocean.

➢ Indonesia: Mount Sabang, Mount Sumeru, Mount Augong
➢ West Indies: St. Vincent, Mount Pelee
➢ West Asia: Mount Ararat, Mount Elbruz.
➢ Spain: Mount La Palma
➢ Africa: Mount Kilimanjaro, Mount Meru
➢ Mediterranean: Mount Etna, Mount Stromboli, Mount Vesuvius, Mount Lipari.
➢ Iceland: Laki fissure, Hekla
➢ Japan: Mount Fuji, Mount Fukutoku
➢ Philippines: Mount Mayon
➢ Hawaiian Islands: Kilauea
➢ India: Barren Island, Narcondam
➢ Turkey: Mount Ararat
➢ Indian Ocean Region: Reunion Island
❖ World volcanoes are present at tectonic plates of the world interacting, it may be converging or diverging of
plates.
❖ By observing the world’s distribution of volcanoes, it appears that volcanoes are associated with the weaker
zones of the earth’s crust, and these are closely associated with seismic events.
 3

❖ Weaker zones are represented by the fold mountain ranges (Andes, Rockies).
❖ As per plate tectonics, there is a close relationship between plate margins and vulcanicity as most of the
world’s active volcanoes are associated with plate boundaries.
❖ Maximum interaction of plates are present in the Pacific(convergent), Atlantic(divergent) and
Mediterranean(convergent).
❖ Constructive or Divergent plate margins - 15% of the world’s active volcanoes.
❖ Destructive or Convergent plate margins - 80% of the world’s active volcanoes.
Three Zones:
Circum-Pacific zone (Pacific ring of fire):
❖ It is a convergent type of interaction of tectonic plates (Ocean-continent and ocean-ocean plates). Subduction
is happening at trenches which leads to volcanism.
❖ It is because of the pressure due to subduction, there is the formation of fold mountains (Rockies and Andes).
❖ In Ocean-continent interaction there is the formation of volcanoes.
❖ In Ocean-ocean interaction there is the formation of Islands like Aleutian island, Kuril island, Philippines,
etc.
❖ This zone is responsible for around 75% of the volcanic activity, also called the “ volcanic zones of the
convergent oceanic plate margins”.
❖ It consists of the eastern and western coastal areas of the Pacific Ocean, Islands, Arcs, and festoons off the
east coast of Asia.
❖ It is also called the “Ring Of Fire”.
❖ It begins from Erebus Mount of Antarctica and runs northwards through Andes & Rockies Mountains to
reach Alaska from where it runs towards the eastern Asiatic coast to include the volcanoes of Island Arcs
and festoons (Sakhalin, Kamchatka, Japan, Philippines, etc).
❖ Most of the volcanoes such as the Aleutian & Hawaii Islands are found in the chains here.
❖ Mount Cotopaxi, the highest
Volcanic Mountain in the
world found here.
❖ Volcanic Eruptions:
primarily due to the
Collision of American &
Pacific Plates.
❖ Ring of Fire:
➢ The Ring of Fire is a
long chain of volcanoes
and other tectonically
active structures that
surround the Pacific
Ocean.
 4

➢ The chain runs up along the western coast of South and North America, crosses over the Aleutian
Islands in Alaska, and runs down the eastern coast of Asia past New Zealand and into the northern
coast of Antarctica.
➢ The Ring of Fire is one of
the most geologically
active areas on Earth and is
a site for frequent
earthquakes and powerful
volcanic eruptions.
➢ Many of these volcanoes
were created through the
tectonic process of
subduction whereby dense
oceanic plates collide with
and slide under lighter
continental plates.
➢ In general, foci of the earthquake in the areas of mid-oceanic ridges, or ring of fire are at shallow depths.
➢ Whereas along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-
seated ones.
Mid-Continental zone:
❖ Also known as the “volcanic zones of convergent continental plate margins.”
❖ It includes the volcanoes of the Alpine Mountain chains, the Mediterranean Sea and volcanoes of fault
zones of East Africa.
❖ Volcanic eruptions occur
due to the convergent type
of the Eurasian Plates &
African and Indian plates.
❖ This belt doesn’t have the
continuity of the volcanic
eruptions as several gaps
are found along the Alps
and the Himalayas
because of compact and
thick crust formed due to
intense folding activity.
❖ The Indian plate is still
subducting, that's why the heights of the Himalayas are increasing.
❖ Mediterranean region: Mount Etna, Mount Vesuvius
❖ Africa: Mount Kilimanjaro, Mount Meru, Mount Birunga.
 5

Mid-Atlantic zone:
❖ It includes the volcanoes along the mid-Atlantic ridge which represents the splitting zones of the plates due
to convection cells.
❖ In this divergent type of interaction, two plates diverge in opposite directions from the mid-oceanic ridge).
❖ These volcanoes mainly of fissure eruptions type occur along the divergent plate margins.
❖ In Iceland, the most active volcanic area is located on the mid-Atlantic ridge.
❖ Examples: Laki Fissure eruption of 1783 A.D and Hekla Volcano-1974.
❖ Active volcanoes- Lesser Antilles, Azores, St Helena.
Hotspot:
❖ Volcanism occurs due to radioactive disintegration of elements in the mantle
causing magma flow upward, in the interior of the Earth, away from the places
of Plates-Interaction.
❖ Regions of the magma rising upward without any subduction.
❖ Magma is generated by the mantle core boundary and is transported upward
through convection by Mantle Plume.
❖ Hotspot volcanism is a type of volcanism that typically occurs at the interior
parts of the lithospheric plates rather than at the zones of convergence
and divergence (plate margins).
❖ A hotspot is a region within the Earth’s mantle from which heat rises through
the process of convection.
❖ Amount of heat concentrated in abnormally
hot magma is maximum due to convection
currents and high levels of radioactivity.
❖ This heat facilitates the melting of rock
(Magma) at the base of the lithosphere, where
the brittle, upper portion of the mantle meets
the Earth’s crust.
❖ This magma often pushes through cracks in the
crust to form volcanoes.
❖ Anomalies: Which has no relation with
regions having crustal interaction.


 1

DAILY
CLASS NOTES
Geography

Lecture - 19
Volcanism (Part 03)
 2

Volcanism (Part 03)

Hotspot:
 Hotspot volcanism is a type of volcanism that typically occurs at the interior parts of the lithospheric
plates rather than at the zones of convergence and divergence (plate margins).
 The Iceland Hotspot and Afar Hotspot which are situated at the divergent boundary are exceptions.
 Hotspot volcanism explains the so-called anomalous volcanism, the type that occurs far from plate
boundaries, like in Hawaii and Yellowstone, or in excessive amounts along mid-ocean ridges, in Iceland.
 Examples of hotspots include the Hawaiian Hotspot, the Yellowstone Hotspot, and the Reunion Hotspot.
 Hotspot volcanism occurs due to abnormally hot centers in the mantle known as mantle plumes.
 Magma is transferred by mantle plumes.
 Most of the mantle plumes lie far from tectonic plate boundaries (Example: Hawaiian Hotspot), while
others represent unusually large volume volcanoes near plate boundaries (Example: Iceland Hotspot).
Mantle Plumes:
 Hot spot volcanism is unique because it does not occur at the boundaries of Earth’s tectonic plates, where
all other volcanism occurs.
 Instead, it occurs at abnormally hot
centers known as Mantle Plumes.
 Mantle plumes are exceptionally hot
areas, they supply hot magma from
the bottom of the mantle to the top of
the mantle and then cause volcanism.
 A mantle plume is the convection of
abnormally hot rock (magma) within
the Earth’s mantle.
 Unlike the larger convection cells in the mantle which change their
position over timescales, the position of the mantle plumes is to be
relatively fixed.
 It originates from the core-mantle boundary where an abnormally
hot plume of rock accumulates.
 The mantle plume is shaped like a mushroom head with a long
conduit (tail) connecting the bulbous head to its base. The head
expands in size as the plume.
 The plume rises through the Earth’s mantle becoming a diapir
(dome-like intrusion forced into brittle overlying rocks) in the
upper mantle (lower parts of the lithosphere).
 It melts the rock at the base of the Lithosphere layer by layer (these layers are called volcanic traps).
 Due to the pressure in the crust by magma leading to the formation of lava, they form a number of
landforms.
 3

 It is away from the plate subduction.

Mantle plume and flood basalt volcanism
 Mantle plumes have been responsible for extensive accumulations of flood basalts and crustal cracks
underneath it.
 Mantle plumes (a few hundred kilometers in diameter ) rise
slowly towards the upside. When a plume head encounters the
base of the lithosphere, it flattens out and undergoes
widespread decompression melting to form large volumes of
basalt magma.
 The basaltic magma may then erupt onto the surface through a
series of fissures giving rise to large igneous provinces, such
as Iceland (laki fissure type), Siberian Traps, Deccan Traps,
and Ontong Java Plateau are extensive regions of basalts.
 These flood basalts create a lot of
pollution.
 Very large amounts of volcanic
material in large igneous provinces
can cover huge areas with lava and
volcanic ash, causing long-lasting
climate change (such as the
triggering of a small ice age).
 The Réunion hotspot (produced
by the Deccan Traps (about 66
million years ago) coincides with
the Cretaceous Paleogene extinction event (also known as the Cretaceous-Tertiary extinction) or the fifth
and the most recent mass extinction.
 Though a meteor impact (Chicxulub
Crater) was the cause of the extinction
event, the volcanic activity may have
caused environmental stresses.
 Additionally, the largest flood basalt
event (the Siberian Traps and
Plateau) occurred around 250 million
years ago and coincided with the
largest mass extinction in history, the
Permian Triassic extinction event.
 4

NOTE:
 A mass extinction event is when species vanish much faster than they are replaced. This is usually defined
as about 75% of the world's species being lost in a 'short' amount of geological time - less than 2.8 million
years. Till now five great mass extinctions have changed the face of life on Earth.

Mantle Plumes- Volcanic Hotspots:

 The mantle plume provides a continuous supply of abnormally hot magma
to a fixed location in the mantle referred to as a hotspot.
 The abnormally high heat of the hotspot facilitates the melting of rock at the
base of the lithosphere.
 The melted rock, known as magma, which is at high pressure, often pushes
through cracks in the crust to form hotspot volcanoes (Example: Mount
Mauna Kea).

Hotspot Volcano Chains:

 A volcano above a hotspot does not erupt forever.
 When the crustal tectonic plate is moving on the mantle
(Asthenosphere), the volcano moves and is eventually cut off from
the hotspot (the plate moves overhead relative to the fixed plume
source).
 Without a source of heat, the volcano becomes extinct and cools.
 This cooling causes the rock of the volcano and the tectonic plate to
become denser. Over time, the dense rock sinks and erodes.
 5

 Some volcano formation Islands are visible and some

will not be visible.
 A new and active volcano develops over the hotspot
creating a continuous cycle of volcanism and forming
a volcanic arc that parallels crust movement.
 The Hawaiian Islands chain in the Pacific Ocean is
the best example.
 The islands and seamounts (submarine mountains) exhibit age
progression, with the youngest near present-day Hawaii and the oldest
near the Aleutian Trench.
Mantle Plumes and Divergence:
 A hotspot causes some divergence below the crust of the earth, and
because of this high divergence of plates creates a tensile
force (Convection cells) in between these.
 Existence of hotspots, hot magma, and convection currents
in place, there is pressure on the crust (tensile force).
 It leads to thinning of the crust and the formation of rift
valleys.
 Example: Afar region hotspot of Africa (Arabian plate,
Somalian plate, Red Sea, Gulf of Aden, and great African
rift valley).
 There is the presence of hotspots also in the Afar region.
Mantle plumes & uplifted landforms:
 As the plume reaches the lithosphere, it spreads out laterally
doming zones of the Earth. Upliftment causes the upland
formation. Example-The Ethiopian Highlands.
 There is a thick crust, it does not break easily.
Mantle Plumes and Thinning of Continental Crust:
 Active hotspot leads to pressure on the crust.
There is neither eruption nor any fissure
formation.
 It causes the upliftment in the crust due to
upward pressure. Example: Ethiopian highlands
(Eastern Africa) etc.
 There can be other reasons also like thin crust,
low magmatic pressure, strong crust, etc.
 In the future if the pressure is more it can also
lead to the formation of volcanic mountains.
 6

Hotspot Volcanic Landforms:

 Volcanic activity at hot spots can create submarine mountains
known as seamounts.
 Hot spot seamounts that reach the surface of the water can
create entire chains of islands, such as the US state of
Hawaii Reunion islands near Madagascar is also an example
of volcanic hotspots.
 Hot spots can also develop beneath continents.
 The Yellowstone hotspot and Tattapani, Himachal
Pradesh (India) has produced a series of volcanic features
that extend in a northeastern direction.
Hot Spring And Geysers:
 These are also manifestations of volcanic activity.
 They result from the interaction of groundwater with
magma or with solidified but still hot igneous rocks at
shallow depths.
 A hot spring is different from a geyser in the sense that a hot
spring gives off steam when the water comes near the surface.
 The colour in these geysers is due to Cyanobacteria.
 A geyser keeps the water boiling underground, which helps create
the pressure that causes the water to rise out of Color Due to the
presence of Silica.
Hazardous Effects of Volcanism:
 High-intensity volcanoes destroy human localities in nearly 3 km
to 4 km area.
 Voluminous hot liquid lavas with high speed (around 48 km/ hr)
bury human structures, kill people and animals, destroy agri farms,
pastures, plug rivers, and lakes, and burn and destroy forests.
 Example: Laki Lava flow in 1783 AD, killed 24 individuals in
Iceland, and destroyed 15 agri farms.
 Volcanic Materials: An immense quantity of volcanic materials like pyroclastic materials, dust and ashes,
and smoke covers large areas and destroys crops vegetation, and buildings, disrupts and diverts natural
drainage systems, and health hazards.
 Loss to human lives: Due to sudden bursting there is no time for humans to evacuate. Example: Mount
Pelee in 1902 destroyed the whole of St Pierre town and killed 28000 inhabitants, Kelut volcano killed 5500
people in 1919.
 Before and after the volcanic eruptions, earthquakes generate destructive Tsunamis which create
disastrous sea waves causing numerous deaths.
 7

 Example: 1883 Krakatoa Tsunamis of around 30- 40 Meters in height killed 36000 people in coastal
areas of Java and Sumatra.
 Radiation Imbalance: The radiation balance of the earth and atmosphere is changed by volcanic
eruptions. Thus it helps in causing climate change, dust and ashes in the sky cause reduction in the insolation
from the sun to earth.
 Example: Krakatoa in 1883 erupted its fragmented materials, dust, and ashes up to 23 km in the sky and
formed a thick veil in the stratosphere, which reduced the solar radiation on earth by 20-30%.
Benefits of Volcanoes:
 Volcanic rocks upon weathering and decomposition can yield very fertile soils.
 The ash and dust are found very fertile for fields and orchards.
 They have a great deal of scenic beauty in the form of geysers, and springs of hot water.
 These geysers and water springs have the potential to be developed as geothermal electricity.
 They add extensive plateaus and volcanic mountains.
 Volcanic activity produces valuable minerals and gases.
Volcanic Landforms:
 Volcanic landforms are divided into
extrusive and intrusive landforms based
on whether magma cools within the crust
or above the crust.
 Rocks formed by the cooling of
magma within the crust are called
Plutonic rocks.
 Rocks formed by the cooling of lava
above the surface are called Igneous
rocks.
 In general, the term ‘Igneous rocks’
is used to refer to all rocks of
volcanic origin.
 Extrusive Volcanic Landforms: The
lava that cools on the surface portions of
the earth assumes different forms. These forms are called extrusive forms.
 Fissure
 Shield
 Composite
 Caldera
 Crater
 Intrusive Volcanic Landforms: The lava that cools within the earth's crust assumes different forms. These
forms are called intrusive forms.
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Extrusive Volcanic Landforms:

 Extrusive landforms are formed from material thrown out during volcanic activity.
 The materials thrown out during volcanic activity includes lava flows, pyroclastic debris, volcanic bombs,
ash and dust, and gasses such as nitrogen compounds, sulphur compounds, and minor amounts of
chlorine, hydrogen, and argon.
 9

Fissure Volcanoes:
 A fissure vent, also known as a volcanic fissure or eruption fissure, is
a narrow, linear volcanic vent through which lava erupts, usually
without any explosive activity.
 The vent is often a few meters wide and may be many kilometers long.
Generally, fissure vents are common in basaltic volcanism.
 Example: Laki fissure (Iceland) etc.
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Shield Type Volcanic Landforms:

 The Hawaiian volcanoes are the most famous
examples.These volcanoes are mostly made up
of basalt, a type of lava that is very fluid when
erupted.
 These volcanoes are not steep, but gentle
slopes.
 They become explosive if somehow water
gets into the vent; otherwise, they are less explosive.
 Example: Mauna Loa (Hawaii), Mount Kilauea, etc.
Composite Type Volcanic Landforms:
 They are conical volcanic landforms.
 Along with andesitic lava, large quantities of
pyroclastic material and ashes find their way to the
ground.
 Andesitic lava along with pyroclastic material
accumulates in the vicinity of the vent openings
leading to the formation of layers, and this makes the
mounts appear as composite volcanoes.
 It has great heights and a steep slope.
 Example: Mount Stromboli etc.
 The highest and most common volcanoes have
composite cones. These are often called Strato- volcanoes.
 Mount Stromboli 'Lighthouse of the Mediterranean’, Mount Vesuvius, Mount Fuji, etc. are examples.
Crater:
 A crater is an inverted cone-shaped vent through which the magma
flows out. When the volcano is not active the crater appears as a
bowl-shaped depression.
 Due to their unstable environments, water gets deposited in these
depressions and some crater lakes exist only intermittently.
 Caldera lakes, in contrast, can be quite large and long-lasting.
Caldera Lake:
 After the eruption of magma has ceased, the crater generally turns
into a lake after some time.
 This lake is called a 'caldera'.
 Examples: Lonar Lake in Maharashtra
 “Lake Toba” (Indonesia) is the largest crater lake in the world.
 11

Intrusive Types:
 Intrusive landforms are formed when magma cools within the crust (Plutonic rocks or intrusive igneous
rock).
 The intrusive activity of volcanoes gives rise to various forms.
 12

 Batholiths
 These are large rock masses formed due to the
cooling down and solidification of hot magma
inside the earth.
 These landforms appear on the surface only after
the denudation processes which remove the
overlying materials.
 Batholiths form the core of huge mountains and
may be exposed on the surface after erosion.
 These are large granitic bodies.
 Laccoliths:
 These are large dome-shaped
intrusive bodies connected by a
pipeline conduit from below.
 These are basically intrusive
counterparts of an exposed dome-like
batholith.
 The Karnataka plateau is spotted
with dome hills of granite rocks. Most
of these, now exfoliated, are
examples of laccoliths or batholiths.
 Lopolith:
 In the earth's crust when the lava moves upwards, a portion of the same may tend to move in a
horizontal direction wherever it finds a weak plane. It may get rested in different forms.
 In case it develops into a saucer shape, concave to the sky body, it is called Lapolith.
 Phacolith:
 A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of anticlines
in the folded igneous country.
 Such wavy materials have a definite conduit to source beneath in the form of magma chambers
(subsequently developed as batholiths). These volcanic landforms are called Phacoliths.
 Sills:
 These are solidified horizontal lava layers inside the earth.
 The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the
thickness of the material.
 The thinner ones are called sheets while the thick horizontal deposits are called sills.
 Dykes:
 When the lava makes its way through cracks and the fissures developed in the land, it solidifies almost
perpendicular to the ground.
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 It gets cooled in the same position to develop a

wall-like structure. These structures are called
dykes.
 These are the most commonly found intrusive
forms in the western Maharashtra area. These
are considered the feeders for the eruptions that
led to the development of the Deccan traps.

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 1

DAILY
CLASS NOTES
GEOGRAPHY

Lecture – 20
Earthquake
 2

Earthquake

Earthquake:
❖ The shaking or trembling of the earth’s surface, caused by the sudden movement of a part of the earth’s
crust is called an earthquake.
❖ It is a form of energy of waves transmitted through the surface
layer of the earth in widening circles from a point of sudden
energy release. It causes seismic waves or earthquake waves
that result in shanking of the ground.
❖ These seismic waves originate in a limited region but
vibrations (Shock waves or seismic waves) spread all over the
surface above the crust.
❖ Destruction of Infrastructure, Loss of lives, firing and collapse of
buildings etc. are the main effects of earthquakes.
❖ About 50,000 large earthquakes to be noticed without the aid of instruments occur annually on the Earth.
❖ Approximately 100 of these are of sufficient intensity to produce substantial damage, if their centers are near
areas of habitation.
❖ An instrument ‘seismograph’ records the waves reaching the surface.
❖ The magnitude is the unit of measurement of an earthquake.
Why does the earth shake?
❖ The release of energy occurs
along a fault. (A fault is a
sharp break in the crustal
rocks.) Rocks along a fault
tend to move in opposite
directions.
❖ Overlying rock strata press
them, and then the friction
locks them together. However,
their tendency to move apart at
some point of time overcomes
the friction.
❖ So, the blocks get deformed,
and eventually, they slide past one another abruptly.
❖ This causes a release of energy, and the energy waves travel in all directions.
❖ The point where the energy is released is called the focus of an earthquake, it is also called a hypocentre
(Focus). It is the place of origin of the earthquake.
 3

❖ The energy waves travel in many directions to reach the surface. The point on the surface which is nearest to
the focus, is called the epicenter. It is the first one to experience the waves. It is a point directly above the
focus. At the epicentre, the maximum damage happens.
Terms Associated with Earthquakes:

❖ Focus: This is the main

point or region inside
the crust where the
earthquake originates.
It is a point where
energy gets released.
❖ Epicenter: It is a point
on the earth's surface
which is vertically
above the focus. The
maximum damage
caused in this region by
the earthquake.
❖ Fault: It is a place where two plates interact, and a gap between two plates or two crustal plates.
❖ Plates: These are tectonic plates that are moving inside the earth, such as ocean plates and continental plates.
Causes of Earthquakes:
There are two types of causes like Natural and Anthropogenic.
❖ Natural causes:
➢ Plate interaction
➢ Slippage of rocks
➢ Volcanism
 4

❖ Anthropogenic:(Humans are responsible)

➢ Nuclear Explosion
➢ Deep Mining
➢ Reservoir
❖ Earthquakes occur most often along geologic faults, narrow zones where rock masses move in relation to
one another. The major fault lines of the world are located at the fringes of the huge tectonic plates that make
up Earth’s crust.
Plate Tectonics:
❖ These occur in the form of convergent, divergent and transform boundaries.

❖ In convergent boundaries, there is subduction of one crust into another one, which is called the zone of
subduction or Benioff zone.
❖ When plates get subducted, there is the release of energy in the form of an Earthquake. Example: Tsunami
in Indian Ocean Region in 2004, Nepal Earthquake, 2015.
❖ In divergent boundaries, both crusts move apart from each other. It leads to the formation of a zone of
disturbance. It causes the origin of the earthquake.
❖ In the transform boundary, there is no subduction and no volcanic activity. But these boundaries have a
large magnitude of the earthquake.
❖ Example: San Andreas Fault is a transform fault where the Pacific plate and North American plate move
horizontally relative to each other causing earthquakes along the fault lines.
 5

Fault Zones:
❖ Because of pushing and pulling force on the rocks, they get pre-stressed. Then the stress of the rock changes
its structure and undergoes
deformation.
❖ The compression force (tensile force)
on the tectonic plate on both sides by
other plates results in shifting of the
position of the plate.
❖ This leads to the sudden release of
energy (Seismic waves) and causes
vibrations or earthquakes in that
region.
❖ The plane of the rock experiences
maximum stress, and due to the stress
the rock breaks down along the plane.
❖ There is a creation of a fault zone
along with the plane. The energy
released at this point is in the form of
earthquakes.
❖ The release of energy can be the small, intermediate, or large magnitude of
earthquakes.
❖ For example: Anatolian Faults.
Volcanism:
❖ Earthquakes from volcanoes are generally less severe and more limited
in extent than those caused by fracturing of the earth’s crust.
❖ Due to the subduction of one plate into another plate along with
convergent boundaries, there is the formation of volcanism. The
subduction creates pressure and resistance on rocks and causes the
release of energy and causes
earthquakes.
❖ At trenches, large-magnitude
earthquakes happen.
Rock Slippage:
❖ Sudden slippage of rock along the
faults and fractures in the earth’s
crust occurs due to constant changes
in the volume and density of rocks
due to intense temperature and
pressure in the earth’s interior.
 6

❖ Two rocks are located upon each other, because of both top and bottom forces on the rocks, slippage happens.
This leads to a change in the position of the rock.
❖ Due to the rock slippage, there is a sudden release of energy (Seismic waves) in the form of earthquakes.
Human-Induced Earthquakes:
❖ Human activities like nuclear
explosions, mining actions,
and the generation of
reservoirs cause earthquakes.
❖ Nuclear explosion tests cause
the release of energy which
results in cracks in the surface.
The rocks get slipped and
cause earthquakes.
❖ In mining, due to digging of
the surface at depth the
overlying rocks get slipped and
create faults. The energy gets
released in the form of
earthquakes.
❖ Due to the construction of the
dams and reservoirs, the water puts a
load on rock and pressure in the
downward direction. It leads to the
development of cracks and rock
slippage, then the release of energy in
the form of earthquakes.
❖ Examples:
➢ In 1967, the 6.3 magnitude
Koynanagar earthquake
occurred near the Koyna Dam
reservoir in Maharashtra and
claimed more than 150 lives.
➢ The 2008 Sichuan earthquake,
which caused approximately
68,000 deaths.
➢ It is believed that the construction
and filling of the Zipingpu Dam may have triggered the earthquake.
 7

Seismic Waves and Earthquake Waves:

❖ Seismic waves are related to vibrations over the earth's
surface.
❖ Earthquake waves are a form of energy that travels
through the layers of the earth and which are released
due to earthquakes, volcanism, landslides and nuclear
explosions.
❖ Seismic waves are the waves through which energy
travels across the crust.
❖ For example, the slipping of land generates seismic
waves and these waves travel in all directions.
❖ Seismic waves are produced when some form of energy stored in Earth’s crust is suddenly released, usually
when masses of rock straining against one another suddenly fracture and slip'.
Types of Seismic Waves:
❖ Earthquake waves are basically of two types: Body waves and Surface waves.
❖ Body waves are generated due to the release of energy at the focus and travel in all directions travelling
through the body of the earth. Hence, the name body waves(P wave & S wave).
❖ The body waves interact with the surface layers, rocks, materials, etc and generate a new set of waves
called surface waves(R wave and L wave).

Classification of Earthquakes (on the Basis of Depth):

There are two types of classification like Shallow focus & Deep Focus.
❖ Wadati Benioff zone: It is a zone of subduction along which earthquakes are common, which are produced
by the interaction of a downgoing oceanic crustal plate against a continental plate
❖ Differential motion along the zone produces numerous earthquakes.
 8

➢ Shallow Focus Earthquakes: The focus is lying at a depth of 70 km depth and these earthquakes occur
mostly in the crustal regions.
✓ These are more frequent and random. The energy dissemination is more and leads to large
magnitude (energy will spread in less area and concentrated in a small area leading to more
destruction) earthquakes.
✓ It causes a huge amount of destruction.
✓ Approximately 75-80% of earthquakes are the shallow focus. Example: In the 2015 earthquake in
Nepal (depth of focus was 20-30 km).
➢ Intermediate Focus Earthquakes: The focus is lying at a depth of 70-300 km. Almost 12-15% of
earthquakes are the intermediate focus.
➢ Deep Focus Earthquakes: The focus is lying at a depth of 300-700 km. Around 3-5% of earthquakes
are deep-focus earthquakes. Shallow-focus earthquakes are found within the earth’s outer crustal layer,
while deep-focus earthquakes occur within the deeper subduction zones of the earth.
✓ Due to the large depth, the energy gets distributed and leads to lesser magnitude earthquakes.
✓ These earthquakes are less destructive in nature.
➢ Example:
✓ The Okhotsk Sea (Russia) Earthquake happened at 630 km depth.
✓ Vanuatu Earthquake in 2004 at 700 km depth.
Effects of Earthquake:
❖ Earthquake cause landslide, damming of rivers, depressions which from lakes
❖ They can cause submergence and emergence of landforms along coastal regions. Example: Coastline of
Kutch.
❖ Lead to change in surface drainage and underground circulation of water.
❖ More devastating features of earthquakes are fires and Seismic waves(tsunamis).
❖ Formation of cracks and fissure especially in the region of the epicentre is common.
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 1

DAILY
CLASS NOTES
Geography

Lecture - 21
Earthquakes (Part 02)
 2

Earthquakes (Part 02)

Types of Seismic Waves:
❖ Earthquake waves are basically of two types:
Body waves and Surface waves.
❖ Body waves are generated due to the release
of energy at the focus and travel in all
directions, travelling through the body of the
earth. Hence, the name body waves.
❖ The body waves interact with the surface
layers rocks, materials, etc and generate a
new set of waves called surface waves.
❖ These waves move along the surface. The
velocity of waves changes as they travel
through materials with different densities.
❖ The denser the material, the higher is the
velocity.
❖ Their direction also changes as they reflect or
refract when coming across materials with
different densities.
❖ Body waves travel in large volume and surface waves travel in small volume.

Body Waves:
❖ These flow inside a complete volume of the body.
❖ There are two types of body waves.
➢ They are called P and S-waves.
 3

Primary Waves (P-Waves):

❖ P-waves move faster in the longitudinal direction and are the first to arrive at the surface. These are also
called ‘primary waves’ (compression waves).
❖ They are faster because they transmit energy in
the medium very easily, so faster than all other
waves.
❖ These are the first to be recorded in
seismographs.
❖ The P-waves are similar to sound waves in the
atmosphere because particle motion is along
the direction of propagation.
❖ These waves are of high frequency.
❖ It generates pressure which causes compression or tension. And it can lead to density changes.
❖ They can travel in all mediums through gaseous, liquid, and
solid materials.
❖ Particles of the medium vibrate along the direction of
propagation of the wave.
❖ Velocity of P waves in Solids, Liquids, Gasses.
❖ Their velocity depends on the shear strength or elasticity of the
material.
❖ It causes less or least damage in the body.
S-Waves:
❖ S-waves arrive at the surface with some time lag in a transversal
direction.
 4

❖ These are called secondary waves (sheer waves). Also called as transverse or distortional waves.
❖ Analogous to water ripples or light waves S-waves.
❖ In this, the particle's motion is perpendicular to the direction of propagation.
❖ These waves are high-frequency waves. Travel at varying velocities (proportional to shear strength) through
the solid part of the Earth's crust, mantle.
❖ Transverse waves are like water ripples.
❖ S-waves can travel only through solid materials. This characteristic of the S-waves is quite important. It has
helped scientists to understand the structure of the interior of the earth.
❖ Reflection causes waves to rebound whereas refraction makes waves move in different directions.
❖ The variations in the direction of waves are inferred with the help of their record on a seismograph.
Surface Waves:
❖ The surface waves are the last to report
on seismographs.
❖ These are moving along the surface only.
These are body waves.
❖ They are the immediate neighbourhood
of the epicenter.
❖ These waves are more destructive
because of the large amount of energy in a
small volume.
❖ These have large magnitudes and low
frequency, so they move very slowly.
❖ They lose energy slowly compared to
body waves.
❖ They cause displacement of rocks, and
hence, the collapse of structures occurs.
❖ Also called long-period waves.
❖ They are low frequency, long wavelength, and
transverse vibration.
❖ Generally affect the surface of the Earth only
and die out at smaller depths.
❖ It has two types of waves:
➢ L-waves:
✓ These waves are fast and highly
destructive.
✓ These are slower than the body
waves.
 5

✓ These travel on the surface only,

thus the energy caused by them
is more.
➢ Rayleigh waves:
✓ They are horizontal
distortions or shaking
types of waves.
✓ They are slow waves.
Importance of Seismic Waves:
These waves are utilized to study:
❖ Interior of Earth: To study the overall composition of the earth (minerals and metals etc), phase change (Solid
and molten), density (by analyzing velocity) and distribution of crust, mantle and core.
❖ Earthquake: To study magnitude, intensity, causes, and future possibility of earthquakes.
Features of Seismic Waves:
❖ Velocity of waves changes with density, density, composition and elasticity (shear strength). If we go down
inside the Earth, the velocity of waves increases due to an increase in density.
❖ The denser the material, the higher the velocity.
❖ Their direction also changes as they reflect or refract when coming across materials with different densities.
❖ It has helped scientists to understand the structure of the interior of the Earth like composition, physical
properties (solid, liquid and gas) etc.
❖ The waves refract while traveling in different mediums. This gives us information about the material inside
the Earth.
Propagation of Earthquake Waves:
❖ Different types of earthquake waves travel in different manners.
❖ As they move or propagate, they cause vibration in the body of the rocks through which they pass.
❖ P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of
propagation.
❖ As a result, it creates density differences in the material leading to stretching and squeezing of the material.
❖ Other three waves vibrate perpendicular to the direction of propagation.
❖ The direction of vibrations of S-waves is
perpendicular to the wave direction in the
vertical plane.
❖ Hence, they create troughs and crests in the
material through which they pass. Surface
waves are considered to be the most
damaging waves.
 6

Measurement of Earthquakes:
❖ The earthquake events are scaled either according to the magnitude or intensity of the shock. The magnitude
scale is known as the Richter scale.
❖ The magnitude relates to the energy released during the earthquake.
❖ The magnitude is expressed in numbers from 0-10.
❖ Intensity refers to the impact felt
in a locality. So, the intensity
scale measures the effects of an
earthquake where it occurs.
❖ Thus, an earthquake may have
different intensities in different
locations which are measured
using the Mercalli Scale Range
of intensity.
❖ The intensity scale is named
after Mercalli, an Italian
seismologist. The intensity
scale takes into account the
visible damage caused by the
event.
❖ The range of intensity scale is
from 1-12.
❖ The Isoseismals are the
contours of equal earthquake
intensity. The area that suffering
strong shaking and significant
damage during an earthquake is
termed as Meizoseismal
region.
❖ An earthquake with a magnitude of 7.5 on the Richter scale releases 32 times the energy than one with a 6.5
magnitude.
❖ An earthquake of magnitude 3 is the smallest which is normally felt by humans.
❖ In 1934, an earthquake in Bihar recorded a magnitude of 8.4 on the Richter scale
❖ In 1954, Good Friday Earthquake-Alaska, the magnitude of the earthquake was 8.4 - 8.6.
❖ Tsunamis are the most disastrous among natural calamities caused by earthquakes. Though their occurrence is
rare, the havoc they cause is tremendous.
❖ The latest is the Japan Earthquake Tsunami of 2011 which caused the death of more than 15000
individuals
 7

Types of Earthquakes:
❖ Tectonic Earthquakes: These are generated due to the sliding of rocks along a fault plane. Example: 1872
California 1923 Sagami Bay Japan.
❖ Volcanic Earthquake: These are confined to areas of active
volcanoes. Example: Etna 1968
❖ Collapse Earthquake: Intense mining activity, sometimes
the roofs of underground mines collapse causing minor
tremors.
❖ Explosion Earthquakes: Ground shaking may also occur
due to the explosion of chemical or nuclear devices.
❖ Reservoir-Induced Earthquakes: The earthquakes that occur in the areas of large reservoirs. Example: 1936
Hoover Dam Earthquake due to Lake Mead.
How do Seismic Waves Help in Defining Earth’s Interior?
❖ The speed of seismic waves varies with the
composition of the medium.
❖ This was enough to conclude that there is
something below earth’s crust which has greater
density and varied composition. It was later
called Mohorovic discontinuity or simply
Moho.
❖ The study of seismic waves helps in determining
that the mantle is denser than the crust and is
viscous, semi-molten material.
❖ P-wave velocities change in the outer core
than in the deep mantle while S-waves do not
travel at all in the
liquid portion of the
outer core.
❖ Time is recorded for
P-waves and S-waves
to travel through the
Earth and arrive at a
seismographic
station.
❖ As we know that the
P waves reach the
seismographs first at
a station, the
difference between the time of P waves and S waves is called S-P Interval.
 8

❖ The S-P interval increases with increasing distance from the epicentre.
❖ At each station a circle on a map can be drawn which has a radius equal to the distance from the epicenter.
Emergence of Shadow Zone:
❖ Earthquake waves get recorded in seismographs located at
far-off locations.
❖ However, there exist some specific areas where the waves
are not reported. Such a zone is called the ‘shadow zone’.
❖ The study of different events reveals that for each
earthquake, there exists an altogether different shadow
zone.
❖ It was observed that seismographs located at any distance
within 105° from the epicenter, recorded the arrival of
both P and S-waves.
❖ However, the seismographs located beyond 145° from the
epicenter, record the arrival of P-waves, but not that of
S-waves.
❖ Thus, a zone between 105° and 145° from the epicenter was identified as the shadow zone for both types
of waves.
❖ The entire zone beyond 105° does not receive S-waves.
❖ The shadow zone of the S-waves is much larger than that
of the P-waves.
❖ The shadow zone of P-waves appears as a band around
the Earth between 105° and 145° away from the
epicenter.
❖ The shadow zone of S-waves is not only larger in extent
but is also a little over 40 percent of the Earth's surface.
❖ For example, we know that the Earth’s outer core is liquid
because s-waves are not able to pass through it. When an
earthquake occurs there is a shadow zone on the opposite
side of the earth where no s-wave arrives.
❖ Similarly, we know that the earth has a solid inner core
because some p-waves are reflected off the boundary between the inner core and the outer core.
Causes
❖ Vulcanicity: Each volcanic eruption is followed by Earthquake and many of the severe Earthquakes cause
volcanic eruptions. Explosive gasses try escaping and push the crustal surface from below with great force-
severe earth tremors. Krakatoa Volcano-between Java & Sumatra-severe EQ-tremors felt upto Cape Horn.
 9

❖ Hydrostatic Pressure And Anthropogenic Causes: Human activities like Pumping of groundwater and oil,
deep underground mining, blasting of rocks, nuclear explosion, storage of huge volume of waters in reservoirs,
construction of large dams, impounding of volume of water in big reservoirs cause disequilibrium of already
isostatically adjusted rocks below the reservoirs. Ex-1931 EQ in Greece- Marathon Dam; MAH-due to
Koyna Reservoir, 1967-Koyna EQ-Satara.
❖ Faulting And Elastic Rebound Theory: Horizontal & vertical movements by endogenic forces are faults and
folds is isostatic disequilibrium in crustal rocks-Earthquakes of varying magnitudes.
❖ Sudden dislocation of rock blocks caused by both tensile and compressive forces trigger immediate tremors
due to sudden maladjustment of rock blocks. Ex: 1950-earthquake-Assam -due to disequilibrium in crustal
rocks.
❖ Elastic Rebound Theory: Underground rocks are elastic like rubber and expand when stretched and pulled.
Rocks continue to be stretched as long as the tensile forces do not exceed the elasticity of rocks, once tensile
forces are more than the elasticity they are broken and immediately occupy their previous positions to adjust
themselves. And all this disturbs the equilibrium of the crustal surface causing earth tremors. 1872-EQ-
California-due to faulting in Ovan Valley;1906-EQ-San Andreas Fault.
❖ Plate Tectonic Theory: Major tectonic events include:
➢ Ruptures and faults along Constructive plate boundaries plate
➢ Faulting & Folding along destructive plate boundaries &
➢ Transform faults along Conservative fault boundaries.
❖ All sorts of disequilibrium are caused due to different types of motions and resulting into earthquakes of varying
magnitudes.
➢ Constructive Boundary: Moderate Earthquake-shallow focus-Mid Atlantic Ridge.
➢ Destructive Boundary: Earthquake-high magnitude & deep focus(700km)-Ex- Pacific Ring.
➢ Conservative boundary: Earthquake-severe Earthquake of California.
World Distribution of Earthquakes:
❖ Seismic centers are closely related to certain zones of the globe; earthquakes are associated with the weaker
and isostatically disturbed areas of the Globe.
❖ Most of the World's earthquakes
occur in the following:
➢ Zones of young fold
mountains.
➢ Zones of fracturing and
faulting.
➢ Junctions of continental
and oceanic margins.
➢ Zones of active volcanoes.
➢ Along different plate
boundaries.
 10

Circum Pacific Belt (Convergent Boundary Earthquake):

❖ Around 65% of the total earthquakes in the World are located in this zone.
❖ There are four ideal conditions for earthquakes:
1. Junction of continental and oceanic margins.
2. Zone of young folded mountains
3. Zone of active volcanoes
4. Subduction zone- convergent plate boundary
❖ Western marginal zones of North and South Americas are isostatically very sensitive because of the convergent
plate boundaries.
❖ The Pacific Plate is being subducted under the American Plate.
❖ It is also a zone of intense volcanic activity. Example: Earthquakes Mexico City 1985 convergent plate
boundaries.
Mid-Continental Belt (Divergent Type of Earthquake):
❖ Mediterranean Belt or Alpine Himalayan Belt is the subduction zones of continental plates.
❖ Around 21% of the total
seismic events of the world
occur here.
❖ It includes the Mediterranean
Sea, northern Africa, Eastern
Africa, the Himalayan, and
Mount Burmese Hills.
➢ Himalayan Region: Zone
of maximum intensity of
seismic tremors because it
lies in the subduction zone
of the Asiatic and Indian
plates. Example: 1991
Uttarkashi Earthquake, 1999 Chamoli Earthquake etc.
➢ Plane Seismic Region: Zone of moderate intensity. Example: 1934 Earthquake of Bihar, 1950 Earthquake
of Assam etc.
➢ Peninsular India: Zone of minimum intensity. Earthquakes in this region are related to active faults below
the Deccan region.
➢ Himalayan Fault Zone: A broad system of interactive faults complex grid of faults extending all along
this colliding zone Earthquake Belt extends:
✓ Sulaiman and Kirthar Zones in West
✓ Himalayas in North
✓ Burmese arc East
 11

Bhuj Earthquake: 26 January 2001:

❖ 8.1 Richter scale as per USA
❖ Deaths- 50000 to 1 Lakh
❖ Affected- 50 lakhs people
❖ Reasons:
➢ Sea floor spreading of Indian Ocean 5 cm/year.
➢ Gradual northwards movement of the Indian Plate.
➢ Major reactivated faults below Kutch Region:
✓ First: East-West running fault between
Ahmedabad and Bhuj.
✓ Second: Running North-South direction from
Mehsana, Ahmedabad to Baroda.
➢ These subterranean faults intersect each other near Viramgam, Santhalpur and Radhanpur.
Mid-Atlantic Ridge Belt (Divergent Type of Earthquake):
❖ It includes all epic centres along the Mid-Atlantic Ridge and several islands nearer to the ridge.
❖ It records shallow focus and moderate earthquakes essentially due to the creation of transform faults fractures
due to the splitting of the plates and movement in opposite directions.
❖ Therefore, the spreading of the seafloor and fissure type of volcanic activity causes earthquakes of moderate
intensity.
❖ These are around 8-10% of earthquakes in the world.
Turkey Earthquakes:
❖ The recent earthquake affected Syria, Jordan and Turkey.
❖ These are all located in the
area of the Anatolian
tectonic block.
❖ It was a 7.8 Richter scale
earthquake.
❖ In 1939, in this region the
Erzincan earthquake
occurred in the Kilkit River
valley. It is highly
earthquake-prone and
damages a lot of areas.
➢ The Arabian plate pushes northward, the Anatolian block moves westward and the African plate moves
northward.
➢ The movement of these plates created a strike-slip and resulted in a very big earthquake.
 12

Hazardous Effects:
❖ Slope Instability and Landslides:
Earthquake shocks-hilly regions-weaker
lithologies & tectonically sensitive=slope
instability=failure and Landslides. Ex-
Peruvian Earthquake of 1970-triggered
collapse of ice caps seated on Mt. Huascaran
near Yungey town-gigantic debris flow-
killed 25000 people.
❖ Damage to Human Structures: The
earthquake also damages the building,
roads, rails, factory, bridges,
telecommunication lines, water pipelines.
❖ Damage to Towns and Cities: Due to highest density of buildings and large agglomerations of human
population-large mag tremors-building collapse-uprooting of electric poles-cables damaged-bending of water
pipes-obstruction of sewer systems. Ex- Kolkata Earthquake of 1737-thousands of buildings were severely
damaged-3 lakh people killed.
❖ Fires: The strong vibrations-severe fires in houses, industries, hotels-due to contact of live electric wires,
churning of blast furnaces. Ex-Sagami Bay-1923-38000 ppl killed by fires. Earthquake of San Francisco-USA-
1906-widespread fires in several parts of the city.
❖ Deformation of Ground Surface: Due to rise and subsidence of ground surface and faulting activity. Ex-1964-
Alaska-displacement of ground surface upto 10-15 mtrs. 1897-Assam Earthquake Fault-19.3 kms long
❖ Flash Floods: Damages to dam-severe flash floods-due to sudden release of blocked water. Ex-Chamoli in the
UK.
❖ Tsunamis: Due to earthquake tremors travelling through sea waters generating high sea waves-Tsunamis. Ex-
Earthquake of Lisbon (Portugal) in 1755-12m high sea waves-killed 30 to 60,000 people.
Earthquake Zones in India:
❖ The Indian subcontinent has a long history of devastating earthquakes, partially due to the fact that India is
driving into Asia at a rate of approximately 47 mm/year.
❖ About 60% of the area of the Indian Subcontinent is vulnerable to earthquakes.
❖ Bureau of Indian Standards is the official agency for publishing the seismic hazard maps and codes.
➢ Seismic Zone II: Area with minor damage earthquakes corresponding to intensities V to VI of MM scale
(MM-Modified Mercalli Intensity scale).
➢ Seismic Zone III: Moderate damage corresponding to intensity VII of MM scale.
➢ Seismic Zone IV: Major damage corresponding to intensity VII and higher of MM scale.
➢ Seismic Zone V: Area determined by pro seismically of certain major fault systems and is seismically the
most active region. Zone V is the most seismically active region, while zone II is the least active region.
 13

❖ Occurrence of the Killari

earthquake in Maharashtra
(1993) resulted in revision of the
seismic zoning map in which the
low hazard zone or Seismic Zone
1 was merged with Seismic Zone
II. Thus Zone I does not appear in
mapping.
Global distribution of Earthquake:
❖ Circum-Pacific Belt: This belt is
along a path surrounding the
Pacific Ocean This zone includes
the regions of great seismic
activity such as Japan, the
Philippines, and Chile. This path
coincides with the "Pacific Ring
of Fire".
❖ Alpine-Himalayan Belt: Another major concentration of strong seismic activity runs through the mountainous
regions that flank the Mediterranean Sea and extends through Iran and on past the Himalayan Mountains.
❖ Mid-Atlantic Ridge: a mid-ocean ridge that extends along the floor of the Atlantic Ocean in southern Europe.
Harmful effects of Earthquake:
❖ Landforms: Ground shaking, differential ground settlement, land and mudslides, soil liquefaction, ground
lurching and avalanches.
❖ Damage of Property: When an earthquake occurs, buildings are greatly damaged. Underground pipelines and
railway lines are damaged or broken. Dams on river collapse, resultant floods cause havoc.
❖ Human Loss: Duration of tremors of earthquake is normally of only a few seconds, but thousands of people
may die in this short period.
❖ Tsunamis: Earthquakes can often result in tsunamis. It wreaks havoc on settlement of coastal areas.
❖ Fountains of Mud: Due to the intense impact of the earthquake, hot water and mud appear on the surface and
take the form of fountains. In the Bihar earthquake of 1934, the fields of farmer were covered by knee-deep
mud and crops destroyed.
❖ Liquefaction: It is the underlying layer of water rich sand that compacts and sends a column of water and fine
sand up and out onto the surface. The depth of lakes etc. become lower and sand is pushed through the ground
and the building sinks into the ground.

 1

DAILY
CLASS NOTES
Geography

Lecture - 22
Distribution of Continents and
Oceans
 2

Distribution of Continents and Oceans

Introduction:
❖ Oceans form 70.8% of the total Surface area of the Globe.
❖ Continents form 29.2% of the total Surface area of the globe
❖ There is a non-uniformity in the distribution of continents and oceans in both hemispheres.

NOTE:
❖ There are various types of questions by geographers like how present distribution of ocean &
continents has arrived at, How earthquakes happens, Why and how mountain ranges form, What is
reason for Islands formation?

Distributional Pattern:
❖ Dominance: Land in the Northern
Hemisphere (NH)-around 83 % of the total
land area of the globe and Oceans in the
Southern Hemisphere (SH)-around
90.6% of the total oceanic area of the globe.
❖ Triangular shape Continents: The bases
in North and Apices in South- Example-
North & South Americas (Base-Cape
horn), Africa (Base-Cape of good hope).
But there are few exceptions such as
Australia & Antarctica.
❖ Triangular shaped Oceans: Indian
Ocean-Base in the South & Apices in the
Bay of Bengal and Arabian Sea Pacific-
Apex-Aleutian Islands and base in the
south.
❖ The North Pole & South Pole are
surrounded by land and sea areas
respectively.
❖ Antipodal Arrangement of continents and
oceans, more than 95% of the total land
area is opposite to water bodies . But there
are exceptions such as Patagonia-opposite
to North China and New Zealand opposite
to Spain & Portugal.
❖ The Great Pacific Ocean occupies almost
1/3rd of the total surface area globe.
 3

Continental Drift Theory:

❖ In 1912, this theory was given by Alfred Wegner, a German meteorologist.
❖ He was observing the ice age fossils and different climates.
❖ In different continents, there are remains of glaciation. He wanted to find the
reason and evidence for this phenomenon.
❖ He came up with two possibilities:
➢ Either climate has moved (from one region to other)
➢ Continents have moved (drifted from old to present positions)
❖ Ultimately he believed more on the second possibility.
❖ Wegner rejected the view of the permanency of continents and accepted the drifting of continents.
❖ Wegener believed in 3 layers of Earth-SIAL, SIMA & NIFE.
➢ SIAL (Silica and Aluminum) - Limited to the continental masses alone.
➢ SIMA (Silica, and Magnesium) - Ocean crust represented by its upper part.
➢ Continents or SIALIC masses were assumed to be floated on SIMA without any resistance offered by
SIMA.
Distribution of Continents and Oceans:
❖ Before 225 million years, all the continents first formed a single landmass and mega ocean surrounded the
same.
❖ The Single large continent was named Pangaea and the mega ocean was called Panthalassa.
❖ Pangaea (SIAL) was floating on the SIMA, it was ruptured/broken due to the Tidal force of the sun and
moon and Gravitational force.
❖ After a few million years, Pangaea began to
break. It initially splits into:
➢ Laurasia (northern continents)
➢ Gondwanaland (southern continents).
❖ When two continents ruptured then the water
came in between the ruptured space. The sea
formed due to rupture is known as the Tethys
Sea.
❖ Evan still, the forces were continuously acting
on the landmass.
❖ Subsequently, these two also broke down to form
7 different continents today.
Evidence of Continental Drift:
❖ Matching of continents (JigSaw Fit): The shorelines of Africa and South America facing each other have
a remarkable and unmistakable match.
 4

➢ Eastern boundary of South America can fit

with the western boundary of Africa.
Similarly, the Eastern boundary of North
America can fit with the Western boundary of
Europe.
❖ Similar Mountain Systems: Similar mountains
formed at the edges of continents . Geologica l
evidence suggests that mountain systems of
western and eastern coastal areas of the Atlantic
are similar & Identical.
➢ Example: Appalachians of North East
regions of North America are compatible with
the mountain systems of Ireland & Wales.
❖ Rock Ages: Rocks of the same ages on different
coasts are the main evidence.
❖ Similar Fossil deposits, Vegetation and Fauna:
Similarity in the fossils & vegetation remains found
on the eastern coast of South America & Western
Coast of Africa.
➢ Common fossils of Lystrosaurus found in
Africa, India, and Antarctica.(refer to the
figure below). Also, similar vegetation and
fauna are found in different continents.
❖ Geodetic (data related to earth) Evidence:
Greenland is drifting toward the west at the rate
(last 50 years geography) of 20 cm/year (calculated by the Bathymetric model and satellite images).
❖ Glacial Deposits: Evidence of carboniferous glaciation (fossils of glaciation) in Brazil, Falkland, South
Africa, Peninsular India, Australia & Antarctica further proves the unification of landmasses. At the south
pole, Gondwnaland was present before drifting.
❖ Observed in parts of continents, which proved that continents were together in the geological past.
Forces Responsible:
❖ Gravitational Differential Forces
❖ Buoyancy forces
❖ Tidal forces
➢ Towards equator movements of the sialic (continental blocks) were due to the gravitational force and
buoyancy.
➢ The westward movement was caused by the Tidal force of the Sun & Moon. As per the theory, the
attraction force of the Sun & Moon dragged the continental crust towards the west.
 5

Buoyancy Force:
❖ It is the force that acts in the opposite direction of the weight and keeps the body over a fluid floating.
❖ This force helps continents to float towards the equator.

Drifting Mechanism:
❖ Pangea was broken into two parts due to Buoyancy,
Gravitational force, and Tidal forces: The northern
part of Laurasia & South Gondwana land.
❖ Intervening space between these two landmasses was
filled up with water and the resultant water body was
called as Tethys Sea.
❖ In the Cretaceous period, Gondwanaland was disrupted
and formed India, Australia, Madagascar, and Antarctica.
North America broke away from Laurasia and drifted
westwards. South America broke away from Africa &
moved westwards.
❖ Due to the Northwards movement of India, the Indian
Ocean was formed & due to the west movement of both
Americas, the Atlantic formed in an S-shape due to the
differential rate of drifting of North & South Americas.
❖ Size of Panthalasa was greatly reduced due to the
movement of continental blocks from all sides and the
remaining portion became the Pacific Ocean.
Mountain Building:
❖ As per Wegener, Mountains formed during the westwards
& equator wards drifting of the continents in the following
manner:
➢ Frontal edges of we stward drifting continental blocks of North & South Americas were crumpled
(compressive force) & folded against the resistance of the rocks of the sea floor-Formed Rockies &
Andes-western cordilleras.
➢ Alpine ranges of Eurasia were folded due to the equator towards movements of Eurasia & Africa
together with Peninsular India.
Origin of Island Arcs:
❖ Wegner has related the process of formation of Island Arcs & festoons (of Eastern Asia, West Indies and
arcs of south Antilles) to the differential rates of continental drift.
❖ When the Asiatic block was moving westwards , the eastern margin of this block couldn’t keep pace with
the westward-moving major landmass, rather lagged behind, which led to the formation of island arcs-
Sakhalin, Kurile, Japan, and the Philippines.
 6

❖ Also some portions of North and South Americas were lagged behind & Island arcs of West Indies & South
Antilles were formed.
Criticism:
❖ Forces: Gravitation, Buoyancy & Tidal forces are not sufficient to drift the continents apart. Tidal force
as invoked by Wegener, for the drift of continents would need to be 10000 Mn times as powerful as today
and it would have stopped earth rotation in a Year.
❖ JigSaw fit: The concept of JigSaw Fit can't be revalidated.
❖ Chronological Sequence: Not elaborated on the direction and Chronological Sequence of the
displacements of the continents.
❖ Unanswered Questions: Why did the process of continental drift not start before the Mesozoic Era? What
about volcanism? How did earthquakes form? How volcanism and earthquakes are present in mountain
regions like Andes, Alphines, and the Himalayas? These were unanswered questions.
Convectional Current Theory (Arthur Holmes 1930)
❖ He was a critic of theories supporting the external forces.
❖ He believed force is created internally by the possibility of convection currents
operating in the mantle portion which creates force.
❖ Due to the heat provided by the core, convection currents travel in an upward
direction.
❖ These currents are also generated due to radioactive elements causing thermal
differences in the mantle portion.
❖ These currents put pressure on the continental crust and lead to the movement of
continental crust.
❖ Convection Currents: Convection is the phenomenon of heat transfer.


 1

DAILY
CLASS NOTES
Geography

Lecture - 23
Continents and Oceans
(Part-02)
 2

Continents and Oceans (Part-02)

Theory of Seafloor Spreading:
 In 1960, Prof. Harry Hess, analyzed the seafloor spreading of the Pacific from Mexico to British Columbia.
 Mid-Oceanic Ridges were situated on the rising thermal convection currents
coming up from Mantle.
 Oceanic crusts move in opposite directions from mid-oceanic ridges and
thus there is continuous upwelling of new molten materials along Mid-
Oceanic Ridges.
 Molten lavas cool down and solidify to form a new crust along the trailing
ends of divergent plates.
 Therefore, it proves that:
 Sea floor spreads along the Mid Oceanic Ridges.
 Expanding crusts are destroyed along the oceanic trenches.

Sea Floor Spreading Evidence (Hess 1961):

 Volcanic eruptions: These eruptions are common
and they bring huge amounts of lava to the surface in
the area of Mid Oceanic Ridges.
 Similar Composition: The rocks on either side of the
crust of mid-oceanic ridges show remarkable
similarities like age, chemical composition and
magnetic properties.
 Age of Rocks: The ocean crust rocks (200 million
years old) are much younger than the continental
rocks (3200 million years old).
 Oceanic crust was unexpectedly thinner than the continental crust because of magma heat continuously
hitting the oceanic crust.

NOTE:
Stages of Seafloor spreading:
 Initially it gets upwarp (Crust is upwarp - initially there is pressure by the magma, and then pressure leads
to upwarping).
 After that at the same time convection cells inside, and they start pulling the crust, so when pulling happens,
after that because of pulling this crust starts breaking down/tearing apart, that tearing apart known as Rift
Valley.
 Because of the continuous tension or tensile force, we do have formation of structure which is known as
Rift Valley formation.
 3

 Widening of the Rift valley leads to formation of the Nascent sea, after that it will be completely widening
further, then it will rupture and lead to volcanoes also, and volcanism lead to the volcanic ridge.

 Sediments: The sediments on the ocean floor are unexpectedly very thin.
 The deep trenches have deep-seated earthquake occurrences while in mid-oceanic ridges, the quake foci
(focus is the place inside Earth's crust where an earthquake originates) have shallow depths.
 Mid-Oceanic ridge: A mid-oceanic ridge is composed of two chains of mountains separated by a large
depression. The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s
surface. Example: Iceland, a part of the mid-relief features of ocean floors Atlantic Ridge.
 Hess argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and
the new lava wedges into it, pushing the oceanic crust on either side.
 The younger age of the oceanic crust as well as the fact that the spreading of one ocean does not cause the
shrinking of the other, the ocean floor thus spreads.
 He further maintained that the ocean floor that gets pushed due to volcanic eruptions at the crest sinks down
at the oceanic trenches and gets consumed.
 The age of the rocks near the mid-oceanic ridge is younger than the age of rocks away from the ridge.
 Circum Pacific Zone (Ring of Fire), is the zone of frequent earthquakes and volcanic activity.
 The concept of seafloor spreading is utilized as a base for the Plate tectonic theory.

What is Trench?
 These areas are the deepest parts of the oceans.
 The trenches are relatively steep-sided, narrow basins.
 They are some 3-5 km deeper than the surrounding ocean
floor.
 These are associated with active volcanoes and strong
earthquakes. That is why they are very significant in the
study of plate movements.
 4

 As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean
and 6 in the Indian Ocean.

African Rift Valley:

 It is also an example of seafloor spreading.
 The Afar hotspot and divergent boundary are
present in this region.
 It is believed that due to the pressure in future, this
region will become separate from the African
mainland.

Plate Tectonic Theory

 Plates: Rigid lithospheric slabs or rigid and solid
Crustal layers are called plates.
 Interactions of tectonic plates (motion, subduction, etc.) are
called plate tectonics.
 Plate Tectonics: Mechanism of evolution, movement, and
motion of plates & resultant reactions.
 Plate Tectonic Theory is based on two major concepts
 Continental Drift.
 The seafloor is spreading.
 Tectonically the plate margins or boundaries are most
important because all tectonic activities occur along the plate
margins. For example: seismic events, vulcanicity, mountain
building, faulting, etc.
 It is the most recent and widely accepted theory which gives
the most satisfactory answers to intricate and puzzling questions
regarding:
 Origin of continents and oceans,
 Formation of mountains,
 Occurrence of earthquakes and
 Eruption of volcanoes.
 In this theory the lithosphere is believed to be broken into a
series of separate plates that move in response to the convection
cells in the upper mantle.
 There are three types of plate boundaries:
1. Constructive: Where two plates diverge (diverging
continents) and continuous upwelling of molten material
and formation of new ocean crust.
 5

2. Destructive: Where plates come towards one

another (converging boundaries or subduction
zones), collide and one plate goes into the mantle
and is consumed. The heavier plate gets
subducted into destructive plate boundaries
(consumption of one plate).
3. Conservative/Transform: Where plates move
past one another along transform faults without
being subdued or overriding.

Major and Minor Plates of the World:

Types of Plate Boundaries and Movements:

Major Plates  Antarctic (and the surrounding oceanic) plate.
 North American plate (with the western Atlantic floor separated from the South
American plate along the Caribbean islands).
 South American plate (with the western Atlantic floor separated from the North
American plate along the Caribbean islands).
 Pacific plate.
 India-Australia-New Zealand plate.
 Africa with the eastern Atlantic floor plate.
 Eurasia and the adjacent oceanic plate.
Minor Plates  Cocos plate
 Nazca plate
 Arabian plate
 Philippine plate
 Caroline plate
 Fuji plate
 Juan De Fuca plate.
 6

Divergent  New Crust is generated as the plates pull away from each other.
Boundary  Example: Mid-Oceanic Ridges, Rift Valleys

Convergent  Crust is destroyed as one plate dives into another. Also called the Subduction Zone.
Boundary 1. Ocean and Continental plate. Example: Andes, Rockies, Atlas Mountain
2. Two oceanic plates. Example: Islands chains of Pacific.
3. Two continental plates. Example: Himalayas
Transform  Crust is neither produced nor destroyed as the plates slide horizontally past each
Boundary other.
 Planes of separation are generally perpendicular to Mid oceanic ridges.
 Example: San Andreas Fault, North America

Plate Tectonics: Three Types of Interaction:

Convergent plate boundary
 It is the boundary where the crust is destroyed as one plate dives under another. The location where the sinking
of a plate occurs is called a subduction zone.
 Ocean-Ocean Convergence:
 When two oceanic plates converge,
the longer (heavier) plate gets subducted
which leads to volcanism and the
construction of new islands (volcanic).
Example: Hawaii island in the Atlantic
Ocean, Kurile islands etc.
 Continent-Continent Convergence:
 There is not much subduction in this
type of interaction, hence there is no
volcanic activity.
 The pressure in the upward direction
(upwarping) due to the convergence
of plates leads to the formation of
Folded mountains.
 Example: Himalayas, Urals etc.
 7

 Ocean-Continent Convergence :
 When the ocean and continent plate
converge the oceanic plate gets subducted
and creates pressure.
 Because the ocean crust has high density
and the continental crust has low density.
 The upward movement of magma due to
pressure leads to the formation of volcanic
mountain chains. Example: Andes
mountains, Alps, etc.

Divergent Plate Boundary:

 The places where the plates move away from each
other are called spreading sites.
 The new crust is generated as the plates pull away
from each other.
 The best-known example of divergent boundaries
(between two oceanic plates) is the Mid-Atlantic
Ridge. At this, the American Plate(s) is/are separated
from the Eurasian and African Plates.
 When there is a divergence between two continental
plates then there is the formation of rift valleys.
Example: Great African Rift Valley etc.
 After the formation of rift valley the further divergence
leads to the formation of a nascent sea.
 On further divergence the mid-oceanic ridge forms.

Transform Boundary:
 The crust is neither produced nor destroyed as the
plates slide horizontally past each other.
 As the eruptions do not take all along the entire crest
at the same time, there is a differential movement of
a portion of the plate away from the axis of the earth.
 Also, the rotation of the earth has an effect on the
separated blocks of the plate portions.
 8

 There will not be any volcanic activity but there will be earthquakes. Example-San Andreas Fault.

Indian Plate:
 India was a large island situated off the Australian coast. The Tethys Sea separated it from the Asian
continent till about 225 million years ago.
 India is supposed to have started her northward journey about 200
million years ago (Pangaea broke).
 About 140 million years ago, the subcontinent was located as south as 50º
S latitude.
 The Tethys Sea separated the Indian plate and the Eurasian plate. The
Tibetan block was a part of the Asiatic landmass.
 India collided with Asia about 40-50 million years ago causing rapid
uplift of the Himalayas (the Indian plate and the Eurasian plate were close
to the equator back then).
 It's thought that India's coastline was denser and more firmly attached to
the seabed, which is why Asia's softer soil was pushed up rather than the
other way around.
 The process is continuing, and the height of the Himalayas is rising even
to this date.
 The northward movement of the Indian tectonic plate pushing slowly against the Asiatic plate is evident
by the frequent earthquakes in the region.
 During the movement of the Indian plate towards the Asiatic plate, a major event that occurred was the
outpouring of lava and formation of the Deccan Traps (shield volcano).
 The shield volcanism started somewhere around 60 million years ago and continued for a long period.


 1

DAILY
CLASS NOTES
Geography

Lecture - 24
Mountains of World
 2

Mountains of World
Mountains:
❖ A mountain is any natural elevation of the earth's surface. The mountains may have a small summit and a
broad base. These are the relief features.
❖ It is considerably higher than
the surrounding area. Some
mountains are even higher than
the clouds. As you go higher,
the climate becomes colder.
❖ Mountains may be arranged in a
line known as a range. Many
mountain systems consist of a
series of parallel ranges
extending over hundreds of
kilometres.
❖ The Himalayas, the Alps and the Andes are mountain ranges of Asia, Europe and South America,
respectively.
❖ These are the significant relief features of the 2nd order on the earth's surface.

❖ There are three types of mountains on the basis of formation (Tectonic formation):
1. Fold Mountains,
2. Block Mountains and
3. Volcanic Mountains.
Fold Mountains:
❖ Fold mountains are created where two or more of Earth's tectonic plates are pushed together. At these
colliding, compressing boundaries, rocks and debris are warped and folded due to compression forces.
❖ The Himalayan Mountains and the Alps are young fold mountains with rugged relief and high conical peaks.
❖ The Appalachians in North America and the Ural mountains in Russia have rounded features and low elevation.
They are very old, folded mountains.
❖ Example: Himalayas, Andes, Rockies.
 3

Block Mountains:
❖ They are created when large areas are broken and displaced vertically. This is the result of Tensile force. The
uplifted blocks are termed as horsts and the lowered blocks are called graben.
❖ The Rhine Valley and the Vosges Mountain in Europe are examples of such mountain systems.
❖ Mauna Kea (Hawaii) in the Pacific Ocean is an undersea mountain. It is higher than Mount Everest being
10,205 meters high.
❖ Examples: Vosges, Black Forest, Salt range-Pakistan, Wasatch Range-Utah province, USA.
Volcanic Mountains:
❖ They are formed due to volcanic activity. Mount Kilimanjaro in Africa and Mount Fujiyama in Japan are
examples of such mountains. Mountains are very useful.
❖ The mountains are a storehouse of
water. Many rivers have their
source in the glaciers in the
mountains. Reservoirs are made
and the water is harnessed for the
use of people. Water from the
mountains is also used for irrigation
and the generation of hydro-
electricity. The river valleys and
terraces are ideal for the cultivation
of crops. Mountains have a rich
variety of flora and fauna.
❖ Example: Mount Stromboli, Mount Aconcagua.
❖ In some mountains, there are permanently frozen rivers of ice. They are called glaciers.
Highlands:
❖ Highlands refer to elevated or mountainous areas of land that are characterised by their higher elevation
compared to the surrounding regions, often accompanied by rugged terrain, steep slopes, and varying
topography.
❖ Example: Ethiopian Highlands.
Types of Mountains on the Basis of Erosional Formation:
❖ The Aravali range in India is one of the oldest mountain systems in the world. The range has considerably
worn down due to the processes of erosion.
❖ Examples: Aravalis, Vindhyas
On Basis of Period of Formation:
❖ Precambrian: 4.5 Billion to 500 million years ago. Example: Laurentian, Algoman Mountain. These are the
oldest mountains
❖ Caledonian mountains: 490 to 390 million years ago. Mountains of Scotland, Iceland and Scandinavia,
Aravallis, Mahadeo hills.
 4

❖ Hercynian mountains: 420-430 million years ago. Formed during permo-carboniferous periods. Mt of
Ireland, Vosges, Altai, Baikal Mt.,Tien shan.
❖ Alpine mountains: 6.5- 2.5 million years ago. Formed during the tertiary period, the Himalayas, Rockies,
Andes, Atlas, and Pamir knot mountains (Taurus, Zagros, Elburg). These are the youngest mountains.
On the basis of group of Mountains:
❖ Mountain Ridge: System of long, narrow, high hills. The slope
of one side of the ridge is steep & the other side is moderate.
Example: Shimla Ridge
❖ Mountain Range: A system of mountains and hills having
several ridges, peaks and summits and valleys. It represents a
long but narrow strip of mt & hills. Example: Himalayas,
Dhauladhar Ranges.
❖ Mountain Chain: Several parallel long & narrow
mountains of different periods. Example: Himalayas (1.5
million years of age, 45 million years of age and 65 million
years of age), Andes.
❖ Mountain System: Consists of different mountain ranges
of the same period. Have similar
structure form & extension.
Example: Shivalis.
❖ Mountain Group: Consists of
several unsymmetric patterns of
different mountain systems
❖ Cordillera: A community of
mountains having different ridges,
ranges, mountain chains and mountain systems. Examples: North Ameri Promo-carboniferous stains
Types (On the Basis of Height):
❖ Low Mountains: 700 to 1000 Meters
❖ Rough Mountains: 1000 to 1500 Meters
❖ Rugged Mountains: 1500 to 2000 Meters
❖ High Mountains: Above 2000 Meters
 5

On the Basis of Location:

❖ Inland Mountains: These are completely surrounded
by land. These can be relict mountains or can be formed
through tectonic processes. Examples: The Himalayas,
Vosges and the Black Forest (Europe), the Kunlun,
Tienshan, Altai mountains of Asia, the Urals of Russia,
the Aravallis, and Satpura.
❖ Coastal Mountains: These are located near a coastline.
Examples: Rockies, Appalachians, Alpine Mountain
ranges, Western Ghats, Eastern Ghats, Andes, cascade
ranges.
❖ Oceanic mountains: These are found on continental
shelves and ocean floors.
❖ If the height of the mountains is considered from the
ocean floor, Mauna Kea (9140 meters), would be the
highest mountain. It is a dormant volcanic mountain in the
Hawaii hotspot volcanic chain.
 6

Continental Mountains:

On the Basis of Mode of Origin:

❖ Tectonic mountains: Due to tectonic forces-tensile & compressive coupled by endogenic forces. Further four
types:
➢ Folded Mountains: Young folded, mature folded, and old folded.
➢ Block Mountains: Originated by tensile forces leading to the formation of rift valleys.
➢ Dome Mountains: Originated by magmatic intrusions and upwarping of the crustal surfaces.
➢ Accumulation Mountains: Form due to volcanic materials.
 7

❖ Circum Erosional or Relict mountains: Circum-erosional or Relict or Residual mountains (Aravallis in

India, Urals in Russia) are the remnants of old fold mountains derived as
a result of denudation (strip of covering).
➢ Example: Vindhyachal, Aravallis, Satpuras, Eastern Ghats,
Western Ghats.
➢ Forces both inside and outside the globe can change the form of the
planet's surface.
➢ Residual mountains may also evolve from plateaus which have been
dissected by rivers into hills and valleys.
Origin of Block Mountains:
❖ They are the result of faulting caused by tensile and
compressive forces coming from within the earth.
❖ It represents the upstanding part of the ground between two
faults or on either side of the rift valley.
❖ These are of two types:
1. Tilted Block Mountains: Having one side steep and
another side gentle slope.
2. Lifted Block Mountains: Flattened summit of tubular
shape and steep-sided slopes.
Fault theory:
❖ They are formed due to the upward movement of the middle
block between two faults. Upthrown block is known as Horst.
❖ They may be formed when side blocks of two faults move downwards
whereas the middle block remains stable at its place.
❖ They can also be formed when the middle block moves downwards. Thus the
side blocks become the horst & block mountains.
Erosion Theory:
❖ When a detailed study of the Great Basin Range of the USA
took place it was opined that they were not formed due to
faulting and tilting, rather they were formed due to differential
erosion.
❖ It was deduced that during the Mesozoic era, these ranges were
subjected to intense erosion and
differential erosion led to the formation
of the given range.
❖ Criticism: Scientists rejected the theory
on the basis of the fact that denudation
may modify a mountain but can't form a
full mountain.
 8

European Mountains:
 9

North American Mountains:

South American Mountains:

 10

Asian Mountains:

Australian Mountains:
 11

African Mountains:

Fold Mountain Ranges:

❖ Folded Mountain Ranges are formed by the folding
of crustal rocks by compressive forces generated by
the endogenic forces coming from within the earth.
❖ It is the highest and most extensive mountain Range
in the world.
❖ Generally found along the margins of the continents-
North-South direction or East-West direction.
Examples: Andes, Alps, Himalayas, Atlas etc.
❖ These can be young and mature.
❖ They can be old and originated before the tertiary
period and greatly denuded
➢ Old fold mountains Examples: Aravallis,
Vindhyachal
➢ New fold mountains Examples: Rockies, Andes, Alps, Himalayas.
➢ Continent-Continent Convergence- Examples: Himalayas, Ural etc.
➢ Ocean-Continental Convergence- Examples: Andes, Rockies etc.
 12

Characteristics of Fold Mountains:

❖ Youngest mountains on Earth's surface.
❖ Formed due to the folding of sedimentary rocks by strong
compressive forces. Marine fossils found denote that
sedimentary rocks were formed due to the deposition and
consolidation of sediments in water bodies.
❖ Sediments found up to greater depth-12000 meters.
❖ Mt extend for a greater length but their widths are far smaller.
➢ Example; Himalayas: Length-2400 kms & Width-400
kms which means that folded mountain ranges are formed
in long, narrow, & shallow seas, known as Geosynclines
❖ Fold mountains are generally arc-shaped, one side concave
slope & other side convex.
❖ Found along the margins of the continents facing oceans.
Ex-Rockies & Andes are found along the western margins
of North & South Americas.
❖ Similarly Alpine mountains are located along the southern
margins of Europe facing the Mediterranean.
❖ NOTE: Orographic Precipitation: Rain, snow, or other
precipitation occurs when moist air is lifted as it moves
over a mountain range.

Pamir Knot:
❖ The word ‘knot’ is meant by the convergence of some of the major mountain ranges of the world.
❖ It is considered as a high plateau that is surrounded by mountains that contain the high grasslands of the
mountains.
❖ Many mountain ranges radiate outwards in different directions from this small area.
❖ The Hindu Kush, the Karakoram Range, the Kunlun Mountains, and the Tian Shan are several South-
Central Asian mountain ranges that seem to be radiating from the area of orogenic uplift known as Pamir
Knot.
❖ It is popularly called the “Roof of the World” because of its position in very high altitudes.

Convection Current Theory:

❖ This theory was given by Arthur Holmes-1928-29.
❖ Major objectives:
➢ To search the mechanism of Mt Building-scientific.
➢ Scientific explanation for the origin of continents and
oceans basins
❖ Orogenic Force: Driving force of mt building-provided by
thermal convection currents.
 13

❖ Main source: for convection currents –excessive heat in substratum due to disintegration of radioactive
elements.
Basis of the theory:
❖ Earth consists of 3 zones or layers-
➢ Granodiorite-upper layer
➢ Amphibolite-intermediate layer,
➢ Eclogite.-lower
❖ These 3 layers further grouped into 2 zones:
❖ Crust-Upper & middle of the intermediate layer.
❖ Mantle represents the molten part of the lower layers.
❖ Disintegration of the radioactive elements generates heat which causes convection currents.
❖ There is maximum concentration of the Radioactive elements in the crust but temperature is not shown high
due to loss of heat by conduction and radiation.
❖ There is very low concentration of radioactive elements in substratum but due to the accumulation of heat
convection currents are generated.
❖ Convective currents originating below the continental crust are more powerful than convective currents below
the oceanic crust due to greater concentration of radioactive elements in continental crust.
Mechanism:
❖ Due to temperature difference, rising convection currents are formed under the Equatorial crust & Descending
Convective currents are formed under the Polar crust.
❖ Currents generated under the equatorial crust will move towards the pole, thus the crusts are carried away with
the convection currents.
❖ There are two situations of the rising convection currents when they reach the lower limit of crustal masses:
➢ Crustal mass where two rising currents diverge in opposite directions, is stretched and thinned due to
tension and ultimately crust is
ruptured and broken into two blocks
which are carried away by lateral
divergent convective currents.
Opening between two blocks
becomes seas. Divergent convective
currents cause continental drifts.
➢ When two lateral convective
currents originating under the
continental and oceanic crust
convergence, compressive force is generated which causes subsidence in the crustal zones.
❖ As per Holmes, the Equatorial crust was stretched and ruptured, due to divergence of rising convective
currents which carried the ruptured crustal blocks towards North & South & Tethys Sea was Formed.-Opening
of Tethys Sea.
 14

❖ Further two sets of Convergent or downward moving currents brought Laurasia & Gondwanaland together
& Tethys sea was compressed and folded into the Alpine Mountain, Closing of Tethys Sea.
Criticism:
❖ Whole of the theory depends on factors about which very little is known.
❖ The whole mechanism of convective currents depends on the heat generated by radioactive elements in the
substratum but several scientists have raised doubts about the availability of the required amount of heat by
radioactive elements.
❖ Horizontal flow of thermal convective currents under the continental and oceanic crusts is also a doubtful
phenomenon bcoz of the lack of the required amount of heat to drive these currents.
❖ Convective currents originate at a few centers only under the continental and oceanic crusts but the question
arises why are not they originating at all places? If this happens, the horizontal movement of these currents
would not be possible.
❖ Metamorphism of Amphibolites into eclogites and resultant downward movement of relatively denser
eclogites is a doubtful phenomenon. Even if we accept the metamorphism of amphibolites into eclogites, the
resultant increase in density from 3 to 3.4 would not be enough.
Plate tectonic theory:
Objective:
❖ Comprehensive theory which offers explanations for relief features and tectonic events-Mountain Building,
Folding & Faulting, Continental Drift, Vulcanicity, and Seismic Events.
❖ It envisages the formation of Mountains due to the collision of plate Boundaries.
Orogenetic Force:
❖ Force to form mountains is provided by the collision of two convergent plates along the destructive plate
boundaries.
❖ Thermal Convective currents originating in the mantle is a competent force for the movement of plates.
Base of Theory:
❖ Plate tectonics-based on two major scientific evidences:
➢ Evidence of Paleo-magnetism.
➢ Evidence of Seafloor spreading.
❖ There are three types of Plate Boundaries:
1. Constructive or Divergent Plate Boundaries
2. Destructive or Convergent Plate Boundaries
3. Conservative Plate Boundaries
Mechanism of theory:
❖ Mountains are formed due to two convergent plates and are always formed along the destructive plate
boundaries.
❖ Two plates moving together under the impact of thermal convective currents collide against each other and the
plate boundary having relatively denser material is subducted under the other plate boundary of relatively
lighter materials. Subduction zone is the Benioff Zone.
 15

❖ Subduction of the plate boundary causes lateral compressive force which ultimately squeezes and folds the
sediments and materials of margins of plates, thus mt are formed
❖ Subducted part of the plate after reaching a depth of 100 kms or more in the mantle is liquefied and thus
expands in vol and this expansion of molten materials causes a further rise in Mountains.
Ocean-ocean convergence:
❖ Subduction of oceanic plates of relatively denser materials results in
the formation of a fold mountain range.
❖ Examples: Island Arcs & Festoons formed by Japanese Islands,
Philippines.
❖ Honshu islands-characteristic example of Ocean-Ocean
Convergence.
Ocean-Continent Convergence:
❖ Collision of Ocean-Continent convergent plates
results in the formation of the Cordillera type of Fold
Mountains. Example-Western Cordillera of North
America
❖ When oceanic plates collide with the continental
plates, oceanic plates being heavier due to denser
materials is subducted below the continental plate
boundary.
❖ Andes & Rockies are formed due to the Subduction
of the Pacific Ocean plate under the American Continental Plate.
Continent-Continent Convergence
❖ Two convergent plates composed of the continental crusts
collide against each other-plate with relatively denser
material-subducted under the plate having comparatively
lighter materials than the former.
❖ Resultant lateral compression squeezes and folds the
sediments deposited on either side of continental plate
margins and thus forms the gigantic fold mountains. Example:
Alps, Himalayas
Formation of the Himalayas:
❖ These are the youngest fold mountain (Tertiary period)
ranges which extend 2400 kms West to East and are of
Arc shape.
 16

❖ Formed due to Continental interaction at Convergent

Boundary.
❖ The Himalayas are lying on the mid continental belt.
❖ Protect the Indian sub-continent from cold, dry, frigid winds
from Siberia.
❖ Acts as a barrier for Monsoon winds-causes rainfall.
❖ It also plays an important role in the formation of the Deserts-
Taklamakan Desert & Gobi Desert.
❖ It is the source of the number of Rivers-Ganga, Yamuna, and
Kali.
❖ 70-65 Million Years Ago- Tethys Geosyncline was present in
place of the Himalayas-bordering Indian Plate in the south and
the Asiatic plate in the North.
❖ 60-30 Million Years Ago- the Indian plate started subducting under
Asiatic Plate.
❖ 30-20 Million Years Ago- Subduction of the Indian plate under Asiatic
caused lateral compression due to which sediments of Tethys geosyncline
got squeezed and folded into 3 parallel chains of Himalayas.
Importance:
❖ It has different freshwater glaciers: Gangotri Glacier, Milam Glacier,
Chemayungdung (Brahmaputra river originates from this), etc.
❖ It is a barrier for the Indian sub-continent from cold, dry, frigid winds
from (Russia) Siberia.
❖ These also regulate rainfall and act as a barrier for Monsoon winds and deflect it, causing rainfall.
❖ It regulates branches of wet winds and causes the distribution of rainfall in India.
❖ During summers or hot weather when the ice melts these passes are navigable, this also creates a threat of
attack from the neighboring countries. Thus the melting of glaciers due to climate change also creates a
security threat for India. Therefore conservation measures to protect against the melting of the ice is the need
of the hour.
❖ It promotes tourism and is a place of history (the presence of various monasteries).
❖ The Tibetan plateau plays an important role in the Indian monsoon. It is formed due to the Indian plate's
interaction with the European plate.
❖ Environment conservation: There are a number of species(plants, animals, Birds), Example-Manas National
Park.
❖ It has strategic importance also due to the presence of various passes like Niti pass, Mana pass, etc. (A
mountain pass is a navigable route through a mountain range or over a ridge).
Formation of Alpine Mountains
❖ Alpine Mt of Europe were formed due to the convergence and collision of the European & African Plates.
❖ Collisions of these two continental plates have been very complex and so was the formation of the European
Alpine Mountains.
 17

Highest Mountain Peaks:


 1

DAILY
CLASS NOTES
Geography

Lecture - 25
Plateaus of World
 2

Plateaus of World
Andes Mountain Ranges:
❖ These are located in South America.
❖ These are formed due to the convergence of Ocean-
Continental plate boundaries.
❖ The Atacama desert is present on its leeward side.
❖ The average alleviation is around 4000 meters.
❖ It is the Longest continental mountain range.
❖ The Highest Mountain Range outside Asia.
❖ Mt Aconcagua is the Highest Peak.
❖ Due to the mountain barrier, It Causes Orographic
Precipitation.
❖ Ojos del Salado (6,893 m) (active volcano) on the
Chile-Argentina border is the highest (active) volcano
on earth.
❖ In the Amazon forest there are Equatorial
rainforests due to daily precipitation.

Rockies:
❖ Rockies are divided into - three parts.
➢ Northern Rockies: Above there are
Arctic Rockies (Brooks range).
➢ Middle Rockies
➢ Southern Rockies
❖ These Rockies range are From British
Columbia to New Mexico.
❖ It Plays a role in the formation of Chinook
winds (Warm winds).
❖ Chinook winds are known as ‘Snow eaters’
because they help in melting snow and
facilitate human activities in the region like
agriculture etc.
 3

❖ It captures moisture-laden winds from the Pacific and Orographic rainfall in the Windward Region.
❖ The Rain Shadow effect led to the formation of Mojave Desert.
❖ Mt. Elbert is the Highest Peak
❖ These are formed due to the convergence of Ocean-Continental plate boundaries.
Ural:

❖ Runs North to South Through West Russia from Arctic Coast to the Ural River.
❖ Formed due to Continent-Continent plate interaction at Convergent Boundary.
❖ Forms a local Boundary between Europe and Russia.
❖ Mughalzhar hills are part of Urals belt in Kazakhstan.
❖ These form a water divide between the rivers of Russia and Europe (due to the high slope).
Atlas:
❖ It is a fold mountain located in Africa.
❖ It passes through Morocco, Algeria and Tunisia.
❖ It brings Orographic rainfall on the windward side of Atlas Mountain.
 4

❖ Westerlies from the Atlantic carry moisture into the region but mountains act as the weather divider between
the coastal grasslands, wetlands and the Sahara Desert.
❖ It separates the Mediterranean climate from the Sahara
Desert due to being located on the leeward side of the Atlas
mountains, cold currents, altitude, etc.
❖ It Extends 2400 km approx.
❖ The highest peak is Toubkal (4,165 meters) in southwestern
Morocco.
❖ It also separated the Mediterranean type of climate from the
dry climate.
❖ One branch of Sirocco winds passes through the Atlas range.
❖ Reason for the formation is the Ocean-Continent plate
interaction.

NOTE:
❖ The Mediterranean type of climate is characterized by hot, dry summers and cool, wet winters.
❖ It is located between about 30° and 45° latitude north and south of the Equator and on the western
sides of the continents.

Himalayas:
❖ It is the youngest fold of the mountain chain.
❖ It extends- an average of 2400 km West to East (the width decreases) and is Arc shaped.
❖ It was formed due to Continental-Continental interaction at Convergent Boundaries (Indian and Eurasian or
Asiatic Plates).
❖ It is a barrier for the
Indian sub-continent
from cold, dry, frigid
winds from (Russia)
Siberia.
❖ These also regulate
rainfall and act as a
barrier for Monsoon
winds and deflect it,
causing rainfall.
❖ It regulates branches
of wet winds and
causes the
distribution of rainfall
in India.
 5

❖ It has a role in the formation of the Deserts-Taklamakan Desert & Gobi Desert.
❖ It is the source of a number of Rivers-Ganga, Yamuna, Indus, Brahmaputra, Kali, etc.
❖ It has different freshwater glaciers- Gangotri Glacier, Milam Glacier, Chemayungdung (Brahmaputra river
originates from this), etc.
❖ The Tibetan plateau plays an important role in the Indian monsoon. It is formed due to the Indian plate's
interaction with the European plate.
Alps:
❖ These are young fold Mountain Ranges.
❖ It separates Northern France and Germany from
the Mediterranean climate. Due to arc type
shape, they separate the marine west coast climate
of Europe from the Mediterranean areas of France,
Italy etc
❖ It is the highest Mountain Range in Europe- 1200
kms from West to East.
❖ Foehn/Fohn winds move in the Alps mountains,
which influences more precipitation in South
Europe and Eurasia.
❖ Mt Blanc is the highest peak.
❖ The mountain range stretches approximately 750
miles (1,200 kilometers) in a crescent shape across
eight Alpine countries: France, Switzerland, Monaco, Italy, Liechtenstein, Austria, Germany, and Slovenia.
❖ These are formed due to the interaction of African and Eurasian Plates.
Highest Mountain Peaks:
 6

Plateaus:
❖ Plateaus are the significant relief features of the second order.
❖ A Plateau is an elevated flat upland. These are flat-topped tabular or rolling lands standing above the
surrounding area.
 7

❖ It may have one or more sides with steep slopes.

❖ These originated due to volcanism, plate tectonics and deposition.
❖ The height of plateaus often varies from a few hundred meters to several thousand meters.
❖ Plateaus, like mountains, may be young or old.
➢ Old plateau: Ranchi Plateau
➢ Young Plateau: Mahabaleshwar
❖ They cover 33% of the surface area of the earth (land).
❖ These tabular uplands have relief features of more than 500 feet.
❖ They can be dissected by rivers and form valleys. Example: Colorado Plateau (USA, by Colorado River).
Deccan Plateau, Chota Nagpur Plateau (North Koel River), etc.
❖ The Deccan Plateau in India is one of the oldest Plateaus in the world.
❖ The East African Plateau in Kenya, Tanzania, and Uganda and the Western Plateau of Australia are other
examples.
❖ Finch Trewartha- Tabular uplands having a relief of more than 500 feet may be arbitrarily defined as a Plateau.
Example: Ranchi Plateau, Shillong Plateau, Tibetan Plateau, Colorado Plateau, etc.
❖ Examples: Tibetan Plateau, Colorado Plateau, Bolivian Plateau, Ethiopian Highlands, Katanga Plateau,
Kimberley Plateau, etc.
General Characteristics of Plateaus:
❖ Large Uplands: These are extensive upland areas characterized by flat and rough top surfaces and steep
side walls that rise above the neighbouring ground surface for at least 300 meters to a few lakh kms. Example:
Deccan Plateau, Siberian Plateau, etc.
❖ River streams Cut Through: The rivers, while descending from the Plateau, form long, narrow and deep
valleys (Gorges and Canyons). Example: Major rivers, while descending the Rawa Plateau, meet Ganga and
Yamuna and have entrenched deep valleys, Kevati Gorge by the Mahana River, Gandikota Canyon by the
Pennar River, Odda Gorge by the Odda River, and Colorado Plateau entrenched by the Colorado River.
❖ Vary in places: They vary from one place to another. Example: Tibetan Plateau is surrounded by hills and
mountains on both sides, Piedmont Plateau in the USA is surrounded by hills and mountains on one side and
bordered by plains and coastal areas on another side.
❖ Register Variations in Features: Plateaus also register wide variations from the standpoint of superficial
materials (composition variation):
➢ Some have thick covers of Basaltic Lavas. Example: Deccan Plateau.
➢ Some are entirely composed of sedimentary rocks. Example: Rewa Plateau, Bhander Plateau, Rohtas
Plateau, etc.
➢ Some are so dissected by the network of streams that they are segmented into numerous parts. Example.
Peatlands of Ranchi Palamu uplands are dissected by the North Koel River and its tributaries.
❖ Very extensive in aerial context (may range from 5 lakh sq. km - 30 lakh sq. km)- Deccan Plateau, Siberian
Plateau.
 8

❖ Flat and Rolling top surfaces- The slope of the side walls is very steep but the top surfaces, except for minor
relief, are more or less flat. Example: Ranchi Plateau, abruptly rises from Damodar Valley.
❖ Plateau surface is also dotted with hill ranges and river valleys. Example: The Tibetan plateau is surrounded
by hills and mountains on both sides, Piedmont Plateau- one side surrounded by mountains and another by a
plain, etc.
❖ Inter-montane Plateaus are plateaus which are surrounded by mountains.

 1

DAILY
CLASS NOTES
Geography

Lecture – 26
Plateaus of World (Part - 2)
 2

Plateaus of World (Part - 2)

Importance of Plateaus:
❖ Mineral Resources: Plateaus are very useful because they are rich in mineral deposits.
➢ Chota Nagpur Plateau in India source of Coal, iron, Manganese.
➢ Deccan Plateau in India source of Manganese,Bauxite, Iron.
➢ Kimberley Plateau in Western Australia source of diamonds.
➢ Katanga Plateau in Africa source of Lead, Copper, Tin, and Gold
➢ Bolivia Plateau in South America is a source of Lead and Tin
❖ Energy: These are sources of many energy resources.
➢ Meghalaya Plateau source of Uranium
➢ Canadian shield region source of minerals like oil and gas.
➢ Chota Nagpur Plateau in India source of Coal.
➢ Hydroelectricity can be generated from the dams or flowing rivers Godavari, Krishna, Cauvery of
Deccan and Mahabaleshwar Plateau and Angel Falls in Venezuela.
➢ In the Plateau areas, there may be several waterfalls as the river falls from a great height. In India, the
Hundru Falls in the Chota Nagpur Plateau on the river Subarnarekha and the Jog Falls in
Karnataka are examples of such waterfalls.
❖ Agriculture: Black soil in the Deccan Plateau is helpful for
cotton , rice, sugarcane. The Russian plateau region is also
good for agriculture.
❖ Tourism: Many Plateaus have scenic spots and are of great
attraction for tourists.Deccan Plateau (Ajanta and Ellora
caves), Colorado Plateau in America grand canyon, and
Meghalaya Plateau (Cherrapunji),Religious places
(Nashik, Trimbakeshwar)
❖ Waterfalls like Jog falls in Karnataka,Hundru falls in
Chota Nagpur Plateau, Angel falls in Venezuela( south
america)
❖ Climate: The Tibet Plateau is the highest Plateau in the
world with a height of 4,000 to 6,000 meters above the Mean
Sea Level. It plays a role in rainfall from the Indian Monsoon.
Likewise, the Shillong plateau in Meghalaya and
Karnataka Plateau play an important role in rainfall.
 3

❖ Fossil Fuels: We can get coal in the Appalachian Plateau, USA, Coal in Kamti Coal fields of Maharashtra
(Deccan Plateau) and Shield region in Canada.
Origin and Evolution of Plateaus:
Plateaus originated in a number of ways, which are as follows:
❖ Thermal Formation: Magma in the interior of the earth generates pressure on the crust, creates
unwrapping due to upwarping
of some portion of an extensive
landmass by a few 100s Meters.
Example: Ethiopian Highlands,
Massif Central in France
Western Patlands of Ranchi
Palamau.
❖ Crustal Shortening: Crustal
thickening at convergent
boundaries due to upliftment
and subduction of two
converging plates leads to the
formation of these plateaus .
Example: Tibetan Plateau.

❖ Volcanic Eruptions: These are formed due to the pressure of mantle plume magma forms cracks in the
crust of the earth's surface. Lava came out from these cracks and then created a layer-by-layer form of
plateaus. Example: Deccan traps Columbia plateau and Siberian traps.
 4

❖ Deposition: These are formed due to deposition of thick covers of Basaltic lavas. Example: Columbian
Plateau, Mahabaleshwar Plateau, Iceland and Loess Plateau of China (upwarping and deposition both).
Thick deposits of loose materials by wind sometimes form Plateaus. Example: Loess Plateau of China.
❖ Upwarping:
❖ Due to down warping of the surrounding areas.
For Example if the southern part of ganga plain is
down warped by a few meters and the northern
part remains stable, then the remaining plane area
associated with Himalaya becomes a plateau.
❖ These are formed due to upward movement of a
crustal plate due to compressive forces from
endogenic forces which leads to upwarping of
some portion of an extensive landmass.
Example: Western Patlands of Ranchi Palamau, Potwar Plateau, Pakistan (upwarping and deposition
both).
❖ Fold Mountain Formation: If less folding occurs, it leads to the formation of Plateaus. Sometimes,
adjoining areas of the mountains are not folded but are raised during the process of mountain building.
Such unfolded but upwarped regions become Plateaus. Example: Tibetan plateau, Cumberland Plateau
west of Appalachian Mountains.
 5

Classification of Plateaus:
On the Basis of Origin:
1. Glacial Plateau: Plateau and mountains
modified and transformed by the glacial
actions like erosion and deposition.In this
process mountains get lowered in height
and their sharp reliefs are rounded.
Example: Chibber Garhwal Plateau,
Marg of Kashmir, etc.
2. Volcanic Plateau: These are formed due to
the accumulation of thick layers of Basaltic
Lava.
➢ These are formed over an area of
7,74,000 km2 in peninsular India due to
the accumulation of an enormous volume
of basaltic lavas that erupted during the Cretaceous period.
➢ Example: Columbia Plateau, USA is the best example of a
lava plateau. It is an Intermontane Plateau because it is
surrounded by mountains on three sides.
➢ Columbia River and its tributaries have dissected the Plateau
to a great extent.
➢ Other examples are the Deccan Plateau, Siberian Plateau, etc.
3. Fluvial Plateau: It was formed due to the continuous deposits of fluvial
sediments brought by the rivers and the sediments are consolidated and
stratified into sedimentary rocks of great thickness.
❖ These sediments are raised upwards, due to earth movements, relative to the
surrounding areas and an upland plateau with an extensive flat surface is
formed. Example: Extensive Kaimur Plateau consisting of Panna
Plateau, Bhandar Plateau, Rohtas Plateau, etc.
4. Aeolian Plateau: It was formed due to the deposition of fine sediments
brought down by the winds.
➢ The enormous volumes of sediments are consolidated in due course of time
and a Plateau is formed. Example: Potwar Plateau of Pakistan, Loess
Plateau of China.
 6

5. Tectonic Plateau: It is the most extensive, highest & complex plateau that originated through the
endogenetic forces coming from deep within the earth.
➢ These plateaus are formed by the horizontal and vertical movements caused by endogenetic forces
called Diastrophic plateaus. Example: Meghalaya Plateau, etc.
On the Basis of Geographical Situation :
❖ Intermontane Plateau: These are surrounded by mountains. Tibetan Plateau, Bolivian Plateau, Peru
Plateau, Columbian Plateau, etc.
❖ Piedmont Plateaus: Plateau formed at
the foothill zone of the extensive
mountains is called the Piedmont
Plateau.
➢ They are surrounded by mountains
on one side and plains or coast on
another. Example: Patagonian
Plateau (andes, south
america),cumberland plateau
appalachian mountains
➢ The side of the Plateau facing the plains is a steep slope and
thus forms an escarpment.
➢ The Appalachian Piedmont Plateau abruptly terminated
into the Atlantic coastal plains.
➢ A sudden break in the slope along the junction of the
Piedmont and coastal plains make a sudden decrease in the
channel gradients of Atlantic-bound rivers, and rivers
entering the Atlantic make stupendous waterfalls.
 7

➢ These are located at one side of the mountain and the

other side with the coast. Appalachian Piedmont,
Patagonian Plateau, etc.
❖ Continental Plateau: It is a very extensive plateau and is
generally away from the mountainous areas but is
surrounded by the Coastal Plains. These are located
inside the continents. Deccan Plateau, Ranchi Plateau,
Shillong Plateau, Columbian Plateau etc.
❖ Coastal Plateau: These are located near the coastal
regions. Coromandel coastal upland of east India.
On the Basis of Shape and Size:
❖ Dome Shaped Plateau: These are formed when the
landmass is uplifted in such a manner that the middle
portion is upwarped and the sides are rounded.
➢ These are generally formed due to endogenetic
forces, mainly during volcanic activities.
Chotanagpur plateau is a typical example.
➢ Ozark Plateau, USA- it was formed due to the
upliftment of the ground surface caused by the
Appalachian Mountain Building during the Permian
Period.
❖ Dissected Plateau: river streams cut the plateaus .Columbia Plateau,
etc, Deccan plateau dissected by Krishna ,Godavari, Cauvery and
Ranchi plateau. Dissection leads to valleys and canyons.
❖ Stepped Plateau: Kaimur Plateau, Panna Plateau etc
❖ Flat-Topped Plateau: Rewa Plateau, Tibetan Plateau, Bhander
Plateau, etc.
 8

On the Basis of Climate:

❖ Dry and Uneven Plateau: These are associated with dry and warm weather. These are also known as
uneven plateaus. The Arabian
Plateau is an example of a dry Plateau.
❖ Humid Plateau: Due to the Funnel
effect in Shillong Plateau, there is a
high amount of rainfall. High rainfall
and high humidity is characteristic of
these plateaus Example: Shillong
Plateau.
❖ Ice-Capped Plateau:These are fully
covered by ice Example: Greenland
Plateau, etc.
 9

On the Basis of stages of Erosion:

❖ Young Plateau: In these erosion is not started or least amount of erosion . Mahabaleshwar, Khandala
upland, Colorado Plateau.
❖ Mature Plateau: In these heavy erosions taking place. Ranchi Plateau, Hazaribagh Plateau, Appalachian
Plateau, etc.
❖ Rejuvenated Plateau: Plateau rises again due to deposition after complete erosion. Missouri Plateau, etc.
Major Plateaus of the World:
Tibetan Plateau:
❖ It is the highest Plateau in the world. Its average height is 5000 meters.
❖ Most extensive Plateau as it covers an area of 8 lakh sq miles.
❖ It is surrounded by Kunlun Mountain in the North, the Himalayas in the
South, Kunlun Himalayas in the West, and the Chinese Mountains in the
East.
❖ There are several enclosed salt lakes here.
❖ It is the source of various rivers- Indus, Brahmaputra, Mekong,
Salween, Yellow, and Yangtze Rivers.The plains formed from
the deposits of these rivers are known as Granaries of the
World.
❖ It has an important role in the development of the monsoon in
India.
Mexican Plateau:
❖ The entire Plateau is covered with playa lakes,
valleys, low hills, and salt lakes.
❖ Southern part is characterized by several
volcanic mountains.
❖ Tectonically, the Plateau is very much sensitive
and unstable.
❖ It is surrounded by the Sierra Madre
Occidental Range.
Columbian Plateau:
❖ River Columbia and its tributary Snake
meet in this Plateau.
❖ It is bordered by the Cascade Range and
the Rocky Mountains and divided by the Columbia River.
 10

❖ This Plateau has been formed due to volcanic eruptions with a consequent coating of basalt lava (Flood
Basalt Plateau).
❖ The Columbia Plateau is located in parts of Washington, Oregon, and the Idaho States of the U.S.A.
Colorado Plateau:
❖ The Colorado Plateau includes Utah, Arizona, and New Mexico.
❖ It is divided by the Colorado River and the Grand Canyon.
❖ It is the largest Plateau in America, and it covers an area of 337,000 sq
km and its highest elevation is about 2,450 meters above sea level.
❖ This Plateau is an example of the Intermontane Plateau. Mesas and
buttes are found here at many places.
❖ The Plateau is known for the groundwater which is under positive
pressure and causes the emergence of springs called Artesian wells.
Siberian Plateau:
❖ The Plateau occupies a great part of central Siberia between the
Yenisei and Lena rivers.
❖ It is located in the Siberian Platform and extends over an area
of 3,500,000 km2 , between the Yenisei in the west and the
Central Yakutian Lowland in the east.
❖ To the South, it is bound by the Altai Mountains, Salair Ridge,
Kuznetsk Alatau, the Eastern and Western Sayan
Mountains, and other mountains of Tuva, as well as the North
Baikal Highlands and Baikal Mountains.
❖ To the North of the plateau lie the North Siberian Lowland
and to the East, the Plateau gives way to the Central Yakutian
Lowland and the Lena Plateau.
❖ The surface of the Central Siberian Plateau is characterized
by the alternation of wide plateaus and ridges, some of the
latter are sharply jagged.
❖ The Central Siberian Plateau covers one-third of Siberia.
❖ The Himalayas protect from the chilled Siberian winds
which flow through this region.
 11

Deccan Plateau:
❖ Large Plateau which forms most of the southern part of India.
❖ It is bordered by the Western and Eastern Ghats.
❖ Volcanic Deccan Traps- Largest volcanic feature on Earth,
Made of multiple basalt layers or lava flows, covers 5,00,000
sq km.
❖ It is known for containing some unique fossils.
❖ It is rich in minerals. Primary mineral ores found in this region are
mica and iron ore, diamonds, gold and other metals in the Golconda
region.
❖ Major rivers here are the Godavari and Krishna which are a source
of Hydroelectricity.
❖ Tourist sites here are Ajanta and Ellora Caves.
❖ Black Soil is found here which is suitable for Cotton and Sugarcane
cultivation.
Kimberley Plateau:
❖ It lies in the north-western part of
Australia.
❖ This Plateau is made of volcanic
eruptions.
 12

❖ Many minerals like iron, gold, lead, zinc, silver and diamond are found here.
❖ Diamonds are very famous from here.
Patagonian Plateau:
❖ Patagonia refers to a geographical region that
encompasses the southern end of South
America, governed by Argentina and Chile.
❖ It is a Piedmont plateau (Arid Landforms)
lying in the southern part of Argentina.
❖ It is a rain shadow desert plateau.
❖ It is an important region for sheep rearing.
World Plateaus:
 13

   
 1

DAILY
CLASS NOTES
Geography

Lecture - 27
Introduction to Climatology
 2

Introduction to Climatology
Climatology:
Note:
 Importance of Climatology in terms of UPSC Pre and Mains:
 Atmosphere
 Temperature(Lapse rate, Temperature inversion)
 Pressure distribution(Pressure belts, Winds)
 Rainfall, cloud, Humidity, Frost, Fog
 Thunderstorm, Tornado
 Climatic phenomena(La-nina, El-nino, Indian ocean dipole, Madden Julian Oscillation)
 Cyclone, Polar Vortex
 Jetstream
 (2-3 questions in prelims, and 2 question comes in mains(20-25 marks)

 Climatology is the study of climate and how it changes over time. It is the study of the changes in weather
and climate and the underlying causes of the changes.
 Climatology is also the study of systematic and regional studies of atmospheric conditions (short term and
long term).
 Climate refers to the sum total of weather conditions and variations over a large area for a long period of
time (10-100 years). For example-Precipitation(Rain, Snow, Hails, Frost), Temperature, Humidity.
 Weather refers to the state of the atmosphere over an area at any point of time (short time(may be for 1 or
2 days).
 The elements of weather and climate are the same, i.e. temperature, atmospheric pressure, wind, humidity
and precipitation.
 This science helps people better understand the atmospheric conditions that cause weather patterns and
temperature changes over time.
 The climate of India is described as the ‘monsoon’ type. In Asia, this type of climate is found mainly in the
south and the southeast.
 Despite an overall unity in the general pattern, there are perceptible regional variations in climatic conditions
within the country.
 Seasons change (It is a variation in Atmospheric condition in a few months) in weather in around 3 to 6
months(few months). It is a mid-term phenomenon. For example: Summer. Winter
 3

Atmosphere:
 Our planet Earth is enveloped by a deep blanket of gasses
extending several thousands of kilometers above the Earth
surface.
 This gaseous cover of the earth is known as the
atmosphere, and they have some weight.The mixture of the
gasses pulled by the gravity of Earth, and then they remain
attached to Earth.
 It includes gasses(Air-O2, N2, CO2, Argon, Ozone and
water vapour) and consists of Solid particles(dust particles,
SOOT, ashes, salt, pollens), etc.
 Aerosols are defined as a combination of liquid or solid
particles suspended in a gaseous or liquid environment.
 Like land (lithosphere) and water (hydrosphere), the
atmosphere is an integral part of the earth.
 Atmosphere, lithosphere, Hydrosphere combined is called
Biosphere.
 Compared to the earth’s radius, the atmosphere appears to be
only a very thin layer of gasses.
 However, because of the force of gravity, it is inseparable from the Earth.
 The density of the atmosphere varies with height. It is maximum at sea level and decreases rapidly as
we go up. The climbers experience problems in breathing due to this decrease in the density of air.
 The atmosphere exerts pressure on the Earth. Some areas experience high pressure while some have
low pressure.
 The air is an integral part of the earth’s mass and 99
percent of the total mass of the atmosphere is confined
to the height of 32 km from the earth’s surface.
 The air is colourless and odourless and can be felt
only when it blows as wind.
 The atmosphere is composed of gasses, water vapour
and dust particles.
 The proportion of gasses changes in the higher layers of
the atmosphere.
Atmospheric Pressure:
 The air exerts pressure on the earth’s surface by virtue of
its weight. This pressure is called atmospheric pressure.
 Atmospheric pressure is the most important climatic element.
 The atmospheric pressure at sea level is 1034 gm per square centimeter.
 4

 As the height increases, the pressure keeps on decreasing.

 At Mount Everest, when people reach the height the nose starts bleeding due to low atmospheric pressure in
the bloodstream.

NOTE:
 Pressure is force per unit area, and the atmospheric pressure is around 1.0132 bar per unit area. The Bar is
the unit of Pressure.

Origin of the Atmosphere:

 The atmosphere is nowadays thought to have been created at the time the Earth was being formed.
 When the earth was formed around 4.5 to 5 billion
years ago, it had initially very high temperatures.
 Due to its slow cooling, the Hydrogen and Helium
gas was stripped out (removal of the primordial
atmosphere). It happened around 5-10 billion years
ago due to the blowing of solar winds.
 After this, degassing happened due to volcanic
eruptions and cooling down.
 The gasses created an umbrella around the earth,
which consist of a number of gasses (Carbon
dioxide, methane, etc.), particles (dust, ash, soot,
etc.), and water vapour.
 The water vapour started condensing around the hygroscopic nuclei.
 Clouds form when the invisible water vapour in the air condenses into visible water droplets or ice crystals.
 For this to happen, the parcel of air must be saturated, i.e. unable to hold all the water it contains in
vapour form, so it starts to condense into a liquid or solid
form.
 It leads to precipitation (rainfall) and the filling up of
depressions on the earth from rainwater.
 The collection of rainwater helps in the formation of oceans.
And in the oceans, life formation happened.
 Due to insolation there is a biochemical reaction in the ocean
with the help of energy provided by the sunlight. It led to the
generation of algae and thus, the evolution of life on Earth.
 The algae and other organisms started performing
photosynthesis and absorbed carbon dioxide and released
oxygen into the atmosphere.
 The atmosphere got modified due to the process of photosynthesis around 2.5 billion years ago.
 5

Role of Earth’s Atmosphere:

 Important Gasses: The atmosphere contains various gasses like oxygen (Respiration-Animals, Amphibians,
Humans and Combustion of fuels), carbon dioxide, nitrogen, etc.
 Plants require for food preparation(photosynthesis)carbon dioxide to survive while animals and many
other organisms need oxygen for
their survival.
 Carbon dioxide is also required for
temperature regulation.
 Nitrogen forms the basic building
block (DNA), and it is also required
for proteins and Amino acids, and
also required for plants, animals,
and humans.
 The atmosphere supplies these life-
giving gasses.
 All life forms need a particular
range of temperature and a specific
range of frequencies of solar
radiation to carry out their
biophysical processes.
 Regulates Solar Radiation: The atmosphere absorbs certain frequencies and lets through some other
frequencies of solar radiation. In other words, the atmosphere regulates the entry of solar radiation. It is
favourable for biochemicals which are important for life for life evolution. Approximately 50 per cent of the
radiation from insolation reaches the earth's proximity (near to the Earth).
 Greenhouse Gas Effect: The atmosphere also keeps the temperature over the earth’s surface within
certain limits. In the absence of the atmosphere, extremes of temperature would exist between day and night
over the earth’s surface. It allows short waves from the sun to pass through and traps the long waves which
increase the temperature on the earth’s surface. This helps in managing the heat budget.

NOTE:
 A heat budget is a perfect balance between incoming heat (short wave radiation) absorbed by the earth and
outgoing heat (long wave radiation) escaping it in the form of radiation.
 If the incoming heat and the outgoing heat are not balanced, then Earth would be getting either too warmer
or cooler.

 Protection from UV rays: Harmful ultraviolet (Ultraviolet) radiation would find its way through if the
atmosphere (ozone in the stratosphere to be specific) were absent. Ultraviolet rays are harmful for Humans,
Animals, Plants, and Material. The Ozone layer is inside the stratosphere.
 6

 Protection from Extra-terrestrial Objects: The

atmosphere also takes care of extra-terrestrial objects like
meteors which get burnt up while passing through the
atmosphere (mesosphere to be precise) due to friction.
Mesosphere does not allow the meteor to reach the Earth
surface.
 Weather is another important phenomenon that dictates the
direction of a number of natural and man-made
processes like plant growth, agriculture, soil formation,
human settlements, etc. the various climatic factors join
together to create weather. The massive weather
phenomena are Winds, clouds, Precipitation (Fog,
Frost, Snow, Hail), Cyclones, Thunderstorms,
Tornadoes. The maximum weather phenomenon
occurs in the atmospheric layer called the
Troposphere (Weather theatre).
Sun- Source of Energy:
 The energy from the sun is a key driver behind
various phenomena like winds, rain, ocean
currents, denudation, cloud formation etc.
 The Sun source of energy in the
form of radiation which is coming
to Earth(Insolation).
 There are four types of radiation
that are emitted from the sun.
 These are as follows:
1. Short-wave radiation (1
lambda =0.4 microns-X-
Rays, Gamma Rays, UV
Rays)
2. Long wave radiation (It is
from 300 microns to 100 metres-For example-Radio communication).
3. Visible rays (V-I-B-G-Y-O-R) (Violet, Indigo, Blue, Green, yellow, orange, red it is 0.4 micron to 0.7
micron).
4. Infrared waves (It responds 0.4 microns to 300 microns, and It is responsible for heat, 50 % radiation
from infrared).
 7

Composition of the Atmosphere:

There are three parts like:
 Gasses:
 There are two types like
permanent(No change-
Example: Nitrogen, Oxygen,
Argon) and
Variable(Changing
proportion in time and space.
Example: Co2, Water vapour,
Ozone, Helium, Hydrogen).
Permanent gasses of the atmosphere:
Constituent Chemical Formula Percentage by Volume
Nitrogen N₂ 78.08%
Oxygen O₂ 20.95%
Argon Ar 0.93%
Carbon Dioxide CO₂ 0.036%
Neon Ne 0.0018%
Helium He 0.0005%
Krypton Kr 0.0001%
Xenon Xe 0.000009%
Hydrogen H₂ 0.000005%
 Solid Particles:
 The solid particles are dust, particulate matter, PM 2.5, PM 10, SOOT, ashes, pollens, salt particles).
 The solid particles are responsible for various phenomena like Reflection, Refraction, and scattering of
light(Example-Orange sun, Blue sky).
 They act as a nuclie for clouds.
 Aerosoles:
 Solid & liquid/vapour suspended in air.
 The atmosphere is a mixture of many gasses. In addition, it
contains ‘aerosols’.
 An aerosol is a suspension of fine solid particles or liquid
droplets, in the air or another gas.
 Aerosols can be natural or anthropogenic.
 Examples of natural aerosols are fog, dust, forest
exudates, and geyser steam.
 8

 Examples of anthropogenic aerosols are haze, particulate air pollutants, and smoke.
 If the suspended particles, water vapour, and other variable gasses were excluded from the atmosphere, then
the dry air is very stable all over the earth up to an altitude of about 80 kilometers.

 1

DAILY
CLASS NOTES
Geography

Lecture – 28
Atmosphere of Earth
 2

Atmosphere of Earth

Atmosphere:
❖ The height of the atmosphere goes up to 30000 kilometers.
❖ The majority of mass of the atmospheres are gasses, water vapour, solids, and aerosols, and this is
concentrated in the zone of 5 to 6 kilometers, with the maximum atmospheric mass concentrated in the
Troposphere.
❖ The various atmospheric studies were done up to 800 kilometers.
Composition of the Atmosphere:
Aerosoles:
❖ The proportion of gasses changes in the higher layers
of the atmosphere in such a way that oxygen will be
almost in negligible quantity at a height of 120
km.
❖ Similarly, carbon dioxide and water vapour are found
only up to 90 km from the surface of the earth.
❖ Nitrogen and oxygen make up nearly 99% of the
clean, dry air.
❖ The remaining gases are mostly inert and
constitute about 1% of the atmosphere.
❖ Besides these gases, large quantities of water vapour and dust particles are also
present in the atmosphere.
❖ These solid and liquid particles are of great climatic significance.
❖ There are various permanent gases (Helium, Hydrogen, etc.) and variable
gasses (water vapour, carbon dioxide, etc.) in the atmosphere.
❖ At polar regions, the water vapour is lowest (around 0.1-0.2%) and at equatorial
latitudes, it is highest (around 4%).
❖ With an increase in height, the composition of gasses varies due
to the effect of solar rays and gravitational force.
Nitrogen:
1. Nitrogen accounts for 78% of total atmospheric volume.
2. It is a relatively inert gas (does not react) and is an important
constituent of all organic compounds.
 3

3. Nitrogen aids in the formation of building blocks such as proteins and amino acids and is also a major
component of chlorophyll, the compound by which plants use sunlight energy to produce sugars from water
and carbon dioxide (i.e., photosynthesis)
4. The main function of nitrogen is to control combustion by diluting oxygen.
5. Plants need Nitrogen for their survival, but they cannot take it directly from the air (Free Nitrogen in the
form of gas).
6. Nitrogen comes in three forms:
a. In the form of Free Nitrogen (N2 gass),
b. In the form of Oxide Nitrogens (Nitric Oxide, Nitrous oxide, and Nitrogen dioxide),
c. In the form of Nitrogen-hydrogen compound (Ammonia-NH3).
7. Nitrogen Fixation is the phenomenon of conversion of free nitrogen molecules into a usable form like
Ammonia.
8. Bacteria (Pseudomonas, Nitrosomonas, Nitrobactor, Azerobacter, etc.) that live in the soil and roots of some
plants, take nitrogen from the air and change its form so that plants can use it.
9. Leguminous is used to describe plants in the legume family, which includes the plants that produce beans,
peas, and lentils. Crops that have nodules in their roots that contain nitrogen-fixing bacteria (Rhizobium)
are called leguminous crops.
10. It is a permanent gas, i.e., its proportion neither increases nor decreases.
11. Animals obtain nitrogen by eating plants and other animals, and humans obtain it from plants and animals.
Oxygen:
❖ Oxygen, although constituting only 21% of the total volume of the atmosphere, is the most important
component among gases.
❖ All living organisms inhale oxygen.
❖ Respiration for Animals, humans, and amphibians (Oxygen through blood rushes to tissue and cells,
providing energy to them).
❖ Besides, oxygen can combine with other elements to form important compounds, such as oxides like
Iron oxide, Aluminum oxide, etc.
❖ Also, combustion (burning) is not possible without oxygen.
❖ Oxygen has an important role in the formation of carbohydrates.
❖ Initially, when Earth formed, oxygen was not available.
❖ Oxygen is available in the atmosphere up to 120 kilometers.
❖ UV rays break down the oxygen and render it useless.
❖ O is nascent oxygen, and O2 is oxygen molecules.
 4

Carbon Dioxide (Green house Gas):

❖ It is the third important gas. It constitutes only about
0.03% of the dry air and is a product of combustion.
❖ It allows shortwave solar radiation to pass through,
and blocks long-wave terrestrial radiation from Earth.
❖ Green plants, through photosynthesis, absorb
carbon dioxide from the atmosphere and use it to
manufacture food and keep other bio-physical
processes going.
❖ Being an efficient absorber of heat, carbon dioxide is
considered to be of great climatic significance.
❖ Carbon dioxide is considered a very important factor in
the “Heat energy budget”.
❖ It acts as a blanket in the atmosphere, which traps the
heat and produces the greenhouse gas effect.
❖ It is a variable gas, and the percentage of carbon dioxide is
changing.
❖ It is very important for Photosynthesis (Green plants
utilise CO2 to produce their food and oxygen utilise too).
❖ Carbon dioxide is a very important meteorological gas as it
is transparent to the incoming solar radiation but opaque to the outgoing terre strial radiation.
❖ It is largely responsible for the greenhouse effect.
❖ With the increased burning of fossil fuels -oil, coal, and natural gas– the carbon dioxide percentage in the
atmosphere has been increasing at an alarming rate.
❖ More carbon dioxide in the atmosphere means more heat absorption.
❖ It could significantly raise the temperature at lower levels of the atmosphere, thus inducing drastic
climatic changes (Global Warming).

Note:
❖ The greenhouse effect is the way in which heat is trapped close to Earth's surface by “greenhouse gases.”
These heat-trapping gases can be thought of as a blanket wrapped around Earth, keeping the planet toastier
than it would be without them.
❖ In the 1800s, CO2's value was somewhere around 290 ppm.
❖ After the industrial revolution in 2020, it was 360-375, and in 2023, it crossed 400 ppm.
❖ The reasons for temperature rises are Industries, Vehicles, Thermal power plants, Urbanisation,
Deforestation, etc.
 5

Ozone:
❖ Ozone is another important gas in the atmosphere
and is actually a type of oxygen molecule
consisting of three instead of two atoms.
❖ It is a bluish, unstable gas with a pungent odour.
❖ The amount of ozone in the atmosphere is
0.00005/volume.
❖ It forms less than 0.00005% by volume of the
atmosphere and is unevenly distributed.
❖ Ozone is found in the Stratosphere (Good
ozone).
❖ It is between 20 km and 25 km in altitude that the greatest concentrations of ozone are found. It is
formed at higher altitudes (90–100 km,
where oxygen is destroyed and forms ‘o’
atoms) and transported downward.
❖ At 50-60 km (zone of the stratosphere),
again, oxygen reacts with ‘o’ atoms.
❖ Oxygen atoms combine with molecules to
form the Ozone layer.
❖ Ozone plays a crucial role in blocking the
harmful ultraviolet radiation from the sun
from reaching the surface of the earth.
❖ Other gasses found in almost negligible quantities in the atmosphere are argon, neon, helium, hydrogen,
xenon, krypton, methane, etc.
❖ Ozone can be “good” or “bad” for health and the
environment, depending on where it’s found in the
atmosphere.
❖ Stratospheric ozone is “good” because it protects living
things from ultraviolet radiation from the sun.
❖ Ground-level ozone is considered “bad” because it can
trigger a variety of health problems, particularly for
children, the elderly, and people of all ages who have lung
diseases such as asthma.
❖ Ozone is another important greenhouse gas, but it has a
very small proportion at the surface.
 6

Ozone Hole:
❖ The ozone hole is not technically a “hole” where no ozone is present, but is actually a region of
exceptionally depleted ozone in the stratosphere over the Antarctic that happens at the beginning of spring
(August–October) in the Southern Hemisphere.
❖ Satellite instruments provide us with daily images of ozone over the Antarctic region. The ozone hole image
below shows the very low values centered over Antarctica on 4 October 2004.
❖ From the historical records, we know that total column ozone values of less than 220 Dobson Units were
not observed prior to 1979.
❖ From an aircraft field mission over Antarctica, we also know that a total column ozone level of less than 220
Dobson Units is a result of catalyzed ozone loss from chlorine and bromine compounds. For these reasons,
we use 220 Dobson Units as the boundary of the region representing ozone loss.
Water Vapour:
❖ Water Vapour is one of the most variable gaseous substances present in the atmosphere – constituting
between 0.02% and 4% of the total volume (in cold, dry, and humid tropical climates, respectively), which
decreases with altitude.
❖ Water vapour also decreases from the Equator towards the pole.
❖ Water vapour formed from water bodies like Lakes, Rivers, seas, and Vegetation (Trees, plants).
❖ The phenomenon of evaporation is the conversion of liq uid to vapour.
❖ Around 90% of moisture content in the atmosphere exists within 5-6 km of the surface of the earth, and
the remaining 10 per cent of water vapour is maximum up to 90 km.
❖ Water vapour is responsible for weather phenomena like rain, clouds, snow, hail, thunderstorms, etc.
❖ Water vapour is another greenhouse gas like CO2 (trapped in terrestrial radiation).
❖ Like carbon dioxide, water vapour plays a significant role in the insulating action of the atmosphere.
❖ It absorbs not only the long-wave terrestrial radiation (infrared or heat emitted by the earth during the
night) but also a part of the incoming solar radiation.
❖ It is the source of precipitation and clouds. On condensation, it releases latent heat of condensation -the
ultimate driving force behind all storms.
❖ The moisture–carrying capacity of air is directly proportional to its temperature.
❖ The water-vapour content of the atmosphere varies from place to place and from time to time because the
humidity capacity of air is determined by its temperature.
❖ At 30 °C (86 °F), for example, a volume of air can contain up to 4.5 per cent water vapour. At -40 °C (-40
°F), however, it can hold no more than 0.2 per cent.
❖ The poles have low insolation, and in the tropics, the region around the equator has high insolation.
 7

❖ When a volume of air at a given temperature holds the maximum amount of water vapour, the air is said to
be saturated.
❖ It thus acts like a blanket, allowing the earth neither to become too cold nor too hot. Water vapour also
contributes to the stability and instability of the air.
❖ The relative humidity is the water-vapor content of the air relative to its content at saturation. Saturated air,
for example, has a relative humidity of 100 percent, and near the Earth, the relative humidity very rarely falls
below 30 percent.
Solid Particles:
❖ The Solid Particles present in the atmosphere consist of sand particles (from weathered rocks and also
derived from volcanic ash), pollen grains, small organisms, soot, and ocean salts; the upper layers of the
atmosphere may even have fragments of meteors which got burnt up in the atmosphere. These solid particles
perform the functions of absorbing, reflecting, and scattering radiation.
❖ The solid particles are, consequently, responsible for the orange and red colours at sunset and sunrise and
for the length of dawn (the first appearance of light in the sky before sunrise) and twilight (the soft glowing
light from the sky when the sun is below the horizon, caused by the reflection of the sun's rays by the
atmosphere). Dusk: the darker stage of twilight.).
❖ The blue colour of the sky is also due to selective scattering by dust particles.
❖ Some of the dust particles are hygroscopic (i.e., readily absorbing moisture from the air) in character, and, as
such, act as nuclei of condensation.
❖ Thus, dust particles are an important contributory factor in the formation of clouds, fog and hailstones.
Structure of the Atmosphere:
❖ Its height is around 16000-29000 km. but the most important part of the atmosphere that is studied is around
800km.
❖ Up to 5.5 km in height, it consists of various gases, water-vapour, and solid particles.
❖ The atmosphere can be studied as a layered entity, as each layer has its own peculiar characteristics.
❖ These layers are systematically discussed below.
❖ There are structures like:
➢ Troposphere (surface to 10 km)
➢ Stratosphere (Up to 50 km)
➢ Mesosphere (Up to 80-90 km)
➢ Thermosphere (Up to 600 km)
Troposphere:
❖ The word troposphere originates from the Greek word ‘Tropos’, which means mixing or turbulence.
Therefore, it is a zone of mixing of gases and turbulence due to vertical winds, which leads to the
formation of cyclones and other weather phenomena.
 8

❖ It is the atmospheric layer between the earth’s surface and an altitude of up to 6-8 km at the Poles and 16-
18 km at the Equator.
❖ Around 75% of the gaseous mass is concentrated in
this layer.
❖ The thickness of the troposphere is greater at the
equator because the heated air rises to greater heights
(due to direct insolation at the equator). Due to the
earth's rotation, there is also centrifugal force acting at the
equator, which lifts the air outwards (at the poles, there is
less centrifugal effect).
❖ The friction layer is around 1 to 3 km, and this is more
resistant to the flow of air /winds.
❖ The troposphere ends with the Tropopause. It acts as a
boundary between the troposphere and the
stratosphere. This layer is marked by constant
temperatures. It is also known as a cold spot and in this
region mixing gets stopped.
❖ The troposphere gets heated up due to terrestrial radiation.
❖ As one goes upwards, the temperature in this layer,
falls at the rate of 6.5°C per kilometre (normal lapse
rate), and reaches -45°C at the Poles and -80°C over the
Equator at Tropopause (greater fall in temperature above
the equator because of the greater thickness of troposphere
– 18 km).
❖ The fall in temperature is called the ‘lapse rate’. (More
about this in future posts).
❖ The troposphere is marked by a temperature inversions,
turbulence, and eddies.
❖ It is also the most significant zone meteorologically in the
entire atmosphere (Almost all the weather phenomena like
rainfall, fog, hailstorms, etc. are confined to this layer). It is
the theater for the weather because all cyclones,
anticyclones, storms, and precipitation occur here, as all
water vapors and solid particles lie within it.
 9

❖ It is also called the convective region (vertical winds) since all convection stops at Tropopause. The
amount of water vapor or moisture decreases with the
increasing height of the troposphere.
❖ Around 99% water vapour is present in this layer.
❖ The troposphere is influenced by seasons and jet
streams.
❖ The maximum concentration of greenhouse gases is in
this region, which helps manage the heat budget
(Greenhouse gas effect).
❖ Helicopters, birds, gas balloons, etc. fly in this region.
All forms of life are present in this layer due to favorable
weather conditions and habitable temperatures.
❖ Moisture is responsible for all weather phenomena

   
 1

DAILY
CLASS NOTES
GEOGRAPHY

Lecture – 29
Layers of Atmosphere
 2

Layers of Atmosphere

Stratosphere:
❖ It lies beyond the troposphere, up to an altitude of 10 to 50 km from the earth’s surface.
❖ The stratosphere is called a stable region.
❖ The temperature in this layer remains constant for some distance but then rises to reach a level of 0°C at 50
km altitude.This rise is due to the presence of ozone in this layer (harmful ultraviolet radiation is absorbed
by ozone).
❖ This layer is almost free from clouds and associated weather phenomena, making conditions most ideal
for flying airplanes(no turbulence in this layer). There is no/little resistance from air.
❖ So, airplanes fly in the lower stratosphere, sometimes in the upper troposphere where the weather is calm.
❖ Aircrafts prefer this layer due to low disturbance, and low density also. As with increasing height density
is falling (Density = mass/Volume).
❖ Weather balloons (weather exploration) are also sent to this layer.
❖ The density of the stratosphere is 0.289kg/metre cube, at the surface it is 1.17 kg/metre cube.
❖ Sometimes, cirrus clouds are present at lower levels in this layer.
❖ In this layer there is no convection, that means no vertical winds are there.
❖ Due to the absence of vertical winds, material reaching this layer stays for a longer time.
Ozonosphere:
❖ It lies at an altitude between 15 km to 35 km from the earth’s
surface and spans the stratosphere and lower mesosphere.
❖ Because of the presence of ozone molecules, this layer reflects
harmful ultraviolet radiation.
❖ The ozonosphere is also called the chemosphere because a lot of
chemical activity goes on here.
❖ Ozone layer absorbs UV Rays, and prevents earth and humans
from ill effects like damage to vegetation, skin cancer, structure
damage.
❖ The Ozone layer is called the Ozonosphere.
❖ Ozone can be “good” or “bad” for health and the
environment, depending on where it’s found in the
atmosphere.
❖ Stratospheric ozone is “good” because it protects
living things from ultraviolet radiation from the sun.
❖ The temperature, here, rises at a rate of 5°C per
kilometer.
 3

❖ Due to the absence of vertical wind movement, the pollutants like chlorine, bromine, CFCs (Chloro-fluoro
carbons), etc. stay for longer periods of time in this layer.
❖ CFCs are found in refrigerators, sprays, propellants, fuel
emission of jets.
❖ There is low temperature and less moisture in the air.
❖ In the stratosphere there are particles, dust, gasses, and
CFCs that stay there for a long time.
➢ These pollutants are responsible for damaging the
ozone layer.
❖ The polar stratospheric clouds form near the tropopause
(due to the presence of water vapour at lower heights in polar regions) and carry various gasses like Chlorine,
Bromine, Fluorine, etc.
➢ Polar stratospheric clouds (PSCs)
play important roles in
stratospheric ozone depletion
during winter and spring at high
latitudes (e.g., the Antarctic ozone
hole).
➢ These particles also provide sites
for heterogeneous reactions that
convert stable chlorine reservoir
species to radicals that destroy
ozone catalytically.
❖ The ozone is converted into oxygen and
free chlorine molecules which again starts the cyclic reaction process.
NOTE:
❖ Inside the stratosphere there are Ozone molecules, and chlorine will interact with ozone then reaction will
happen (oxygen and chlorine single molecule will be separated, and it will break this oxygen, and form
ClO).
❖ Again chlorine will interact with ozone and form O2+CLO, and they will interact with each other and form
Cl2+O2.
❖ Other possibilities are also like (O2+Cl-O, and O2+Cl-O = ClO will meet some O (CL-O+O) will form O2
again.
❖ Cl-O+O=Cl (free chlorine) + O2.
❖ Free chlorine is ready to attack again on ozone molecules.
❖ The damage of the ozone layer is called Ozone depletion, and for depletion of ozone there is one treaty sign
which is called Montreal protocol.
 4

Montreal Protocol:
❖ The Montreal Protocol on Substances that Deplete the Ozone Layer is the landmark multilateral
environmental agreement that regulates the production and consumption of nearly 100 man-made chemicals
referred to as ozone-depleting substances (ODS).
❖ When released into the atmosphere, those chemicals damage the stratospheric ozone layer, the Earth’s
protective shield that protects humans and the environment from harmful levels of ultraviolet radiation from
the sun.
❖ Adopted on 16 September 1987, the Protocol is to date one of the rare treaties to achieve universal
ratification.
❖ The Montreal Protocol phases down the consumption and production of the different ODS in a stepwise
manner, with different timetables for developed and developing countries (referred to as “Article 5
countries”).
❖ The Kigali Agreement is an amendment to the Montreal Protocol, which is signed by countries to phase
out Ozone Depleting Substances.
Mesosphere:
❖ This is an intermediate layer beyond the ozone layer
and continues up to an altitude of 50 kms to 80 km
from the earth’s surface.
❖ This layer is a very cold layer, and the cold layer is
very low temperature.
❖ The temperature in this layer falls gradually to -
100°C to -135 degree celsius (in troposphere
temperature is 25 degree celsius, and at the
stratosphere it is around -50 degree celsius) at 80 km
altitude.
❖ It has a significant density (not higher than the
troposphere) which destroys meteorites (burn up in
this layer upon entering) entering the earth’s surface from space.
❖ This layer is difficult to study as very little/less knowledge is available. Sounding Rockets are being used
for studying this layer for exploration(space and Weather).
❖ Polar Mesospheric Clouds(Rare clouds) are found here, which are strange because they create no weather
phenomenon.
❖ In the mesosphere temperature decreases with height.
❖ There are no reasons for heating in this layer so this is the coldest layer.
 5

Thermosphere:
❖ In the thermosphere, the temperature rises very rapidly with increasing height.
❖ Ionosphere is a part of this layer. It extends between
80-600 km.
❖ This layer helps in radio transmission. In fact, radio
waves transmitted from the earth are reflected back to
the earth by this layer.
❖ The layer has extremely low pressure.
❖ The temperature increases as the height increases in
the thermosphere. This layer has a maximum
temperature. (Though the temperature is high, the
atmosphere is extremely rarefied – gas molecules are
spaced hundreds of kilometres apart. Hence, a person or an object in this layer doesn’t
feel the heat).
❖ This layer is also called a hot zone because the Sun is emitting UV rays , and these
UV rays are transmitting energy to the Thermosphere.
❖ The temperature in hot zones is 1700 to 1800 degree celsius.
❖ It absorbs the maximum amount of insolation.
❖ Due to an increase in the temperature, this layer keeps on expanding upward(pulling
up of the atmosphere due to expansion) and hence
there is maximum friction.
❖ Person would not feel warm because of the
thermosphere’s extremely low pressure.
❖ The International Space Station and satellites orbit
in this layer(in low earth orbit somewhere around 400
km).
❖ The maximum absorption of UV rays is absorbed
here.
❖ Auroras are observed in the lower parts of this layer.
➢ Electrons come in contact with X-rays and UV
rays and air molecules get energized(increase the
energy). The electrons, thus, shift to the higher
orbit after getting energy.
➢ Energy gets released in the form of colorful light, this colourful light called as Auroras.
➢ To get back to their initial state from the energized state, these electrons release energy in the form of
light. This phenomenon helps in forming Auroras.
 6

Ionosphere:
❖ This layer is located at 400 km of height, and is an
electrically charged layer.
❖ This layer is characterized by the ionization of atoms.
❖ It is a part of the thermosphere.
❖ The concentration of ions here is more due to X-rays
and UV rays from sunlight.
❖ High energy rays will ionize the particles(positive and
negative charge).
❖ Because of the electric charge, radio waves transmitted
from the earth are reflected, absorbed and bent back to
the earth by this layer.
❖ Temperature again starts increasing with height because
of radiation from the sun.
❖ Solar Flares(storms) and Coronal Mass Ejections cause severe disruptions in radio communication
because it changes the density of the ions.
Exosphere:
❖ This is the uppermost layer of the atmosphere extending beyond the ionosphere above a height of about 600
to 80,000 kms.
❖ Geostationary orbits/Satellites are there at the
height of 36000 kms.
❖ The maximum temperature go upto 5500 degree
celsius.
❖ This layer is vacuum(nothing) like there is no air,
particles, no gasses, no vapours, and this is just open
space.
❖ The air is extremely rarefied and the temperature
gradually increases through the layer.
❖ Light gasses like helium and hydrogen float into
space from here.
❖ Temperature gradually increases through the layer as it is exposed to direct sunlight.
❖ This layer coincides with outer space (no clear cut boundaries).
   
 1

DAILY
CLASS NOTES
Geography

Lecture - 30
Temperature Distribution
Around the World
 2

Temperature Distribution Around the World

Temperature:
❖ Temperature is the degree of hotness or coldness of the body or a region or a place or a waterbody.
❖ The main cause of the temperature is the Sun.
Sun as the Source of Energy:
❖ Sun is the ultimate source of energy (heat), and it is the cause of temperature.
❖ The Sun is a source of unequal energy and it is the cause of differential energy.
❖ The differential heat received from the sun by different regions on earth is the ultimate reason behind all
climatic phenomena.
❖ Understanding the patterns of distribution of temperature in different seasons is important for
understanding various climatic features like wind systems, pressure systems, precipitation, etc.
❖ Such factors include an increase or a decrease in temperature, an increase or a decrease in altitude, and
various heat transfer processes such as convection and radiation are responsible for Normal lapse rate.
❖ Incoming solar radiation is generally defined in the form of Insolation, and Insolation comprises X-Rays,
UV-Rays, Electromagnetic waves, Radio waves and Infrared waves.
Insolation:
❖ Earth intercepts only one in two billion parts of solar radiation. This intercepted radiation is called
Insolation, i.e., it is the proportion of Solar energy received or intercepted by the earth.
❖ Insolation is responsible for temperature over the
different regions.
❖ It is energy per unit of time and per unit area falling at
a place. It varies from the tropics to the poles.
❖ The insolation is more at the equator and less at the
poles.
❖ Some heat within the core and mantle is transferred to the
surface and ocean bottoms through volcanoes, springs, and
geysers.
❖ The Earth receives short-wave radiation from the Sun
and reflects back long-wave radiation (infrared radiation) while cooling.
❖ The wavelength is inversely proportional to the frequency of the waves.
The Electromagnetic Spectrum:
 3

Ways of Transfer of Heat Energy:

❖ Heat is a form of energy that raises the temperature of a body.
❖ Heat transfer is the process of transfer of heat from a hot body to a cold body.
❖ This process happens naturally.
❖ The heat energy from solar radiation is received by the earth through three
mechanisms:
➢ Radiation: Radiation is the transfer of heat from one body to another
without actual contact or movement. It is possible in relatively
emptier space, for instance, from the sun to the earth through space. The
maximum mode of heat transfer in a body is through this
mechanism. The major source of heat transfer from Radiation.
Radiation does not require any medium.
➢ Conduction: Conduction is the transfer of heat through matter by
molecular activity. Heat transfer in iron and other metals is by
conduction. Generally, denser materials like water are good
conductors and a lighter medium like air is a bad conductor of
heat. In Conduction to bodies are in contact.
➢ Convection: It is the transfer of heat energy by the actual
transfer of hot matter or substance from one place to another.
(Heat transfer by convection cycles in the atmosphere as well as
oceans).
 4

Factors Affecting Temperature Distribution:

❖ Angle of inclination of the Sun’s Rays: The Angle of
Incidence or the Inclination of the Sun’s Rays. The sun
rays are overhead in the tropics, and the oblique rays are
in the polar regions. Maximum heat received by tropics
zone(Torrid zone-It makes hotter), Low heat received by
Temperate zone, and least amount of heat received by
Polar regions(Frigid zone-It makes cold region). The
insolation is 70 watt/meter square in the polar region
and in the tropics area it is 320 watt/meter square.
❖ Duration of Sunshine: The amount of heat received
depends on the duration of day or night; clear sky or
overcast, summer or winter, etc.
➢ The sun rays are vertically overhead at the tropics, thus, these areas receive more sunshine.
➢ Angle of inclination is perpendicular (high) to the tropics and low at the poles.
❖ Transparency of Atmosphere: The atmosphere contains gasses, Pollens, soot, dust, vapours, salt which have
an impact on the transparency. Aerosols (smoke, soot), dust, water vapour, clouds, etc. affect transparency of
temperature. For example hilly regions like Nainital, Mussoorie
have sunny days and cold nights.
➢ Greenhouse gasses trap long-wave radiation in the
atmosphere and cause an increase in the temperature
(Global warming).
➢ If the wavelength (X) of the radiation is more than the
radius of the obstructing particle (such as a gas), then a
scattering of radiation takes place. Example: Orange colour
sun, etc
➢ If the wavelength is less than the obstructing particle
 5

(such as a dust particle), then total reflection takes place.

➢ Absorption of solar radiation takes place if the obstructing particles
happen to be water vapour, ozone molecules, carbon dioxide molecules, or
clouds.
➢ Most of the light received by the Earth is in the form of scattered light.
❖ Land-Sea Differential: Land sea differential is differences in temperature of
land (continents) & Sea (coast). Albedo of land is much greater than the albedo of oceans and water
bodies. Example: Snow-covered areas reflect up to 70%-90% of insolation.
➢ Average penetration of sunlight is more in water, up to 20 meters, than in land – where it is up to one
meter only. Therefore, the land cools or becomes hot more rapidly compared to the oceans.
➢ In oceans, a continuous convection cycle helps in heat
exchange between layers keeping diurnal and annual
temperature ranges low.
➢ The specific heat of water is 2.5 times higher than landmass,
therefore water takes longer to get heated up and to cool down.
➢ Land also absorbs the heat, however, it cannot reflect it like
water bodies.
➢ Ocean currents transfer heat and distribute it to other areas
whereas land is stationary and there is no transfer of heat.
➢ Thus, in the coastal areas, the temperature is neither high nor
low whereas, in continental areas, the temperature varies to a
large extent.
➢ Land heat is concentrated and water heat is spreading up to the
depth so that other factor is concentration of heat in the top
layer of land.
➢ Land reflection is much lower than water so that there is a huge Reflection in Water.

NOTE:
❖ Specific heat is the quantity of heat required to raise the temperature of one gram of a substance by one
degree Celsius. Specific heat of water is more than the specific heat of land.
❖ There are reasons of the Land sea Differential:
➢ Slow heating and cooling of water.
➢ Reflection of light/radiation.
➢ Transmission of heat upto depth(Vertical distribution).
➢ Flow of water(Horizontal heat distribution).

❖ Prevailing Winds:
 6

➢ Prevailing Winds are winds which help in heat

distribution by mixing cold and warm winds.
➢ Winds transfer heat from one latitude to another. They
also help in the exchange of heat between land and
water bodies.
➢ There are three types of winds: planetary winds (easterly
and westerlies) and local winds (Chinook, Fohen, etc.),
Periodic winds(Which flow in the interval of day and
night).
➢ The oceanic winds have the capacity to take the
moderating influence of the sea to coastal areas which
is reflected in cool summers and mild winters. This
effect is pronounced only on the windward side (the side
facing the ocean).
➢ The leeward side or the interiors do not get the
moderating effect of the sea, and therefore experience
extremes of temperature.

NOTE:
❖ The land and sea absorb and transfer heat differently.
➢ During the day the land heats up faster and becomes warmer than the sea. Therefore, over land
the air rises giving rise to a low pressure area, whereas the sea is relatively cool and the pressure
over sea is relatively high. Thus, pressure gradient from sea to land is created and the wind
blows from the sea to the land as the sea breeze.
➢ In the night the reversal of condition takes place. The land loses heat faster and is cooler than
the sea. The pressure gradient is from the land to the sea and hence land breeze results.

❖ Aspects of Slope: The direction of the slope and its angle

control the amount of solar radiation received locally.
➢ Slopes more exposed to the sun receive more solar radiation
than those away from the sun’s direct rays.
➢ Slopes that receive direct sun rays are dry due to loss of
moisture through excess evaporation.
➢ These slopes remain barren if irrigational facilities are
absent. But slopes with good irrigation facilities are good
for agriculture due to abundant sunlight available.
➢ They are occupied by dense human settlements.
❖ Ocean Currents:
➢ Ocean currents involve accrual movement of ocean water.
 7

➢ Depth of 400 metres there are surface currents, and beyond

this there are depth currents.
➢ Ocean currents influence the temperature of adjacent land
areas considerably. Warm currents cause high
temperatures, and transport heat from the equator to the
poles. The cold currents cause low temperature, and
transport cold water from the poles to the equator.
➢ For example in the USA there is California currents, which
is cold current and there is also Gulf stream which is warm
current.
❖ Altitude: In the troposphere with the increase in height, the pressure
falls, the effect of greenhouse gasses decreases, and hence temperature
decreases (applicable only to the troposphere).
➢ The normal lapse rate is roughly 1⁰ C for every 165 meters of
ascent.
❖ Earth’s Distance from the Sun: During its revolution around the Sun,
the Earth is farthest from the sun (152 million km on 4th July). This
position of the earth is called aphelion.
➢ On 3rd January, the earth is the nearest to the sun (147 million
km). This position is called perihelion. Therefore, the annual
insolation received by the earth on the 3rd of January is slightly
more than the amount received on the 4th of July.
➢ However, the effect of this variation in the solar output is
masked by other factors like the distribution of land and sea and
the atmospheric circulation.
➢ Hence, this variation in the solar output does not have a great
effect on daily weather changes on the surface of the earth.
❖ Latitudinal Heat Balance: The amount of insolation received
varies from one latitude to another.
➢ Regions within the equator and 40° North and 40° South
latitudes receive abundant sunlight and thus, more heat is
gained than lost. Hence, they are energy surplus regions.
➢ Regions beyond 40° North and South latitudes lose more
heat than that gained from sunlight. Hence, they are energy
deficit regions (This is because of slant sunlight and high
albedo of polar regions).
➢ Going by this logic, the tropics should have been getting
progressively hotter and the poles getting progressively colder and the
 8

planet would have been inhospitable except for a few regions near mid-latitudes. But, in reality, this does
not happen.
➢ The atmosphere (planetary winds) and the oceans (ocean currents) transfer excess heat from the tropics
(energy surplus region) towards the poles (energy deficit regions) making up for heat loss at higher
latitudes.
➢ Most of the heat transfer takes place across the mid-latitudes (30° to
50°) and hence, much of the stormy weather is associated with this
region.
➢ Thus, the transfer of surplus energy from the lower latitudes to the
deficit energy zone of the higher latitudes maintains an overall
balance over the earth’s surface.
Heat Budget:
❖ We have incoming and outgoing solar radiation (radiation goes from the atmosphere as well as Earth).
❖ Heat budget is the account of incoming and radiations to and from Earth.

NOTE:
❖ For example Sun is giving 100 units of insolation, so out of the 100 units 6 units gets scattered in space
itself, and then balance will come down and 27 units reflected from the clouds, and then some portion of
insolation fall upon the mountains and 2 units reflected from the snow covered peaks also, and then
come to further down and some 14 units absorbed by the atmosphere and finally 51 units reaches the
Earth surface(absorbed by the Earth). This is called Incoming Radiations.
❖ So here 23 units will be coming out in the form of Radiations, apart from this somewhere 9 units will
come up in the form of convection and turbulence, somewhere 19 units will be release in the form of
climatic phenomena, so that 51 units come out of the Earth and 49 units are lying at the Atmosphere.
This is called Outgoing Radiations.

❖ The earth receives a certain amount of

Insolation (short-wave radiation) and gives
 9

back heat into space by terrestrial radiation (long-wave radiation).

❖ Through this give and take the earth maintains a constant temperature.
❖ The earth receives a certain amount of Insolation (short waves) and gives back heat into space by terrestrial
radiation (longwave radiation).
Through this give and take, or the heat budget, the earth maintains a constant temperature.


 1

DAILY
CLASS NOTES
Geography

Lecture - 31
Temperature and Pressure
Distribution
 2

Temperature and Pressure Distribution

The Mean Annual Temperature Distribution:
 Isotherm: It is an imaginary line/curve joining
places/regions having equal/same temperatures.
 It is represented by temperature. [For example 20
degree Isotherm(Suppose there is temperature like T1 is
20 degree and T2 is 21 degree, and X represents
distance between two Isotherms, T2-T1/X=Temperature
Gradient)].
 The rate of change of temperature is across land and
water bodies with distance.
 The horizontal or latitudinal distribution of temperature
is shown with the help of a map with isotherms.
 Effects of altitude are not considered while drawing an isotherm. All the temperatures are reduced to sea
levels.
General Characteristics of Isotherms:
 Generally follow the parallels of latitude: Isotherms
have a close correspondence with the latitude
parallels mainly because the same amount of
insolation is received by all the points located on the
same latitude.
 Sudden bends at ocean–continent boundaries: Due to
differential heating/Cooling of land and water,
temperatures above the oceans and land masses vary
even on the same latitude. (we have seen how land-sea
differential affects temperature distribution). The
sudden bends occur due to the abrupt change in the temperature.
 Narrow spacing between isotherms indicates a rapid change in temperature (high thermal gradient).[There
is T2-T1/x=21-20/x=1/x(lesser value of temperature
gradient)].
 Wide spacing between isotherms indicates a small or
slow change in temperatures( Temperature gradient
=delta T/deltaX=T2-T1/X).[Note: More spacing
means low thermal gradient. So, T2-T1/y=21-
20/3x=1/3x(more spacing, but temperature gradient
will be less).
 3

 The highest temperatures occur over the tropics and sub-tropics (high insolation). The lowest
temperatures occur in polar and subpolar regions in continents due to the effect of continentality.
 Diurnal and annual range of temperatures are highest in the interiors of continents due to the effect of
continentality (in continental interiors there will be no moderating effect of oceans).
 Diurnal and annual range of temperatures are least in oceans. (High specific heat of water and mixing of
water keep the temperature range low).
 Low-temperature gradients are observed over the tropics (because the sun is almost overhead the entire
year) and high-temperature gradients over the middle and higher latitudes (the sun’s apparent path varies
significantly from season to season).
 Temperature gradients are more closely spaced over the eastern margins of continents. (This is because of
warm ocean currents)
 Temperature gradients are more on the western margins of continents. (This is because of cold ocean
currents).
 The isotherms are irregular over the northern hemisphere due to an enhanced land-sea contrast.
Because of the predominance of land over water in the north, the northern hemisphere is warmer.
 The thermal equator (ITCZ) lies generally to the north of the geographical equator.
 While passing through an area with warm ocean currents, the isotherms show a poleward shift.
(Example: North Atlantic Drift and Gulf Stream combined with westerlies in Northern Atlantic;
Kuroshio Current and North Pacific current combined with westerlies in Northern Pacific).
 Mountains also affect the horizontal distribution of temperature. For instance, the Rockies and the
Andes stop the oceanic influence from going inwards into North and South America.
Seasonal Temperature Distribution:
 The global distribution of temperature can well be understood
by studying the temperature distribution in January and
July.
 The temperature distribution is generally shown with the help
of isotherms. The Isotherms are joining lines with places
having equal temperature.
 In general, the effect of the latitude on temperature is well
pronounced as the isotherms are generally parallel to the
latitude. The deviation from this general trend is more
pronounced in January than in July, especially in the Northern
Hemisphere.
 In the Northern Hemisphere, the land surface area is much larger than in the southern hemisphere.
Hence, the effects of landmass and ocean currents are well-pronounced.
Seasonal Temperature Distribution – January
 During January, it is winter in the northern hemisphere and summer in the southern hemisphere.
 4

 The western margins of continents are warmer than their eastern counterparts, since the Westerlies are
able to carry high temperatures into the landmasses.
 The temperature gradient is close to the eastern margins of continents. The isotherms exhibit a more
regular behavior in the southern hemisphere.
In the Northern Hemisphere:
 The isotherms deviate to the north over the ocean and to the south over the continent. This can be seen
in the North Atlantic Ocean.
 The presence of warm ocean currents, Gulf Stream and North Atlantic drift, make the Northern
Atlantic Ocean warmer and the isotherms show a poleward shift indicating that the oceans are warmer
and are able to carry high temperatures poleward.
 An equator-ward bend of the isotherms over the northern continents shows that the landmasses are
over-cooled and that polar cold winds are able to penetrate southwards, even in the interiors. It is much more
pronounced in the Siberian plain.
 Lowest temperatures are recorded over northern Siberia and Greenland.
In the Southern Hemisphere:
 The effect of the ocean is well pronounced in the southern hemisphere. Here the isotherms are more or
less parallel to the latitudes and the variation in temperature is more gradual than in the northern hemisphere.
 The high-temperature belt runs in the southern hemisphere, somewhere along 30°S latitude.
 The thermal equator lies to the south of the geographical equator (because the Intertropical Convergence
Zone or ITCZ has shifted southwards with the apparent southward movement of the sun).
Seasonal Temperature Distribution – July:
 During July, it is summer in the northern hemisphere and winter in the southern hemisphere. The
isothermal behavior is the opposite of what it is in January.
 In July, the isotherms generally run parallel to the latitudes. The equatorial oceans record warmer
temperatures, more than 27°C. Over the land, more than 30°C is noticed in the subtropical continental region
of Asia, along the 30° N latitude.
In the Northern Hemisphere:
 The highest range of temperature is more than 60° C
over the north-eastern part of the Eurasian continent. This
is due to continentality. The least range of temperature,
3°C, is found between 20° S and 15° N.
 Over the northern continents, a poleward bend of the
isotherms indicates that the landmasses are overheated and
the hot tropical winds are able to go far into the northern
interiors.
 The isotherms over the northern oceans show an
equatorward shift indicating that the oceans are cooler
and are able to carry the moderating effect into tropical
 5

interiors. The lowest temperatures are experienced over Greenland.

 The highest temperature belt runs through northern Africa, west Asia, north-west India, arid
southeastern USA. The temperature gradient is irregular and follows a zig-zag path over the northern
hemisphere.
In the Southern Hemisphere:
 The gradient becomes regular over the southern hemisphere but shows a slight bend towards the equator at
the edges of continents.
 Thermal equator now lies to the north of the geographical equator.
General Temperature Distribution:
High temperatures:
 High temperatures occurred in a tropical and
sub region. It is due to the high insolation and
the resultant increase and temperature of the
region.
 Lowest temperatures occurred in the polar
region. It is due to the lowest insolation.
 Annual Range of temperature. It is the
difference between the maximum and
minimum temperature in a given year. The
maximum range of temperature occurs in the
continental area. When the continents are
heated they get overheated and when the
continents are cool they are overcooked. The range of temperature is minimum in the oceanic region.

NOTE:
 In summer continents are heated fast, then there is maximum temperature, and in winter it cools fast so
there is low temperature.
 In summer oceans heat slowly, and similarly in winter it cools slowly(the range between high temperature
and minimum temperature is low compared to continents).

 Diurnal range of temperature means the difference between the temperature of day and night at any
place. Continents are high and Oceans are low range.
Temperature gradient:
 Temperature Gradient is the rate of change of temperature with distance.
 Temperature gradient is low in the Tropic region. As the sun's rays are overhead in the region. Almost the
same temperature(High Temperature, but noticeable change will be very less) has occurred and the change
in temperature is very less.
 The Temperature gradient is high in the Middle latitude and Higher latitude(Noticeable change in
temperature).As the sun rays fall obliquely in the region,due to this the temperature difference is visible.
 6

Nature of Isotherm in Northern and Southern

Hemisphere:
 Northern Hemisphere:
 It is dominated by the land.
 Isotherms are irregular due to the presence of more
land and the non-uniform presence of ocean and
land.
 Southern Hemisphere:
 It is dominated by the ocean.
 Isotherms are regular due to the presence of the
oceans and the uniform presence of the ocean and
low amount of land.
Shifting of Isotherms:
 During Summer:
 During the summer the Northern Hemisphere is
heated.
 From ocean to continent isotherm shifts towards
poleward because continents are overheated.
 From the Continent to Ocean Isotherms shift
toward the equatorward. It is due to the low
temperature of the adjacent sea, and the ocean
heating up slowly as compared to the land.
 During Winter:
 During the winter the Northern Hemisphere is
cooled and the Southern Hemisphere is heated.
 From ocean to continent isotherm shifts towards
equatorward because continents are overheated.
 From the Continent to Ocean Isotherms shift
toward the Poleward. It is due to the ocean
heating up slowly as compared to the land.
Effects Of Ocean Currents:
 Ocean currents influence the temperature of adjacent
land areas considerably. Warm currents transport heat
from the equator to the poles. The cold currents
transport cold water from the poles to the equator.
 For example Gulf stream and North Atlantic drift are
the eastern coast of North America. These ocean
 7

currents make this entire region warmer. Because of

this Norwegian port is Ice free(Because of the North
Atlantic Drift, and Isotherm also follows the same
pattern). North Atlantic Drift causes relatively high
temperatures in polar areas as compared to other places
in the same latitude so it is causing Isotherm shifting.
Pressure Distribution:
 Air: It is an admixture of various glasses, solids,
aerosols and water vapors.
 Air Pressure or Atmospheric pressure: It is the weight of air per unit area covered.
 Atmospheric pressure is exerted by the atmosphere on the surface and all living bodies.
 Units: N/meter square, Square CentiMeter, Square Meter or Square Inches(Psi).
 P(atm): 1.0132 Bar=1013.2 Millibars=14.7 Pounds/Sq.Inch(Patm=760 mm of hg)
 Air Pressure is Measured by Aneroid Barometer, Barograph, and Fortin Barometer.
 Reduction rate of pressure is 34Mb at every 300 Meter.


 1

DAILY
CLASS NOTES
Geography

Lecture - 32
Pressure Distribution around
the World
 2

Pressure Distribution around the World

Pressure Distribution:
 There is a complex relationship between density, temperature and pressure. As height increases the
temperature, density and pressure also decrease.
 Wind flows from high pressure to low temperature.
 Pressure difference between two points generates wind.
 Pressure Gradient: It is the rate of the change of the pressure with the distance between two points.
 Isobars: The imagery lines or curves joining the points/places/regions of equal/same pressure.
Pressure Systems:
 Air expands when heated and gets compressed when cooled. This results in variations in the atmospheric
pressure.
 The differences in atmospheric pressure cause the movement of air from high pressure to low
pressure, setting the air in motion.
 Atmospheric pressure also determines when the air will
rise or sink.
 Air in horizontal motion is wind.
 The wind redistributes the heat and moisture across
latitudes, thereby, maintaining a constant temperature for
the planet as a whole.
 The vertical rising of moist air forms clouds and brings
precipitation.
 Atmospheric pressure also determines when the air will
rise or sink.

NOTE:
 In the low pressure: Increasing pressure outward and follow of air from outside to inside.
 In the high pressure: Decreasing pressure outward and flow of air from inside to outside.
 Pressure systems can be Thermally induced (Warm rises, cold falls), and Dynamically induced (Cold air
mass makes warm air mass rise).

High-Pressure System:
 A high-pressure system has higher pressure at its
center than the areas around it.
 Winds blow away from high pressure. Swirling in
the opposite direction from a low-pressure system,
the winds of a high-pressure system rotate clockwise
 3

north of the equator and counterclockwise south of the equator.

 This is called anticyclonic flow. Air from higher in the atmosphere sinks down to fill the space left as air
is blown outward.
 In the given diagram H, denotes the location of a high-pressure system. They are oftentimes associated
with clear blue skies.
 It is anticyclonic in nature.
 Divergence of air mass creates the stable conditions in the surrounding atmosphere.
Low-Pressure System:
 A low-pressure system has lower pressure at its center than the areas around it.
 Winds blow towards the low pressure, and the air rises in the atmosphere where they meet.
 As the air rises, the water vapour within it condenses, forming clouds and often precipitation. Because
of Earth’s spin and the Coriolis effect, winds of a low-pressure system swirl counterclockwise north of the
equator and clockwise south of the equator. This is called cyclonic flow. L denotes the low-pressure system
area.
 Low-pressure systems "suck" air into them because
nature wants everything to have equal pressure. By
doing this, they generally create winds and
undesirable weather.
 It is cyclonic in nature.
 Convergence of air mass creates unstable
conditions of the atmosphere.
Convergence and Divergence:
 Generally, over a low pressure area the air will
converge and rise. Over high pressure areas, the air
will subside from above and diverge at the surface.
 Convergence and divergence, in meteorology, the accumulation or drawing apart of air, as well as the rate
at which each takes place. The terms are usually used to refer specifically to the horizontal inflow
(convergence) or outflow (divergence) of air.
 Convergence: The convergence of horizontal winds causes air to rise. It causes cyclonic conditions which
create an unstable atmosphere(cloud, rain, thunderstorm, cyclone).
 Divergence of horizontal winds causes downward movement of the air (subsidence). It causes
anticyclonic conditions which create a stable atmosphere.
Air Pressure:
 Air is made up of a number of mixed gasses.
 Since air has mass, it also has weight.
 It exerts pressure on the earth’s surface which varies from
place to place and from time to time.
 4

 The pressure of air at a given place is defined as a force exerted in all directions by virtue of the weight of
all the air above it.
 The weight of a column of air contained in a unit area from the mean sea level to the top of the
atmosphere is called atmospheric pressure.
 Pressure is a force and the atmospheric pressure is expressed in various units. A new unit known as the
millibar ( mb) was adopted to express atmospheric pressure by meteorological stations in 1914.
 A normal atmospheric pressure equivalent to 14-7 lb. per square inch in weight or a reading of 29-9
inches of mercury in the column is 1013.2 millibars.
Vertical Variation of Pressure:
 In the lower atmosphere (troposphere), pressure decreases rapidly with height.
 The length of the troposphere is somewhere around 10 Km from the surface of the earth.
 It is cold at the top of the troposphere, that’s why nearly all of the water vapour, clouds, and dust
particles in the atmosphere are found in the troposphere.
 With an increase in temperature, the pressure decreases and vice-versa. Thus, pressure and temperature
are inversely proportional.
 In the lower atmosphere, the pressure decreases rapidly with height.
 At the height of Mt. Everest, the air pressure is about two-thirds less than what it is at sea level.
 The decrease in pressure with altitude, however, is not constant because the air column keeps on
reducing with height.

NOTE:
 When air is compressed, then all the air molecules are in a closed pact, this closeness causes more
rubbing and creates lots of friction. More friction leads to high temperatures.
 When air molecules have more space between them, they don't rub as much. Less rubbing leads to less
friction and, in turn, low heat generation i.e., lower temperatures.
 Winds move from high pressure to low pressure, currents move from high potential to low potential, and
also heat flows from high temperature to low temperature, but air doesn't escape upwards because gravity
pulls it down, and there's also a force pushing it up due to differences in pressure (pressure gradient
force). Here, F pr = F gravity (‘F’ pressure is balanced by ‘F’ gravity).
 Pressure Gradient Force pulling air/fluid from high pressure to low pressure.
 Pressure reduction rate is 34 millibar every 300 meter rise in elevation.
 Causes of temperature reduction with height are more greenhouse gases that trap heat at lower
altitudes, so it's hotter down there, outgoing radiation cools things down, and pressure decreases with
increasing height (Low temperature because less friction).

NOTE:
 Insolation heats up the surface then air gets heated up and heat makes air molecules gain energy and
 5

move apart, then the expansion of air happens for constant mass (density=mass/volume, and mass is
constant and volume is increasing then density is decreased), then volume is increased and lighter air will
rise.
 When the low pressure exists, there will be a convergence region, and rising air parcels creates cyclonic
conditions.
 Suppose the temperature is -50 degree celsius at 10 km height, air gets cooled (cold air), it is heavy air and
the tendency to fall down (intermolecular energy decreases), then falls downward. And this place is a zone
of high pressure and then the entire region is divergent and this makes anticyclonic conditions. This type of
situation is called a calm situation (no rainfall).

 Since the factors controlling air density–temperature, amount of water vapour, and gravity are variable,
there is no simple relationship between altitude
and pressure.
 In general, the atmospheric pressure decreases on
average at the rate of about 34 millibars every 300
meters of height.
 The vertical pressure gradient force is much
larger than that of the horizontal pressure
gradient.
 But, it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not
experience strong upward winds.
 Due to gravity, the air at the surface is denser and hence has higher pressure. Since air pressure is
proportional to density as well as temperature, it follows that a change in either temperature or density will
cause a corresponding change in pressure.
 The pressure decreases with height.
 At any elevation, it varies from place to place
and its variation is the primary cause of air
motion, i.e. wind which moves from high-
pressure areas to low-pressure areas.
 A rising pressure indicates fine, settled
weather, while a falling pressure indicates
unstable and cloudy weather.
Horizontal Distribution of Pressure:
 Pressure gradient is the difference between the high-pressure and low-pressure zone.
 The force which pushes the wind from one place to another place is called pressure gradient force.
 Small differences in pressure are highly significant in terms of the wind direction and velocity.
 Horizontal distribution of pressure is studied by drawing isobars at constant levels.
 Isobars are lines connecting places having equal pressure.
 6

 In order to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to
sea level for purposes of comparison.The spacing of isobars expresses the rate and direction of pressure
changes and is referred to as pressure gradient.
 Close spacing of isobars indicates a steep or strong pressure gradient, while wide spacing suggests a
weak gradient.
 The pressure gradient may, thus, be defined as the decrease in pressure per unit distance in the direction in
which the pressure decreases most rapidly.
 There are distinctly identifiable zones of homogeneous horizontal pressure regimes or ‘pressure belts’.
 During summers, there is low pressure in the Tibetan region and High pressure in the Indian Oceans which
leads to the formation of South-west
monsoon winds.
 At the low-pressure zone, winds rise and
high-pressure winds descend.
 In July month, the insolation is maximum
at the Tropic of Cancer, and due to heat,
there is a low-pressure zone in this area.
 At low-pressure zones there is the vertical
upward movement of winds (Ascending)
and at high pressures, these ascending
winds move downward (descending).
 In the southern hemisphere, the isobars
are in a straight line, due to the absence of
landmass (continents) and less obstruction.
Atmospheric Circulation:
 It is the set of wind systems that flows around the earth.
 It is responsible for heat transport from the equator to the poles and various weather phenomena like
rain, cloud, storms etc.
 It also transports moisture from oceans to the land which results in the origin of monsoons, cyclones etc.
Pressure Belts:
 The seven pressure belts are:
1. Equatorial Low,
2. The two Sub-tropical Highs,
3. The two Sub-polar Lows,
4. The two Polar highs.
 Except for the equatorial low, all others form
matching pairs in the northern and southern
hemispheres.
 7

Factors Affecting Atmospheric Pressure:

 Thermal Factor (differential heating of the latitude).
 Dynamic Factor (like Coriolis Force and Transfer of Angular Momentum from the equator to the pole).
 The force exerted by the rotation of the earth is known as the Coriolis force.
 Thus, the horizontal winds near the earth's surface respond to the combined effect of three forces – the
pressure gradient force, the frictional force, and the Coriolis force.
 In addition, the gravitational force acts downward.

NOTE:
 The Hadley cell is a large-scale atmospheric circulation in which air rises near the equator, flows poleward
at a high altitude, descends in the subtropics, and then flows equatorward near the surface
 In the Ferrel cell, air flows poleward and eastward near the surface and equatorward and westward at higher
altitudes; this movement is the reverse of the airflow in the Hadley cells. Ferrel’s model was the first to
account for the westerly winds between latitudes 35° and 60° in both hemispheres.
 The Polar cells, which extend from between 60 and 70 degrees north and south, to the poles. Air in these
cells sinks over the highest latitudes and flows out towards the lower latitudes at the surface.


 1

DAILY
CLASS NOTES
GEOGRAPHY

Lecture – 33
Atmospheric Circulation
 2

Atmospheric Circulation

Pressure Belts of World:

Equatorial Low-Pressure Belt or ‘Doldrums’:

❖ It lies between 10° North and 10° South
latitudes.
❖ Its width may vary between 5° North and 5°
South and 20° North and 20° South.
❖ This belt happens to be a zone of
convergence of trade winds from two
hemispheres from sub-tropical high-pressure
belts.
❖ This belt is also called the Doldrums,
because of the extremely calm air
movements, that means only vertical
winds, there are no horizontal winds.
❖ The position of the belt varies with the
apparent movement of the Sun.
❖ Equatorial low pressure belt is also known as
Variable belt, and It migrates 20 degree North to 20 degree south.
 3

Formation:
❖ As this region lies along the equator, it
receives the highest amount of
insolation.
❖ Due to intense heating, the air gets
warmed up and rises over the
equatorial region (convection), This is
called Thermally induced.
❖ Whenever there is vertically upward
movement of air, the region at the
surface will be at low pressure.
❖ Thus, the belt along the equator is
called the equatorial low pressure belt.
Climate:
❖ This belt is also known
as Inter-Tropical
Convergence Zone
(ITCZ) because the
North-East and South-
East trade winds
flowing from Sub-
tropical High-Pressure
Belts converge here in
the equatorial belt of
low atmospheric
pressure.
 4

NOTE:
❖ At the height of 10 to 12 kms the temperature will be -50 to -55 degree celsius, and Vapour will freeze
there(condensation happened).
❖ Condensation is a process of vapour convert in liquid.
❖ The position of the ITCZ varies seasonally because it follows the Sun, it moves north in the Northern
Hemisphere summer and south in the Northern Hemisphere winter.
❖ It forms calm and light surface winds and a strong upward air movement.
❖ Thus, in spite of the calm and light winds, the Doldrums are characterized by stormy and turbulent
weather, having
thunderstorms·,
heavy rains, and
squalls.
❖ There is the formation
of a cumulonimbus
cloud.
❖ This is a high
instability zone due to
the formation of
cyclones, etc.
❖ Since these regions are
devoid of wind, they are avoided by sailing ships.
❖ The example of equatorial regions are Mexico, Brazil, Venezuela, Indonesia, SouthEast Asian Nation,
❖ In this region rainfall occurs all day.
❖ In this region Lush green Vegetation is there like Ebony, Mahogany, Epiphytes(Trees with canopies, and
sunlight does not reach ground).
❖ In this region high biodiversity is there like Amazon forest, Republic of congo forest, Indonesia forest.
Sub-Tropical High-Pressure Belt or Horse Latitudes:
❖ The subtropical highs extend from near the tropics to about 30 to 35° North and South.
Formation:
❖ After saturation (complete loss of moisture) at the ITCZ, the air moving away from the equatorial low
pressure belt in the upper troposphere becomes dry and cold.
❖ This dry and cold wind subsides at 30° North and South.
❖ The existence of these pressure belts is due to the fact that the rising air of the equatorial region is deflected
towards the poles due to the Earth’s rotation.
❖ In this region divergence happens due to falling winds.
❖ So, the high pressure along this belt is due to the subsidence of air coming from the equatorial region which
descends after becoming heavy.
 5

❖ They lose their energy because of so much travel, so they fall down(Dynamically induced).
❖ The high pressure is also due to the blocking effect of air at upper levels because of the Coriolis force.

Climate:
❖ The subsiding air is warm and dry, therefore, most
of the deserts are present along this belt, in both
hemispheres.
❖ A calm condition (anticyclonic) with feeble winds
is created in this high pressure belt.
❖ The descending air currents feed the winds blowing
towards adjoining low pressure belts.
❖ Descending winds are getting pressurized due to
increasing pressure, and high heat due to
molecular friction.
❖ This region is weather less, no rain, no clouds, dry
air, only dry vegetation (xerophytic vegetation) is
there because of descending wind, and they cause desert formation.
❖ Examples of vegetation are Cactus, Babool, Acacia etc.
❖ The major deserts are Mohave (North America), Patagonia (South America), Sahara (Africa), Thar (India),
Victoria, and Stony (Australia).
❖ This belt is frequently invaded by tropical and extra-tropical disturbances.
❖ The pressure at the surface is 1.0132 Bar, and at the height of 8 to 10 km the pressure is 0.282 Bar.
 6

Horse Latitudes:
❖ The corresponding latitudes of the sub-
tropical high-pressure belt are called horse
latitudes.
❖ In the early days, the sailing vessels with a
cargo of horses found it difficult to sail
under the calm conditions of this high-
pressure belt.
❖ No horizontal winds there so it means no
ship movements are there.
❖ They used to throw horses into the sea when
fodder ran out. Hence the name horse
latitudes.
 7

Sub-Polar Low Pressure Belt:

❖ It extends along 60° latitudes (55 -65
degrees) in both the hemispheres.
❖ These belts are not thermally
induced; instead the winds coming
from the sub-tropics and polar
regions converge in this belt and
rise upward.
❖ These belts are Dynamically
induced (Cold Air mass lifts warm
Air mass along front).
❖ Owing to low temperatures in these
latitudes, the sub polar low pressure
belts are not very well pronounced
year long.
❖ On long-term mean climatic maps,
the sub-polar low-pressure belts of
the northern hemisphere are
grouped into two centers of
atmospheric activity: the Iceland low and the
Aleutian depression (Aleutian low).
❖ Such belts in the southern hemisphere surround
the periphery of Antarctica and are not as well
differentiated.
❖ In the Southern Hemisphere, this low-
pressure belt is more pronounced due to the
vast presence of the ocean and is also referred
to as the Sub-Antarctic Low.
Formation:
❖ These are dynamically produced due to
Coriolis Force produced by the rotation of
the earth on its axis.
❖ Ascent of air as a result of the convergence of westerlies and polar easterlies.
❖ Sub polar low-pressure belts are mainly encountered above oceans.
Seasonal Behavior:
❖ During winter, because of a high contrast between land and sea, this belt is broken into two distinct low
centers – one in the vicinity of the Aleutian Islands and the other between Iceland and Greenland.
❖ During summer, a lesser contrast results in a more developed and regular belt.
 8

Climate:
❖ The area of contrast between
cold and warm air masses
produces polar jet streams
which encircle the earth at 60
degrees latitudes and are
focused in these low pressure
areas.
❖ Due to a great contrast between
the temperatures of the winds
from subtropical and polar source regions,
extra tropical cyclonic storms or lows’
(temperate cyclones or frontal cyclones)
are produced in this region.
❖ Snowfall, and light rainfall also occurs in
this region because of dynamic lifting of
warm air mass.
❖ Dynamic lifting means warm air mass is
lifted by cold air mass.
❖ The vegetation are big long trees Oak,
Fir, Ash, Mapple.
Polar High-Pressure Belt:
❖ The polar highs are small in area and
extend around the poles.
❖ They lie around poles between 85 – 90° North and South latitudes (Arctic and Antarctic regions).
❖ Because of low temperature, air compresses, and its density increases. Hence, Descending winds cause high
pressure here throughout the year.
❖ Polar High pressure belts formation is because of both Thermal and dynamic effects.
Formation:
❖ The air from sub-polar low-pressure belts, after saturation, becomes dry. This dry air becomes cold while
moving toward the poles through the upper troposphere.
❖ The cold air (heavy) on reaching the poles subsides creating a high-pressure belt at the surface of the earth.
Climate:
❖ The lowest temperatures are found over the poles.
❖ In this region vegetations are mostly Lichens, Mosses, etc.
 9

General Circulation of the Atmosphere:

Hadley Cell:
❖ The air at the Inter
Tropical
Convergence Zone
(ITCZ) rises because
of the convection
currents caused by
low pressure. Low
pressure, in turn,
occurs due to high
insolation.
❖ In this region winds
are descending.
❖ The winds from the
tropics converge at
this low pressure zone.
 10

❖ The converged air rises along with the convective cell. It

reaches the top of the troposphere up to an altitude of 14
km and moves towards the poles.
❖ This causes accumulation of air at about 30°North and
South.
❖ The circulation cell closest to the equator is called the
Hadley cell.
❖ Winds are light at the equator because of the weak
horizontal pressure gradients located there.
❖ The warm air rises at the equator producing clouds and causing instability in the atmosphere.
❖ This instability causes thunderstorms to develop and release large amounts of latent heat.
❖ Latent heat is just energy released by the storms due to changes from water vapor to liquid water droplets as
the vapor condenses in the clouds, causing the surrounding air to become more warm and moist, which
essentially provides the energy to drive the Hadley cell.
Ferrel Cell:
❖ In the middle latitudes(30 to 35 degree), the circulation is that of sinking cold air that comes from the poles
and the rising warm air that blows from the subtropical high.
❖ At the surface, these winds are called westerlies and the cell is known as the Ferrel cell.
❖ The Ferrel cell has air motion opposite to planetary rotation. At the surface, this forms the southwesterly
prevailing westerlies.
❖ The Ferrel cells and Hadley cells meet at the horse latitudes.

Polar Cell:
❖ At polar latitudes, the cold dense air subsides near the poles and blows towards middle latitudes as the polar
easterlies. This cell is called the polar cell.
 11

❖ The smallest and weakest cells

are the Polar cells, which extend
from between 60 and 70 degrees
North and South to the poles.
❖ Air in these cells sinks over the
highest latitudes and flows out
towards the lower latitudes at the
surface.
❖ These three cells set the pattern
for the general circulation of
the atmosphere. The transfer of
heat energy from lower latitude to
higher latitudes maintains the general circulation.

   
 1

DAILY
CLASS NOTES
Geography

Lecture - 34
Winds of the World
 2

Winds of the World

Winds:
❖ When air moves in a definite direction, it is called wind. Winds are a result of pressure differences (Horizontal
motion of air due to pressure differences).
❖ Winds help in the heat transfer from the heat surplus region to heat deficient region.
❖ Winds also help in the transfer of moisture, and are responsible for various types of climatic phenomena
(Rainfall, Clouds, Cyclones, Monsoon, El-Nino, Jetstreams).
❖ Surface winds flow at the height of 1 to 3 kms.
❖ If the winds move from west to east, they are called Westerlies.
❖ If they move from east to west, they are called Easterlies.
❖ The direction of the wind is also affected by the Coriolis effect.
❖ The coriolis force deflects the wind to its right direction in the Northern Hemisphere and to the left
direction in the Southern Hemisphere.
Classification of Winds:
❖ Permanent Winds:
➢ They are also called
Primary winds or
Prevailing winds or
Planetary Winds.
➢ They are present or
flowing all around the
year.
➢ Examples: Trade winds,
Westerlies and Polar
Easterlies.
❖ Secondary or Periodic Winds/Variable winds:
➢ Variable winds are not permanent winds.
➢ Seasonal Winds: These winds change their
direction in different seasons. For example
monsoons in India.
➢ Periodic Winds: Land and sea breeze (Flow
due to pressure differential between the land
and sea), mountain and valley breeze etc.
❖ Local Winds:
➢ These blow only during a particular period of
the day or year in a small area.
 3

➢ Winds like Loo, Mistral, Foehn, Bora, Chinook, sirocco, Fohn, etc. are examples of local winds.
Factors/Forces affecting Winds:
❖ Pressure Gradient force:
➢ The difference in atmospheric pressure produces a force.
➢ The rate of change of pressure with respect to distance
is the pressure gradient.
➢ It helps the winds to move from high-pressure to low-
pressure zones.
➢ The pressure gradient is strong where the isobars are
close to each other and weak where the isobars are
apart (Suppose there is two isobars are there, one is
having 1060 mb (Pressure higher) and another one is
having a pressure of 1040 mb (Pressure lower), so wind move from high pressure to low pressure).
➢ Pressure Gradient forces are responsible for flow of winds between two isobars.
➢ It is generated due to the existence of pressure differences between two regions (high and low pressure).
➢ Winds blowing due to pressure differences.
❖ Frictional Force:
➢ It affects the speed of the wind. It is greatest at the surface and its
influence generally extends upto an elevation of 1-3 km. It acts in
the opposite direction of the wind.
➢ Friction acts at the contact point of winds with the surface.
➢ Over the sea surface, the friction is minimal and is maximum at
land.
❖ Gravitational Force:
➢ The vertical pressure gradient force is much larger than that of the horizontal pressure gradient.
➢ It pulls down the winds from elevation to the grand surface.
➢ But, it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not
experience strong upward winds.
➢ Due to gravity, the air at the surface is denser and hence has higher pressure. Since air pressure is
proportional to density as well as temperature, it follows that a change in either temperature or density will
cause a corresponding change in pressure.
❖ Coriolis Force:
➢ The rotation of the earth about its axis affects the direction of the wind (Effects of deflection of winds
due to rotational motion of Earth). This force is called the Coriolis force after the French physicist who
described it in 1844.
 4

➢ It deflects the wind to the right

direction in the northern
hemisphere and to the left in the
southern hemisphere (This is the
Ferrels law of deflection).
➢ The deflection is more when the
wind velocity is high.
➢ The Coriolis force is directly
proportional to the angle of
latitude.
➢ It is maximum at the poles and
is absent at the equator.
➢ The Coriolis force acts
perpendicular to the pressure
gradient force.
➢ The pressure gradient force is
perpendicular to an isobar.
➢ The higher the pressure
gradient force, the more is the
velocity of the wind and the
larger is the deflection in the
direction of the wind.
➢ As a result of these two forces operating perpendicular to
each other, in the low-pressure areas the wind blows
around it.
➢ At the equator, the Coriolis force is zero and the wind
blows perpendicular to the isobars.
➢ The low pressure gets filled instead of getting intensified.
➢ That is the reason why tropical cyclones are not formed
near the equator.
➢ It affects only the direction, not the speed of the winds.
➢ It only acts on the objects in motion. Ex. flowing wind
etc.
Primary or Prevailing Winds:
❖ These are the planetary winds which blow extensively over continents and oceans.
❖ The two most well-understood and significant winds for climate and human activities are trade
winds/Easterlies and westerly winds.
❖ Examples of Primary Winds are:
 5

➢ The Trade Winds

➢ The Westerlies
➢ The Polar easterlies
Trade Winds:
❖ The trade winds are those blowing from the sub-tropical high-pressure areas towards the equatorial low-
pressure belt.
❖ Therefore, these are confined to a region between 30
degrees North and 30 degrees South throughout the
earth’s surface. They are also called Tropical easterlies.
❖ They flow as the north-eastern trades in the northern
hemisphere and the south-eastern trades in the southern
hemisphere.
❖ They flow from 5 to 35 degrees (from the Subtropical
high pressure belt to Equatorial low pressure belt), they
are surface winds and generated due to the falling winds
(Descending winds at sub tropical high pressure belt).
❖ They are most persistent among all winds.
❖ This deflection in their ideally expected north-south direction is explained on the basis of Coriolis force and
Ferrel's law.
❖ Since they travel from high latitude to low latitude areas, they gradually become hot and dry and hence
have a great capacity to hold
moisture.
❖ The trade winds from two hemispheres
converge near the equator and due to
the convergence, they rise and cause
heavy rainfall.
❖ The eastern parts of the trade winds
associated with the cool ocean
currents are drier and more stable
than the western parts of the ocean.
❖ All tropical cyclones form on the eastern side of the continent due to the impact of trade winds. Example:
Bay of Bengal, Philippines etc.
❖ These winds try to move the surface (warm) water and bring it to the eastern margin of the continent.

NOTE:
❖ Easterlies bring warm water to the eastern margin of the continent.
❖ We can also say Easterlies take away warm water from the west margin of the continent.
 6

Westerlies:
❖ The westerlies are the winds blowing from the
subtropical high-pressure belts towards the sub-
polar low-pressure belts (the extent of westerlies
are 30-35 degree to 60-65 degree).
❖ Their directions are opposite to trade winds and that
is why they are also called Anti-trade winds.
❖ They blow from south-west to north-east in the
northern hemisphere and north-west to south-east
in the southern hemisphere under Coriolis Effect.
❖ Westerlies push the warm surface water toward
the western margin of the continents.
❖ It blows from lower latitudes to higher latitudes.
❖ The westerlies of the southern hemisphere are stronger and more persistent due to the vast expanse of
water and lesser obstructions from
continents.
➢ While in the northern
hemisphere, these are irregular
because of uneven relief of vast
land masses.
❖ The westerlies are best developed
between 40 degrees and 65 degrees
in the Southern Hemisphere.
❖ These latitudes are often called
Roaring Forties, Furious Fifties, and Shrieking Sixties.
❖ The poleward boundary of the westerlies is highly fluctuating. There are many seasonal and short-term
fluctuations.
❖ These winds produce wet spells and variability in weather.
❖ These winds cause the formation of temperate cyclones and heavy rainfall on western coasts(Western
margin of continents).
Polar Easterlies:
❖ The Polar easterlies are winds blowing from north-east to
south-west direction in the Northern Hemisphere and
south-east to north-west in the Southern Hemisphere.
❖ They blow from the polar high-pressure areas to the sub-
polar lows region.
❖ They are very cold, dry in nature as they originate in polar
areas and do not cause much rainfall.
 7

❖ These winds give birth to cyclones, frontal rainfall (temperate cyclones) when they come in contact with
westerlies (they mix with relatively warm westerlies and cause Frontal rainfall and cyclones).
❖ The Polar Easterlies are more regular in the southern hemisphere in comparison to the northern
hemisphere.
❖ They bring frequent changes in weather conditions and cause heavy rainfall.

 1

DAILY
CLASS NOTES
GEOGRAPHY

Lecture – 35
Winds of the World (Part 02)
 2

Winds of the World (Part 02)

Classification of winds:
Secondary or Periodic Winds:
❖ Seasonal Winds: These winds change their direction in different seasons. For example monsoons in India.
❖ Periodic Winds: Land and sea breeze, mountain and valley breeze etc.
Local winds:
❖ These blow only during a particular period of the day or year in a small area.
❖ Winds like Loo, Mistral, Foehn, Bora, etc are examples of local winds.
Secondary or Periodic Winds:
❖ These winds change their direction with a change in season. Monsoons are the best example of large-scale
modification of the planetary wind system.
❖ Other examples of periodic winds include land and sea breeze, mountain and valley breeze, cyclones and
anticyclones, and air masses.
Monsoons:
❖ The word monsoon is derived
from the Arabic word
‘mausim’ which literally
means season.
❖ ‘Monsoon’ refers to the
seasonal reversal in the wind
direction during a year.
❖ They blow from sea to land
during summers and from
land to sea during winters, due
to the differential in heating of continents and the oceans.
❖ During summer, the trade winds of the southern hemisphere are pulled northwards by an apparent northward
movement of the sun and by an
intense low-pressure core in the
north-west of the Indian
subcontinent.
❖ While crossing the equator, these
winds get deflected to their right
under the effect of Coriolis force.
❖ These winds now approach the
Asian landmass as south-west
monsoons.
 3

❖ Since they travel a long distance over a vast expanse of water, by the time they reach the south western coast
of India, they are over-saturated with moisture and cause heavy rainfall in India and neighboring countries.
❖ During winter, these conditions are reversed and a high pressure core is created to the north of the Indian
subcontinent. Divergent winds are produced by this anticyclonic movement which travels southwards towards
the equator.
❖ This movement is enhanced by the apparent southward movement of the sun. These are north-east or
winter monsoons which are responsible for some precipitation along the east coast of India.
❖ The monsoon winds flow over India,
Pakistan, Bangladesh, Myanmar
(Burma), Sri Lanka, the Arabian
Sea, Bay of Bengal, southeastern
Asia, northern Australia, China and
Japan.
❖ Outside India, in the eastern Asiatic
countries, such as China and Japan, the
winter monsoon is stronger than the
summer monsoon.
❖ The Tropic of Cancer receives maximum sun rays during summer, thus the low-pressure zone is created
in the Tibetan plateau region.
❖ The high-pressure zone is formed in the Indian Ocean
region and winds flow from the high pressure to low
pressure.
❖ The winds flow from the Southwest direction to the
northeast direction. Thus Indian Monsoon is also known as
the Southwest monsoon(In the month of June/July).
❖ During winter, high pressure is formed in the Tibetan region
and low pressure is formed in the Indian Ocean.
❖ The winds now flow in the reverse direction from Northeast
to southwest direction(Wind flow from land to sea).
❖ It will bring rainfall(November, December) to the Tamil
Nadu coast. This type of monsoon is known as the Northeast
monsoon(North easterlies).
Types of Local Winds:
❖ Periodical Winds: The winds originating from diurnal temperature and pressure variation are known as
Periodical and they generally complete their cycle in a day/24 hours like Land/Sea Breeze and Mountain
Valley Breeze.
❖ Non-Periodical Winds: Only present during a particular season and are classified as Hot and Cold Winds.
 4

Land Breeze and Sea Breeze:

❖ It influences only a narrow strip of 20-30 km along the
coast.
❖ The land and sea absorb and transfer heat differently.
❖ During the day, the land heats up faster and becomes
warmer than the sea.
❖ Therefore, over land the air rises because of convection
giving rise to a low-pressure area (Convergence), whereas
the sea is relatively cool and the pressure over the sea is
relatively high.

NOTE:
❖ In the sea breeze when air rises at the land (low pressure) then it will accumulate the warm air, and
divergence occurs (high pressure). It means winds lose energy and start falling down.
❖ In the land breeze, rising of the winds happens (low pressure) at the sea surface then it will accumulate the
warm air, and divergence occurs (high pressure), and this movement is land to sea that is called Land breeze.
❖ Thus, a pressure gradient from sea to land is created and the wind blows from the sea to the land as the
sea breeze. Sea breezes are the surface winds.
❖ In the night, the reversal of the condition takes place.
❖ The land loses heat faster and is cooler than the sea (relatively cold).
❖ The pressure gradient is from the land to the sea and hence land breeze results (Winds blowing from land
to sea due to pressure difference generated because of differential cooling).
 5

❖ This affects both the breezes stage up to 15 miles from the coast.
❖ In coastal cities like Mumbai, Puducherry, etc. these land and sea breezes distribute the temperature and
maintain moderate weather conditions. It also helps in the reduction of pollution.

Valley Breeze and Mountain Breeze:

❖ In mountainous regions, during the day the slopes get heated up, and to fill the resulting gap the air from
the valley floor blows up the slope (these regions start acquiring low pressure-winds flow from to high
pressure to low pressure areas). Plain areas/valley areas(divergence) are relatively less hot. This wind is known
as the valley breeze.
➢ Winds blowing from valley floor to Mountain slopes due to pressure difference generated because of
differential heating.
❖ During the night the slopes get cooled and the dense air
descends into the valley as the mountain wind.
➢ The plain areas are relatively less cold/relatively warmer,
so these winds blow from high to low pressure also called
Air drainage.
➢ Winds blowing from the mountain slope to valley floor
due to the pressure differential existing between mountain
slope and valley.
❖ The cool air, of the high plateaus and ice fields draining into
the valley, is called katabatic wind (Mountain breeze).
❖ Another type of warm wind (katabatic wind) occurs on the
leeward side of the mountain ranges.
❖ The moisture in these winds, while crossing the mountain
ranges condenses and precipitates.
❖ When it descends down the leeward side of the slope the dry
air gets warmed up by an adiabatic process. This dry air may
melt the snow in a short time.
 6

Tertiary or Local Winds (Non-Periodical):

❖ Local differences in temperature and pressure produce local winds.
❖ Such winds are local in extent and are confined to the lowest levels of the troposphere.
 7

Tertiary of local winds:

❖ Loo
❖ Foehn or Fohn
❖ Chinook
❖ Mistral
❖ Sirocco
❖ Harmattan
Loo:
❖ It originates from the Thar desert and has north westerly to
westerly direction.
❖ In the plains of north and North west India(Rajasthan) and
Pakistan, sometimes a very hot and dry, and dusty wind blows
from the west in the months of May and June, usually in the
afternoons. It is known as a loo.
❖ It sucks the moisture from the human body(Injurious to health).
❖ Its temperature invariably ranges between 45 degrees Celcius and
50 degrees Celcius.
❖ It may cause sunstroke to people.
Foehn or Fohn:
❖ Foehn is a hot wind of local importance in the Alps.
❖ Foehn is a beneficial wind. It is a strong, gusty, dry, and warm wind that develops on the leeward side of a
mountain range due to adiabatic heating.
❖ Foehn winds melt snow and make tourist regions
habitable. For example Switzerland.
❖ As the windward side takes away whatever
moisture there is in the incoming wind in the form
of orographic precipitation, the air that descends on
the leeward side is dry and warm (Katabatic Wind).
❖ The temperature of the wind varies between 15 - 20
degrees Celsius.
❖ The wind helps animal grazing by melting snow
and aids the ripening of grapes, wheat sowing (It
will regulate weather which can become more
comfortable).
❖ This plain region will become frost free.
❖ The Foehn winds are present throughout the winter
and due to the presence of such winds the temperature increases, and the valleys of Switzerland are called
 8

Climatic Oasis during the winter season.

Chinook:
NOTE:
❖ These winds carry the moisture (windward side) and generate the cloud, and occur a good amount of
rainfall), when they cross these areas they will change their nature (lose moisture after rainfall).
❖ The eastern slopes (leeward side) of the Rocky Mountains are snow covered having the temperature of -
5 degree celsius to -10 degree celsius.
❖ These are warm and dry winds blowing on the eastern
slopes (leeward side) of the Rocky Mountains.
❖ They are the result of adiabatic heating which occurs
due to downslope compression on the leeward side, as
the mountain barrier creates frictional drag which tends
to pull the air from the higher level down on the leeward.
❖ Air forced down is heated adiabatically and at the same
time its relative humidity is also lowered.
❖ The temperature in Chinook is so warm that it can
remove the underlying snow cover/ice and sometimes these winds are so dry that in spite of their below-
freezing temperatures, the entire snow cover on the ground disappears, by the process of sublimation. Thus,
these winds are also known as Chinook, which literally means Snow Eater. These conditions become normal
after Chinook winds, and it helps in wheat sowing, and also provides green pasture after snow melting.
❖ Chinook leads to the meltdown of the water, and due to the sudden rise in temperature the water is
evaporated.
❖ Thus, there occurs a reduction in the soil moisture, which is a negative aspect of Chinook.
Mistral:
❖ It is a harmful Wind.
❖ Mistral is one of the local names given to such winds that blow from the
Alps over France towards the Mediterranean Sea.
❖ It is channeled through the Rhine Valley.
❖ It is very cold and dry and blows with high speed.
❖ It brings blizzards into southern France.
Sirocco:
❖ Sirocco is a Harmful Wind. It is the warm, dry, dusty local wind that blows
from the Sahara Desert over the central Mediterranean, and southern Italy, and may even reach Spain.
❖ As the Sirocco descends through the northern slope of the Atlas Mountain, they become extremely warm
and dry.
 9

❖ Sirocco picks up the moisture from the

Mediterranean sea and rains in the south of Italy.
❖ These winds carry red sand particles from the Sahara
which causes red colour rainfall in southern Europe
(Italy) and this rainfall is known as Blood Rain.
❖ Sirocco is so much laden with sand and dust that the
atmospheric visibility reduces almost to zero and they
are much injurious to Agriculture and fruit crops.
❖ Sirocco has different names like Khamsin (Egypt),
Gibli (Libya), Chille (Tunisia).
❖ The Sirocco causes dusty dry conditions along the northern coast of Africa, storms in the Mediterranean Sea,
and cool wet weather in Europe.
Harmattan:
❖ It is the warm and dry winds and travels through the Sahara Desert.
❖ It blows from the northeast or east in the western Sahara and is strongest from late November to mid-
March.
❖ West coast of Africa is very warm because of humidity
(So it makes the situation very uncomfortable).
❖ It usually carries large amounts of dust, which it can
transport hundreds of miles out over the Atlantic Ocean; the
dust often interferes with local aircraft operations.
❖ Harmattan has very high speed winds, and the ability to
uproot the trees.
❖ As compared to the humid tropical air, the trade wind is also
known as “doctor wind” due to its strong dryness (Harmattan is very dry and ready to absorb the moisture,
and it makes weather pleasant).
   
 1

DAILY
CLASS NOTES
Geography

Lecture – 36
Temperature Inversion
 2

Temperature Inversion

Temperature Inversion:
❖ Temperature inversion is a reversal of the
normal behaviour of temperature in the
troposphere, in which a layer of cool air
at the surface is overlain by a layer of
warmer air. (Under normal conditions,
the temperature usually decreases with
height).
❖ Terrestrial radiation is radiation emitted by
the earth.
❖ Inversion is usually of short duration
but quite common nonetheless.
❖ In normal conditions, as we go up,
temperature decreases with a normal
lapse rate. It is 6.5°C per
1000m(This is the temperature
reduction with altitude).
❖ When a relatively colder air layer
overlies the air layer below, then the
situation is Normal Lapse Rate.
❖ Against this normal rule, sometimes,
instead of decreasing, the temperature
may rise with the height gained.
❖ The cooler air is nearer the earth
and the warmer air is aloft. This rise
of temperature with height is known
as Temperature inversion(Reversal
of normal phenomena, when relatively
warm air overlies the cold air layer,
then this situation is Temperature
Inversion).
❖ There are various types of inversion,
these are as follows:
➢ Advectional Inversion/Dynamic Inversion:
 3

✓ When winds are in motion horizontal/vertical.

✓ It consists of Frontal, Valley Inversion, and Horizontal air inversion.
➢ Non-advectional Inversion:
✓ Non-advectional inversion means,air is not in motion.
✓ It can be ground or surface inversion and upper air inversion(Ozone layer in Stratosphere).
➢ Mechanical Inversion:
✓ Convective inversion also occurs in this type of inversion(Equatorial low pressure belt).
✓ The subsidence of winds(30-35 degree) occurs in this type of inversion(Sub tropical low pressure
belt).
❖ Over polar areas, a temperature inversion is normal throughout the year.
❖ The inversion takes place in hills and mountains due to air drainage.
➢ Cold air at the hills and mountains, produced during the night, flows under the influence of gravity.
➢ Being heavy and dense, the cold air acts almost like water and moves down the slope to pile up deeply
in pockets and valley bottoms with warm air above. This is called air drainage.
Ideal Conditions for Temperature Inversion on the Ground:
❖ Long nights, so that the outgoing terrestrial radiation is greater than the incoming radiation(Cooling of
air near the Earth). The heat of the day is radiated off
during the night, and by early morning hours, the
earth is cooler than the air above.
❖ Clear skies, which allow the unobstructed escape of
radiation(no radiation trapped, it means cooling the
Earth). In the cloudy sky inversion gets disturbed
because of trapping of the heat.
❖ The benefit of clear sky is easy exit of terrestrial
radiation, less trapping of heat, and forming of the
inversion.
❖ Calm and stable air, so that there is no vertical mixing
at lower levels(Easily different temperature layers can
exist). [If suppose there is no calm condition, different
layers will mix and the inversion situation will be
disturbed].
❖ Snow-covered regions reflect the incoming radiation
and less heat will be trapped in the atmosphere. Therefore the nearest layer to the ground will be cooler.
 4

❖ High Humidity, so that sufficient moisture is

present.
❖ Dry air is free from water vapours (less moisture)
and thus traps lesser radiation and less heat. It will
create good conditions for the ground or surface
inversion.
Types of Temperature Inversion:
1. Ground Inversion (Surface Temperature
Inversion):
➢ A ground inversion develops when air is
cooled(The height of 30 to 40 feets) by
contact with a colder surface until it
becomes cooler than the overlying air; this
occurs most often on clear nights when
the ground cools off rapidly by
radiation.
➢ If the temperature of surface air drops
below its dew point, fog may be formed.
➢ This kind of temperature inversion is very common in higher latitudes.
➢ Surface temperature inversion in lower and middle latitudes occurs during cold nights and gets destroyed
during daytime.
2. Upper-Air Inversion:
➢ Upper air inversion occurs due to the presence of warm air in the stratosphere and cold air in the
tropopause(Upper air inversion occurs above troposphere due to Ozonlayer because the ozone layer
absorbs UV rays, and gets heated up).
✓ A warm layer of the ozonosphere lies over and above the cold layer of air (Troposphere +
stratosphere).
✓ Low temperature is between 0 to 15 kms, and
high temperature between 15 to 35 kms.
➢ Stratosphere: The stratosphere is the atmospheric
layer above the troposphere, extending from about
10 to 50 kilometers above the Earth's surface.
➢ Warm air: In the stratosphere, temperature
generally increases with altitude due to the
presence of ozone, which absorbs solar radiation
and warms the surrounding air.
 5

➢ Tropopause: The tropopause is the boundary between the troposphere and the stratosphere, located at an
altitude of about 10-15 kilometers (-50 degree celsius).
➢ Cold air: The tropopause acts as a barrier where the temperature gradient changes. Above the tropopause,
the temperature remains relatively constant or even decreases with altitude, creating a layer of cold air.
➢ Inversion: When warm air gets trapped in the ozone layer of the stratosphere due to absorption of
radiation from the Sun, the warm air encounters the cold air (which is at around -50 degrees C) of the
tropopause (a layer just below the stratosphere), and hence the temperature inversion starts taking place at
this layer. It is also known as the thermal inversion.

3. Valley Inversion/ Intermontane Valley (Air Drainage Type of Inversion):

➢ In the intermontane valley, the temperature increases with the
elevation instead of decreasing.
➢ Sometimes, the temperature in the lower layers of air increases
instead of decreasing with elevation. This happens commonly
along the sloping surface.
➢ Here, the surface radiates heat back to space rapidly and cools
down at a faster rate than the upper layers. As a result, the
lower cold layers get condensed and become
heavy.
➢ The sloping surface underneath makes them move
towards the bottom where the cold layer settles
down as a zone of low temperature while the
upper layers are relatively warmer.
➢ Hence, the heavy cold air wind subsiding into the
valley is also known as the Katabatic Winds.
 6

➢ This condition, opposite to the normal vertical distribution of temperature, is known as Temperature
Inversion.
➢ Inversion happens when warm air overlies the cold air layer(Temperature warmer is greater than
temperature colder).
➢ In other words, the vertical temperature gets inverted during a temperature inversion.
➢ This kind of temperature inversion is very strong in the middle and higher latitudes. It can be strong in
regions with high mountains or deep valleys also.
➢ Valley floors are subjected to frostbite.
➢ Hence, in the valley regions, generally, farmers avoid the cultivation of certain crops due to frost action.
Example: Cultivation of coffee on slope etc.
4. Frontal Inversion (Advectional Type of Temperature Inversion):
➢ Interaction of two different masses of air (warm
air and cold air) is called front.
➢ A frontal inversion occurs when a cold air mass
undercuts a warm air mass and lifts it aloft; the
front between the two air masses then has warm air
above and cold air below.
➢ Air Mass is a huge volume of air(It can be upto 1000
square kilometers).
➢ In addition, humidity may be high,
and clouds may be present
immediately above it.
➢ This kind of inversion has a
considerable slope, whereas other
inversions are nearly horizontal.
➢ This type of inversion is unstable
and is destroyed as the weather
changes.
➢ These fronts can be seen in the
temperate region of 60 to 65 degree
latitude (Sub polar low pressure belt-mixing of the warm and cold air).
➢ These inversions lead to the formation of frontal cyclones.
➢ It is an Advectional inversion.
➢ In temperate regions, these types of inversion are common. Example: Canada, etc.
 7

NOTE:
❖ Warm air mass lies above the cold air due to the meeting of air masses.

5. Subsidence Inversion (Upper Surface Temperature Inversion):

➢ A subsidence inversion develops when a widespread
layer of air descends.
➢ The layer is compressed and heated by the resulting
increase in atmospheric pressure, and as a result, the
lapse rate of temperature is reduced.
➢ If the air mass sinks low enough, the air at higher
altitudes becomes warmer than at lower altitudes,
producing a temperature inversion.
➢ This inversion is generally seen in the region of 30 to 35-
degree latitude.
➢ This region experiences stable weather conditions (no
clouds seen).
➢ Subsidence inversions are common over the
northern continents in winter (dry
atmosphere) and over the subtropical
oceans; these regions generally have
subsiding air because they are located under
large high-pressure centers.
➢ This temperature inversion is called upper
surface temperature inversion because it
takes place in the upper parts of the
atmosphere.

NOTE:
❖ Subsidence means air falls down from greater height, as it reaches near the surface (1 Kms), and it
compresses due to high pressure, and because of compression it gets heated(temperature increases) then it
becomes warm air. This implies that warmer air overlies colder air, and this is known as temperature
inversion.

6. Convective Inversion:
➢ As the ground heats up during the day, warm air rises (because the wind is light) where it reaches a
certain altitude.
 8

➢ This situation is known as instability

conditions.
➢ The vacuum created by the rising
warm air is filled by the heavy cold
air. Hence, a zone of turbulence is
created in this region where the warm
air lies above the cold air. This is
known as convective inversion.
➢ After warm moist air reaches up,
condensation happens.
➢ In this region various weather conditions are experienced such as instability, cloud formation,
rainfall, and cyclonic conditions.
➢ It is commonly experienced in Equatorial regions during day time.
Effects of Temperature Inversion:
❖ Inversions play an important role in determining cloud forms, precipitation, and visibility.
❖ An inversion acts as a cap on the upward movement of air from the layers below. As a result, convection
produced by the heating of the air from below is limited to levels below the inversion.
❖ Diffusion of dust, smoke and other air pollutants is likewise limited. Thus, it promotes atmospheric stability.
❖ Temperature Inversion also determines precipitation, cloud forms, etc.
❖ In regions where a pronounced low-level inversion is present, convective clouds cannot grow high enough
to produce showers and hence less rainfall. This causes problems in agriculture and its productivity. It is
dangerous for crops like coffee, cotton, etc.
❖ Visibility may be greatly reduced below the inversion due to the accumulation of dust and smoke particles
because the air near the base of an inversion tends to be cool.
❖ Inversions also affect diurnal variations in temperature.
Diurnal variations tend to be very small.
❖ Fog is formed due to the situation of warm air above and cold
air below and hence visibility is reduced.
❖ Surface is very cold (Winter nights), it cools down the air
near the surface, so the nearby layer cools down. When
moisture condenses it will form the micro droplets, and these
micro drops are fog.
➢ This inversion commonly lasts for a few hours until the
sun comes up and begins to warm the earth.
 9

➢ Temperature is greater than pressure.

➢ The heavy air also contains various harmful gasses and pollutants like dust, smoke, pollen, ash,
smoke, etc.
➢ Pollutants act as hygroscopic nuclei which attract water vapor around it.
➢ Fog and smoke lead to the formation of Smog. It reduces visibility which is a cause of accidents on
roads and delays in flights.
➢ Examples: London Smog in 1952(Killed around 3000-4000 people), Muse Valley in Belgian in 1930
etc.
❖ It causes frost (ice particles) when the condensation of
warm air due to its cooling by cold air below occurs at
a temperature below freezing point. Temperature is
less than freezin points, it means less than 0 degree.
❖ It causes atmospheric stability which stops upward and
downward movement of air.
❖ The convective clouds can not grow high enough, and
hence there are no showers, hence less rainfall. This
causes problems in agriculture and its productivity.
➢ It is harmful to potatoes, peas and coffee cultivation
because of frequent incidents of frostbite.
➢ Therefore important activities like the construction of habitats (Homes and hotels) and the cultivation of
coffee to avoid the harmful effects of temperature inversion in the valley.
❖ In winter the cold air containing pollutants is heavy and sinks near the surface and leads to ground level
air pollution. It is very harmful to people and leads to various lung-related diseases such as bronchiectasis,
asthma, congestion and other breathing issues.
❖ In the region of Himalaya, Alps, Andes the plantation of frost sensitive crops-on slopes like Agriculture,
Habitat, Hotels.

   
 1

DAILY
CLASS NOTES
Geography

Lecture - 37
Jet Streams
 2

Jet Streams
Geostrophic Wind:
❖ The velocity and direction of the wind are the net result of the wind generating forces i.e., the Coriolis Force,
Pressure Gradient Force, and Frictional
Force.
❖ The winds in the upper atmosphere, 2-3 km
above the surface, are free from the frictional
effect of the surface and are controlled by the
pressure gradient and the Coriolis force.
❖ An air parcel initially at rest will move from
high pressure to low pressure because of the
Pressure Gradient Force (PGF).
❖ However, as that air parcel begins to move, it is
deflected by the Coriolis force to the right in
the Northern Hemisphere (to the left in the southern hemisphere).
❖ As the wind gains speed, the deflection increases until the Coriolis force equals the pressure gradient force (2–
3 km above the ground, friction is low and winds travel at greater speeds).
❖ At this point, the wind will be blowing parallel to the isobars (perpendicular to Pressure Gradient Force).
❖ When this happens, the wind is referred to as Geostrophic Wind.
Jet Streams:
❖ The Jet Stream is a geostrophic wind blowing
horizontally through the upper layers of the
troposphere, generally from west to east, at an altitude of
6-13 km above the surface of the earth.
❖ Jet Streams develop where air masses of different
temperatures meet. So, usually surface temperatures
determine where the Jet Stream will form.
➢ Greater the difference in temperature, the faster
the wind velocity inside the jet stream.
❖ Jet Streams extend from 20 degrees latitude to the poles
in both hemispheres.
Origin of Jet Streams:
❖ Jet streams are caused by the temperature difference between tropical air masses and polar air masses.
❖ Tropical air is warm and polar air is cold, and this is due to its relative locations.
❖ So, what happens in one part of the world depends on what is happening elsewhere, after all the atmosphere is
one complete environment, it is connected with so many factors.
 3

Where Does the Jet Stream Begin?

❖ It originates in the tropics, which means around the equator.
❖ This region is warm because the
Insolation falls overhead at the
equator. Hence, it is a place that
fuels the jet stream.
❖ At the Equator, the warm air rises
up in the atmosphere and goes to the
region between the troposphere
and stratosphere.
❖ From here, this warm air gets
drawn toward the colder air
found in the north and south pole.
❖ Now at higher altitudes, the warm
air cools and sinks, drawing more and more warm air behind it.
❖ Meanwhile the cooled air flows towards the equator creating a simple looping mechanism. It’s endless
repetition.
 4

Features of Jet Streams:

❖ Narrow Bands: These are narrow concentrated bands of winds. The stream of air is very narrow (a few hundred
km across) and the air in the stream is directed towards the axis of the stream making it very narrow.
❖ Circumpolar (situated around or inhabiting one of the earth's poles): Jet streams are winds that circle around
the earth with poles as their centers.
❖ High Velocity: Pressure gradient increases with altitude and
creates high velocity winds at higher altitudes.
➢ The friction in the upper troposphere is quite low due to
less dense air. Hence, the Jet streams flow at great velocities.
➢ Temperature also influences the velocity of the jet stream.
➢ The greater the difference in air temperature, the faster the jet
stream, which can reach speeds of up to 250 mph (402 kph)
or greater, but average about 110 mph (177 kph).
➢ The jet streams have an average velocity of 120 kilometers
per hour in winter and 50 km per hour in summer.
➢ These jet streams also have cores where the
speed is much greater.
❖ Geostrophic Streams: The direction of the jet
stream is determined by pressure gradient force
(temperature contrast creates pressure gradients) and
Coriolis force.
➢ The direction of the jet stream is perpendicular
to the pressure gradient force.
➢ The winds surrounding jet streams are of
comparatively low velocities.
❖ Are a part of upper-level westerlies: Jet streams
are produced due to winds flowing from the tropics towards the poles. In polar jet streams, wind flows from
the temperate region towards the polar region, and in sub-polar jet streams winds flow from sub-tropics
towards the temperate region.
➢ Anything moving from the
tropics towards the poles deflects
towards their right in the
northern hemisphere and
towards their left in the
southern hemisphere due to the
Coriolis effect.
 5

❖ Upper Tropospheric: Jet streams flow just below the Tropopause.

➢ Polar jet streams flow 6–9 km above the ground and Sub-
tropical jet streams flow 10–16 km above the ground.
❖ Concentrated bands of Meandering: When the temperature
contrast is maximum, the jet stream flows nearly straight.
➢ But when the temperature contrast reduces, the jet stream
nearly follows the meandering path.
➢ Therefore, meandering depends on temperature contrast
(Temperature difference).
➢ A Meander is called a peak or ridge if it is towards the
poles and trough if it is towards the equator.
Rossby Waves:
❖ The meandering jet streams are called Rossby Waves.
❖ Rossby waves are a natural phenomenon
in the atmosphere and oceans due to the
rotation of the earth.
❖ Rossby waves are formed when high-
intensity polar air moves Equatorwards
while tropical air moves poleward.
❖ The existence of these waves explains the
low-pressure cells (cyclones) and high-
pressure cells (anticyclones).
Permanent Jet Streams:
❖ There are Two Permanent
Jet Streams – subtropical
jets at lower latitudes and
polar front jets at mid-
latitudes.
1. Subtropical Jet Stream
(STJ):
➢ The sub-tropical jet
stream is produced by
the earth’s rotation
(Coriolis force) and
temperature
contrast between
tropical and sub–tropical regions.
➢ At the equator, the rotation produces the greatest velocity in the atmosphere.
 6

➢ As a result, the rising air which spreads out northwards and southwards moves faster than the latitudes over
which it is blowing.
➢ It is deflected to the right in the northern hemisphere and to the left in the southern hemisphere, and
at about 30° latitude, it becomes concentrated as the subtropical jet streams.
➢ During winter, the STJ is nearly continuous in both hemispheres.
➢ The STJ exists throughout the year in the southern hemisphere.
➢ However, it is intermittent in the northern hemisphere during summer when it migrates north.
➢ The STJ can be temporarily displaced when strong mid-latitude troughs extend into subtropical latitudes.
➢ When these displacements occur, the subtropical jet can merge with the polar front jet.
➢ STJ is closely connected to the Indian and African summer monsoons.
2. Polar Front Jet (PFJ):
➢ The polar front jet is produced by a temperature difference and
is closely related to the polar front.
➢ It has a more variable position than the sub-tropical jet.
✓ In summer, its position shifts towards the poles, and in
winter towards the equator.
➢ The jet is strong and continuous in winter.
➢ It greatly influences climates of regions lying close to 60 degree
latitude.
➢ It determines the path and speed and intensity of temperate
cyclones.
Temporary Jet Streams:
❖ Temporary Jet Streams are Somali Jet, Polar Night Jet Stream, and The African Easterly Jet.
The Tropical Easterly Jet or African Easterly Jet:
❖ There are also major high-velocity winds in the
lower troposphere called Low-Level Jets (LLJs).
❖ In the tropics, the most prominent of these are the
Somali Jet and the African Easterly Jet.
❖ The TEJ is a unique and dominant feature of the
northern hemisphere during summer over
southern Asia and northern Africa.
❖ The TEJ is found between 5° and 20° North.
❖ It is continuous in its position, direction, and
intensity from June till the beginning of
October.
❖ The TEJ is the upper-level venting system for
the strong southwest monsoon.
 7

❖ The establishment and maintenance of the TEJ are not fully understood but it is believed that the jet may be
caused by the uniquely high temperatures and heights over the Tibetan Plateau during summer.
❖ During the south Asian summer monsoon, the TEJ induces secondary circulations that enhance convection over
South India and the nearby ocean.
The Somali Jet:
❖ Among the most well-known of the tropical LLJs is
the Somali Jet, a southwesterly jet.
❖ The Somali jet occurs during the summer over
northern Madagascar and off the coast of
Somalia.
❖ The jet is most intense from June to August.
❖ It is a major cross-equatorial flow from the southern
Indian Ocean to the central Arabian Sea.
❖ A split in the axis of the jet over the Arabian Sea, the
more northern branch intersecting the west coast of
India near 17° North, while the southerly branch
moves eastward just south of India.
❖ The jet remains relatively steady from June to September before moving southward to the southern
Indian Ocean during the winter.
Polar Night Jet Stream:
❖ Also called the stratospheric subpolar jet stream, it develops in winter due to the sharp temperature gradient
in the stratosphere around the poles at the height of 30km.
❖ These jet streams become very strong westerly circulation with high wind velocity during winters but their
velocity decreases during summers and the direction becomes easterly.
Influence of Jet Streams on Weather:
❖ Jet streams help in the maintenance of latitudinal heat balance by mass exchange of air.
❖ PFJ influences the mid-latitude weather disturbances. Usually, there are severe storms when jet streams
interfere with surface wind systems.
❖ Jet streams also influence the path of temperate cyclones. They have an influence on the distribution of
precipitation by temperate cyclones.
❖ Sub-tropical jet streams and some temporary jet streams together influence Indian Monsoon patterns.
❖ Jet streams also exercise an influence on the movement of air masses which may cause prolonged Drought or
Flood conditions.
Jet Streams and Weather in Temperate Regions:
❖ PFJ plays a key role in determining the weather because they usually separate colder air and warmer air.
❖ Jet streams generally push air masses around, moving weather systems to new areas and even causing
them to stall if they have moved too far away.
 8

❖ PFJ plays a major role in determining the path

and intensity of frontal precipitation and
frontal cyclones or temperate cyclones.
❖ Weak PFJ also results in the slipping of polar
vortex into temperate regions.
❖ Polar jets have distinct peaks (ridges) and
troughs.
➢ Ridges occur where the warm air (at high
pressure) pushes against the cold air.
➢ Troughs occur where cold air (at lower
pressure) drops into warm air.
✓ This condition occurs due to a weak
jet stream which is weak due to lesser temperature contrast between sub-tropics and temperate regions.
❖ These ridges and troughs give rise to jet
streaks which are the main reason behind
cyclonic and anticyclonic weather
conditions at the surface.
❖ The winds leaving the jet streak are rapidly
diverging, creating a lower pressure at the
upper level (Tropopause) in the
atmosphere.
❖ The air below rapidly replaces the upper
outflowing winds. This in turn creates low
pressure at the surface. This surface low
pressure creates conditions where the
surrounding surface winds rush inwards.
❖ The Coriolis effect creates the cyclonic rotation that is
associated with depressions.
❖ The winds entering the jet streak are rapidly converging, creating a
high pressure at the upper level (Tropopause) in the
atmosphere.
❖ This convergence at the upper troposphere leads to divergence
(high pressure) at the surface (anticyclonic condition).
❖ The Coriolis Effect creates the anticyclonic rotation that is
associated with clear weather.
 9

Indian Monsoon Mechanism and the Role of Sub-Tropical Jet Streams:

❖ The burst of monsoons depends upon the upper air circulation which is dominated by Sub-Tropical Jet
Streams (STJ).
❖ The south west monsoon coming into India is related to the tropical easterly stream. It blows between 8 degree-
35 degree North latitudes.
❖ The north east monsoon (winter monsoon) is related to the subtropical westerly Jet Stream which blows
between 20 degree and 35 degree latitudes in both hemispheres.

Seasonal Migration of Sub-Tropical Jet Streams:

❖ In winter, Sub-Tropical Jet Streams flow along the southern slopes of the Himalayas and in summer shifts
northwards dramatically, flowing along the edge of Himalayas in early June and in late summer (July-
August) along the northern edge of the Tibetan Plateau.
❖ The periodic movement of the Jet Stream often indicates the onset and subsequent withdrawal (STJ returns
back to its position – south of Himalayas) of the monsoon.
❖ Northward movement of the subtropical jet is the first indication of the onset of the monsoon over India.
How Does it Trigger The North East Monsoon?
❖ The westerly Jet Stream, is a cold wind which pushes down wind to the surface creating high pressure on the
surface.
❖ Dry winds from this high pressure area (north western part of India) start blowing towards the low
pressure area (Bay of Bengal).
❖ These winds in turn bring cold waves in winter in the northern part of the country including UP and
Bihar.
❖ After reaching the Bay of Bengal, westerly under the influence of Ferrel’s cell takes the form of north east
monsoon.
❖ When this wind reaches the coast of Tamil Nadu, it causes rainfall with the humidity received from the Bay
of Bengal.
 10

Jet Streams and Aviation:

❖ Jet streams are used by aviators if they have to
fly in the direction of the flow of the jet streams,
and avoid them when flying in opposite directions.
❖ Jet streams may also cause a bumpy flight full of
turbulence, because the jet stream is sometimes
unpredictable and can cause sudden movement.


 1

DAILY
CLASS NOTES
Geography

Lecture - 38
Humidity and Lapse Rate
 2

Humidity and Lapse Rate

Water Vapour in Atmosphere:

 Water vapour in the air varies from zero to four per cent by volume of the atmosphere (averaging around
2% in the atmosphere).
 Water vapour is gaseous form of H₂O.
 Water vapour is the most important constituent of the Atmosphere.
 90 percent of the water vapour exists up to 6 kms height.
 The maximum amount of insolation (sun’s energy) absorbed by water vapour.
 It plays an important role in the heat budget also.
 The major climatic phenomena are clouds, rainfall, fog, mist, snow, hails, cyclones, frost, thunder and
tornadoes.
 The amount of water vapour (Humidity) is measured by an instrument called a Hygrometer.
Significance of Atmospheric Moisture:
 Water vapour absorbs radiation - both incoming and terrestrial. It, thus, plays a crucial role in the
earth’s heat budget.
 The amount of water vapour present decides the quantity of latent energy stored up in the atmosphere
for the development of storms and cyclones.
 The atmospheric moisture affects the human body’s rate of cooling by influencing the sensible
temperature.

NOTE:
 There are three states/phases like Liquid, Solid, Gas.
 Latent Heat is the amount of energy required to convert liquid phase/state into gaseous state.
 Condensation is the process by which water vapor in the air is changed into liquid water; it's the
opposite of evaporation (coming from a high energy to low energy state).
 Condensation is conversion of vapours into liquid along with release of latent heat of Condensation.
 In condensation phenomena, if vapours condense, it may lead to fog, mist, frost, rain, hail, snow, clouds.

Evaporation:
 Evaporation is the process by which water changes from a liquid to a gas or vapor.
 It is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric
water vapor.
 Evaporation may be taken as the starting point in the hydrological cycle. The source of energy for
evaporation is mainly solar radiation.
 The Ocean covering 71 % of the earth’s surface holds 97% of the earth’s water reserves. The ocean
contributes 84% of the annual total and the continents 16%.
 3

 Evaporation increases with temperature, dryness and movement of air.

 Evaporation decreases with cloud cover.
 Evaporation of Water - 64 % by Transpiration, 6 % by Soil Evaporation, 3 % by Water bodies.
 The highest annual evaporation occurs in the sub-tropics of the western North Atlantic and North
Pacific because of the influence of the Gulf Stream and the Kuroshio Current, and in the trade wind
zone of the southern oceans.
 The maximum evaporation occurs in the equatorial region because of high insolation and luxuriant
vegetation, and equators have 4-5 % of water vapour.
 The minimum insolation/minimum heat occurs in the poles, and poles have 0.2 % of water vapour.
 The maximum humidity(Moisture)is in the Equator region, and minimum humidity occurs in the polar
region.

Humidity:
 Absolute Humidity
 Specific Humidity
 Relative Humidity
 Humidity capacity
 Dew point
Humidity:
 Humidity is the amount of water vapour present in a given volume of air at a given place and time.
 Humidity is important for Biodiversity(Flora and Fauna), Humans, and Environment.
 4

Absolute Humidity:
 The actual amount of water vapour present in the atmosphere is known as Absolute Humidity.
 It is the weight of water vapour (gms) per unit volume of air and is expressed in gm/m3.
 The absolute humidity differs from place to place on the surface of the earth.
 The ability of the air to hold water vapour depends entirely on its temperature.
○ Warm air can hold more moisture than cold air.
 Absolute humidity=Weight of moisture/Volume of air(Absolute humidity can increase by adding more
moisture or by reducing volume(by contraction and expansion)).
 For example: 5 Grams of vapour present in the 1 cm cube of air, humidity at x region is 5 grams/cm cube.
 Absolute humidity of the ocean is more than Absolute humidity of the land.
Specific Humidity:
 It is expressed as the weight of water vapour per unit weight of air. It is expressed in gm/ kg (amount of
vapour (grams) present in a given weight of air (kg).
 Specific Humidity =Weight of Moisture/Weight of Air.
 Specific humidity is a yardstick for a geographer to measure moisture in Air.
 Since it is measured in units of weight, the specific humidity is not affected by changes in pressure or
temperature.
 Specific Humidity at poles is 0.2gm/kg of air and at equator it is around 18 gm/kg of air. Specific
Humidity is high at the equator because of maximum evaporation.
 For example: Somewhere15 grams of water vapour of 1kg of air, so specific humidity is 15 gm/kg.
 Absolute Humidity and Relative Humidity are variable whereas Specific Humidity is constant.

NOTE:
 Humidity Capacity Definition: Humidity capacity represents the maximum amount of water vapor that
air can hold in a given volume at a specific temperature.
 Temperature Influence: When
temperature increases, the volume of air
expands, allowing it to hold more
moisture.
 Capacity for Moisture: Humidity
capacity indicates the air's capability to
retain moisture.
 Geographic and Seasonal Variation:
Humidity capacity varies with location
and season. It is higher at the equator,
during summer, and in daytime
compared to the poles, winter, and nighttime.
 5

 Formula: Humidity capacity is calculated as the maximum weight of moisture that can be accommodated
per unit volume of air, expressed as grams per cubic centimeter (gm/cc).
 Example (Calculation): For instance, if air has a volume of 1 cubic centimeter and contains 2 grams of
water vapor, the absolute humidity is 2 grams/cc, while the maximum humidity capacity at that
temperature is 5 grams/cc.
 Relative Humidity: Relative humidity is determined by comparing the actual vapor content (2gm/cc in
this example) to the maximum capacity (5gm/cc), expressed as a percentage. In this case, the relative
humidity is 40%.

Relative Humidity:
 The ratio of water vapour in air at a particular temperature to the total amount of water vapour required
to saturate the same air at the same temperature(The ratio of actual vapours present in a given volume to
maximum vapour present in the same volume).
 Relative humidity increases with an increase in water vapour in the air and decreases with an increase in
temperature.

Relative Humidity (RH)= Actual Humidity (AH) / Humidity Capacity (HC)

 It is greater over the oceans and least over the continents.

 Relative humidity is measured by a Hygrometer.
 Relative humidity determines the amount and rate of evaporation of a place and, hence, it is an important
climatic factor.
 Air containing moisture to its full capacity at a given temperature is said to be ‘saturated’.
 At this temperature, the air cannot hold any additional amount of moisture. Thus, the relative humidity
of the saturated air is 100%.
 Relative humidity can be changed in either of two ways:
 By adding moisture through evaporation (by increasing absolute humidity): if moisture is added by
evaporation, the relative humidity will increase and vice versa.
 By changing the temperature of air (by changing the saturation point): a decrease in temperature
(hence, decrease in moisture-holding capacity/decrease in saturation point) will cause an increase in
relative humidity and vice versa.
Role of Relative Humidity:
 Excess humidity (70-80%) will result in diseases due to the presence of thriving conditions for bacteria, and
pathogens, low human development.
 For example- Equatorial forests like the Amazon are home to various bacteria, and also in Central
America, Indonesia, Mexico, Democratic republic of Congo.
 It may also result in rusting as witnessed in coastal areas.
 6

 Deficient humidity(20-25%) adversely impacts the development of a region. For Example- In the Sahara,
Arab Desert, agriculture is underdeveloped due to the deficiency of moisture. The same holds true for the
Thar desert in India.
Condensation:
 The transformation of water vapour into liquid water droplets is called condensation.
 Condensation is caused by the loss of heat (latent heat of condensation, opposite of latent heat of
vaporization).
 When moist air is cooled, it may reach a level when its capacity to hold water vapour ceases (Saturation
Point = 100% Relative Humidity = Dew Point reached). Then, the excess water vapour condenses into
liquid form.
 If it directly condenses into solid form, it is known as sublimation.
 In free air, condensation results from cooling around very small particles termed as hygroscopic
condensation nuclei.
 Particles of dust, smoke, pollen and salt from the ocean are particularly good nuclei because they
absorb water.
 Condensation also takes place when the moist air comes in contact with some colder object and it may
also take place when the temperature is close to the dew point.
 Condensation, therefore, depends upon the amount of cooling and the relative humidity of the air.
 Condensation is crucial to the water cycle because it is responsible for the formation of clouds.
 These clouds may produce precipitation, which is the primary route for water to return to the Earth's
surface within the water cycle.
 Condensation is the opposite of evaporation.
 After condensation, the water vapour or the moisture in the atmosphere takes one of the following forms -
dew, frost, fog and clouds.


 1

DAILY
CLASS NOTES
Geography

Lecture - 39
Different types of Condensation
 2

Different types of Condensation

Condensation:
❖ The transformation of water vapour into liquid water droplets is called condensation.
❖ Condensation is caused by the loss of heat (latent heat of condensation, opposite of latent heat of vaporization).

Forms of Condensation:
❖ There are different types of condensation like Fog, Dew, Frost, Mist, Smog, Clouds, and Rain.
❖ Condensation can take place when the dew point is-
➢ Lower than the freezing point.
➢ Higher than the freezing point.
❖ White frost, snow and some clouds (cirrus clouds) are produced when the temperature is lower than the
freezing point.
❖ Dew, fog and clouds result even when the temperature is higher than the freezing point.
❖ Forms of condensation may also be classified on the basis of their location, i.e. at or near the earth’s surface
and in free air.
❖ Dew, white frost, fog and mist come in the first category, whereas clouds are in the second category.
❖ Rain is a form of precipitation.

Term Formation Appearance Location Impact

Fog Condensation of water Reduces visibility, Common in valleys, Disrupts transportation,

vapor close to the ground, creates a thick, near bodies of may require special
often in cool and calm opaque layer near water, and during precautions when driving,
conditions. the surface. temperature and can create a
inversions. mysterious ambiance.
 3

Dew Occurs when the ground Small water Appears on surfaces Generally harmless, may
loses heat at night, causing droplets that form such as grass, make surfaces wet, which
nearby air to cool. Moisture on objects, typically leaves, and car can be slippery, and is
in the air condenses on in the morning or at windshields. often considered
surfaces like grass and night. refreshing.
leaves.

Frost When the temperature drops Ice crystals form on Occurs in colder Can damage crops, create
below freezing, water vapor various objects, climates during hazardous driving
in the air directly forms ice such as plants, winter, especially conditions, and result in
crystals on surfaces. windows, and the overnight. slippery walkways.
ground.

Mist Tiny water droplets Creates a fine, thin Can occur near Reduces visibility but
suspended in the air, layer of water bodies of water, usually not as significantly
typically formed when droplets in the air, during rain, or in as fog, making it less
warm, moist air meets a reducing visibility humid conditions. disruptive.
cooler surface or when but not as thick as
humidity is high. fog.

Smog A type of air pollution that Appears as a thick, Typically found in Harmful to human health,
results from the combination yellowish-brown densely populated contributes to respiratory
of smoke and fog, often in haze with reduced urban areas with issues, and can have
urban areas with high levels visibility and high levels of environmental impacts.
of air pollution. unpleasant odor. industrial and
vehicular emissions.

Cloud Formed by the condensation Varies in Found at various Influences weather

of water vapor in the appearance, altitudes in the sky, patterns, provides shade,
atmosphere, usually when including fluffy ranging from low- and plays a critical role in
warm, moist air rises and white clouds, dark level clouds near the the Earth's climate system.
cools at higher altitudes. storm clouds, and ground to high-level
thin wispy clouds. clouds at great
heights.

Dew:
❖ When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than
nuclei in the air above the surface) such as stones, grass blades and plant leaves, it is known as dew.
❖ Dew is very small in size.
❖ The ideal conditions for its formation are a clear sky, calm air, high relative humidity, and cold and long
nights.
 4

❖ For Dew Formation, the dew point must be above the freezing point.
❖ Dew point is the temperature at which condensation of the air starts.
❖ Dew point temperature is the temperature at which the first droplet of water is formed.
❖ It is the temperature at which the moisture of the air gets converted into the liquid droplet.
❖ Dew point of temperature is the property of air.
❖ Conditions for the formation of dew are the following:
➢ Clear sky (no clouds)
➢ Presence of moisture in the air (good condensation).
➢ Cold and long night.
➢ Calm weather conditions (no weather conditions).

NOTE:
❖ The ambient room temperature is currently 25 degrees Celsius. Initially, there is no glass containing cold
water, but once a glass of cold water with a temperature of 10 degrees Celsius is introduced, the surrounding
air comes into contact with the glass.
❖ As a result, the temperature of the air begins to decrease gradually, first to 24 degrees, then to 23, 22, 21, and
eventually reaching 20 degrees Celsius. This specific temperature, 20 degrees Celsius, is indicative of the
dew point property, where the air reaches a state of saturation and transitions into a liquid form.

Frost:
❖ Frost is when the excess moisture is deposited in the form of minute
ice crystals instead of water droplets.
❖ It is frozen microdroplets of water on the surface of leaves of plants.
❖ Frost forms on cold surfaces when condensation takes place below the
freezing point (0° C), i.e. the dew point is less than or equal to the
freezing point.
❖ The ideal conditions for the formation of frost are the same as those for
the formation of dew, except that the air temperature must be at or below the freezing point.
❖ Frosts are generally visible in the hilly areas, high latitudes.
❖ It is dangerous for coffee, tomato, peas, cotton and potato cultivation.
Fog:
❖ It is a special type of thin cloud that consists of microdroplets of
water which are suspended in the air near the ground surface.
❖ It is formed when the temperature of warm-moist air comes in
contact with a cooled surface & moisture gets condensed in the form
of microdroplets that are suspended in the air.
❖ Moisture gets condensed around the hygroscopic nuclei.
 5

❖ When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden,
condensation takes place within itself on fine dust particles.
❖ So, the fog is a cloud with its base at or very near to the ground.
❖ The visibility becomes poor to zero because of the fog and mist.

❖ In urban and industrial centres, smoke provides plenty of nuclei which help the formation of fog and mist.
Such a condition when fog is mixed with smoke is described as smog.
❖ Colour- it becomes yellow due to dust particles and whitish, gray due to the presence of soot or smoke.
❖ Region: Lower Latitudes(Tropics and subtropics)- During winters and rainy seasons, Higher Latitudes
(Temperate zone)- All-year fog is formed, in the Coastal Areas sea fogs are there, and hilly fogs are in the
hilly regions, 60 to 65 degree latitudes there are frontal fogs are there.

Types of Fog:

❖ Advection Fog: It is formed by condensation of warm air when it moves horizontally over a cold surface.
These fogs are thick and persistent. It occurs over warm and cold water mixing zones in oceans. Example:
New Foundland, has dense fog and is a good fishing ground(meeting of labrador and Gulf stream current).
The conditions of the formation of this are the following:
➢ Motion of air
➢ Presence of moisture in air
 6

➢ Temperature difference

❖ Frontal or Precipitation Fog: It is produced due to the

convergence of warm and cold air masses where the
warm air mass is pushed under by the heavier cold air
mass. Precipitation in the warm air mass condenses to
produce fog at the boundary of the two air masses. For
example 60 degree latitudes.
❖ Sea Fog: It is formed over the sea surface due to a large
amount of moisture in the air which cools down as Sea Fog.
For example California current.
❖ Steam Fog: It forms when a light wind of very cold air
mixes with a shallow layer of saturated warm air
immediately above the warmer water (like steam flowing).
It is found in the Great Lakes Region (USA).

❖ Hill Fog: It is formed when winds blow air up a slope, adiabatically cooling it as it rises, and causing the
moisture in it to condense (Formed when moist air contacts cold hill slope). This often causes freezing fog on
mountaintops, where the cloud ceiling would not otherwise be low enough.
❖ Radiation Fog: It results from radiation and cooling of the ground and adjacent air. These fogs are not very
thick. It is usual in the winter. The conditions for formation are the following-
➢ Long nights (winters)
➢ Cloudless or clear sky
➢ Clam conditions of wind
➢ Presence of moisture in the air.
 7

FOG CLASSIFICATION

Summary:

Fog Type Description

Radiation Fog Forms on clear, calm nights when the Earth's surface loses heat rapidly, cooling the air near the
ground to its dew point. Common in valleys and low-lying areas.

Advection Occurs when warm, moist air moves over a colder surface (e.g., cold ocean current). The warm
Fog air cools, condenses, and forms fog. Common along coastlines.

Hill Fog Forms when moist air is lifted over elevated terrain, leading to cooling and condensation. Also
known as upslope fog.

Sea Fog Develops when warm, moist air from the sea moves over a colder ocean surface, causing the
air to cool and fog to form. Common in coastal areas with temperature differences.

Steam Fog Results from cold air passing over warmer water, causing the water to evaporate rapidly and
condense into fog. Often seen over lakes and rivers during cold weather.

Frontal Fog Associated with the lifting of warm, moist air over a colder air mass along a frontal boundary.
Forms in the vicinity of weather fronts.
 8

Mist:
❖ The difference between mist and fog is that mist contains more moisture than fog.
❖ In the mist, each nuclei contains a thicker layer of moisture.
❖ Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface.
❖ Mist is also formed by water droplets, but with less merging or coalescing. This means mist is less dense and
quicker to dissipate.
❖ Fogs are drier than mist and they are prevalent where warm currents of the air come in contact with cold
currents.
❖ Fogs are mini clouds in which condensation takes place around nuclei provided by the dust smoke and the salt
particles.
❖ In the mist, visibility is more than one kilometer but less than two kilometres.

Haze:
❖ Haze is an atmospheric phenomenon where dust, smoke, salt, ashes, gasses and other dry particles obscure
the clarity of the sky. (No condensation, no moisture,only pollutants)
❖ Smog is similar to haze but there is condensation in smog.
❖ Moisture is present in case of smog.
❖ Sources for haze particles include farming, vehicle traffic, industry, and wildfires.
❖ Pollutants such as dust, soot, smoke etc. are suspended in the air due to construction, coal burning, vehicle
pollution industries and wildfires.

Smog:
❖ Smog is a combination of smoke and fog (smoky fog) caused by the burning of large amounts of coal,
vehicular emission and industrial fumes (Primary pollutants).
❖ Smog contains soot particulates like smoke, sulphur dioxide, Nitrogen dioxide and other components.
❖ At Least two distinct types of smog are recognised: Sulphurous smog and photochemical smog.

Clouds:
❖ Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water
vapour in free air at considerable elevations.
❖ These are the accumulates or aggregates of microparticles of water and ice.
❖ It is formed due to the condensation of water particles.
❖ Clouds are caused mainly by the adiabatic cooling of air below its dew point.
❖ As the clouds are formed at some height over the surface of the earth, they take various shapes.
❖ According to their height, expanse, density, and transparency or opaqueness clouds are grouped under four
types:(i) cirrus; (ii) cumulus; (iii) stratus; (iv) nimbus.
 9

❖ There are both high clouds and low clouds.

➢ The low clouds reflect the incoming radiation (insolation).
➢ The high clouds absorb the long wave radiation from the earth.
❖ The weight of the clouds is in billion tons. The primary function of the clouds is to maintain the heat budget.
❖ These also cause precipitation in the form of rain, snowfall, etc.
❖ Cloudburst, a sudden, very heavy rainfall, usually local in nature and of brief duration.
❖ Color: Generally white, gray, black, yellow etc.
❖ Weight: millions of tons, when the moisture exceeds the capacity of the cloud it leads to rainfall.
❖ They are dynamic, and carried away by winds.
❖ They can move at a slow and very high speed (V=240 km/hr) too.
❖ Clouds contain water droplets, ice particles and hygroscopic nuclei (Dust, smoke, soot pollens etc.)

Types of Clouds (Based on Shapes and Size):

Type of Cloud Features

Cirrus ❖ Cirrus clouds are formed at high altitudes (8,000 - 12,000m).

❖ They are thin and detached clouds having a feathery appearance.
❖ They are always white in colour.
❖ No rain.

Cumulus ❖ Cumulus clouds look like cotton wool.

❖ They are generally formed at a height of 4,000 - 7,000 m.
❖ They exist in patches and can be seen scattered here and there.
❖ They have a flat base.
 10

Type of Cloud Features

❖ It is a cloud of combination and causing rain.

❖ For example Cumulous, and cumulonimbus.

Stratus ❖ Low altitude 1000-4000 m.

❖ As their name implies, these are layered clouds covering large portions of the
sky.
❖ This is blanket type.
❖ Frontal clouds are mostly stratus clouds.
❖ Temperate cyclones are also there.
❖ These clouds are generally formed either due to loss of heat or the mixing of
air masses with different temperatures.

Nimbus ❖ Nimbus clouds are black or dark gray.

❖ They form at middle levels or very near the surface.
❖ These are extremely dense and opaque to the rays of the sun. Sometimes the
clouds are so low that they seem to touch the ground.
❖ They are causing heavy rainfall and tropical cyclones.
❖ They are shapeless masses of thick vapour.

Types of Clouds (Based on Height):

❖ A combination of these four basic types can give rise to the following types of clouds:
➢ High clouds – Cirrus, cirrostratus, cirrocumulus;
➢ They primarily reflect the outgoing long wave terrestrial radiation and cause heating.

➢ Middle clouds – Altostratus and altocumulus;

➢ Low clouds – Stratocumulus and nimbostratus (long duration rainfall cloud) and
➢ Clouds with extensive vertical development – Cumulus and Cumulonimbus (thunderstorm cloud).
➢ Low clouds primarily reflect the incoming solar radiation and promote cooling of the earth 's atmosphere.
 11

Primary and Secondary Pollutant:

❖ A primary pollutant is an air pollutant emitted directly from a source.
❖ A secondary pollutant is not directly emitted as such, but forms when other pollutants (primary pollutants)
react in the atmosphere.
❖ Examples of a secondary pollutant include ozone, which is formed
➢ When hydrocarbons (HC) and nitrogen oxides (NOx) combine in the presence of sunlight;
➢ Does not combines with oxygen in the air; and
➢ Acid rain, which is formed when sulfur dioxide or nitrogen oxides react with water.

Sulphurous Smog:
❖ Sulfurous smog is also called "London smog," (first formed in London).
❖ Sulfurous smog results from a high concentration of Sulfur Oxides in the air and is caused by the use of sulfur-
bearing fossil fuels, particularly coal (Coal was the main source of power in London during the nineteenth
century. The effects of coal burning were observed in the early twentieth century).
❖ This type of smog is aggravated by dampness and a high concentration of suspended particulate matter in the
air.
Photochemical Smog:
❖ Photochemical smog is also known as "Los Angeles smog”.
❖ Photochemical smog occurs in urban areas that have large numbers of automobiles, factories, (Nitrogen
oxides are the primary emissions).
❖ It requires neither smoke nor fog. This type of smog has its origin in the nitrogen oxides and hydrocarbon
vapours emitted by automobiles and other sources, which then undergo photochemical reactions in the lower
atmosphere.
❖ Nitrogen Dioxide + Sunlight + Hydrocarbons
Ozone (Ozone in the stratosphere is beneficial, but
near the earth's surface it results in global warming as
it is a greenhouse gas).
❖ Reactions involved: When fossil fuels are burnt, a
variety of pollutants are emitted into the earth's
troposphere. Two of the pollutants that are emitted are
hydrocarbons (unburned fuels) and nitric oxide (NO).
❖ Surface-level ozone concentrations are considered
unhealthy if they exceed 70 parts per billion for eight
hours or longer; such conditions are fairly common in
urban areas prone to photochemical smog.
Effects of Photochemical Smog:
❖ Photochemical smog causes serious health problems. Both Ozone and PAN act as powerful eye irritants.
❖ Ozone and nitric oxide irritate the nose and throat and their high concentration causes headache, chest
pain, dryness of the throat, cough and difficulty in breathing.
 12

❖ Photochemical smog leads to cracking of rubber and extensive damage to plant life. It also causes
corrosion of metals, stones, building materials, rubber and painted surfaces.

Effects of Smog:
❖ The atmospheric pollution levels of Los Angeles, Beijing, Delhi, Mexico City and other cities are increased
by inversion that traps pollution close to the ground.
❖ It is usually highly toxic to humans and can cause severe sickness, shortened life or death.
❖ Smog is a combination of airborne particulate matter, like soot, and invisible toxic gasses including ozone
(03), carbon monoxide (CO), sulfur dioxide (S02), which are carcinogens (cancer causing agents).
❖ Temperature inversions are accentuated and precipitation is reduced.
❖ Smog related Haze lowers visibility.

   
 1

DAILY
CLASS NOTES
Geography

Lecture - 40
Precipitation
 2

Precipitation
Precipitation:
 The process of continuous condensation in free air helps the condensed particles to grow in size. When the
resistance of the air fails to hold them against the force of gravity, they fall onto the earth’s surface.
 Thus, after the condensation of water vapour, the release of moisture is known as precipitation.
 This may take place in liquid or solid form.

 Precipitation in the form of drops of water is called rainfall when the drop size is more than 0.5 mm.
 It is called Virage when raindrops evaporate before reaching the earth while passing through dry air.
 Drizzle is light rainfall with a drop size being less than 0.5 mm, and when evaporation occurs before
reaching the ground, leading to foggy conditions. It is referred to as Mist.
 When the temperature is lower than 0° C, precipitation takes place in the form of fine flakes of snow and is
called Snowfall.
 Moisture is released in the form of hexagonal crystals.
 These crystals form flakes of snow. Besides rain and snow, other forms of precipitation are Sleet and Hail,
though the latter is limited in occurrence and are sporadic in both time and space.
 Sleet is frozen raindrops and refrozen melted snow water. When a layer of air with a temperature above
freezing point overlies a sub freezing layer near the ground, precipitation takes place in the form of sleet.
 Raindrops, which leave the warmer air, encounter the colder air below. As a result, they solidify and reach
the ground as small pellets of ice not bigger than the raindrops from which they are formed.
Sometimes, drops of rain after being released by the clouds become solidified into small rounded solid
pieces of ice and reach the surface of the earth. These are called Hailstones.
 These are formed by the rainwater passing through the colder layers.
 Hailstones have several concentric layers of ice one over the other.
 3

Type of Precipitation:

Precipitation Description Formation Process

Type

Rainfall Precipitation in the form of water Condensation of water vapor into larger liquid droplets,
drops, with drop size exceeding 0.5 typically occurring at temperatures above freezing (0°C).
mm.

Virga Raindrops evaporate before Raindrops that undergo evaporation due to dry air
reaching the Earth while passing conditions, often not reaching the ground and creating
through dry air. streaks in the sky.

Drizzle Light rainfall with drop size less Consists of small, fine liquid droplets that are less than
than 0.5 mm, leading to foggy 0.5 mm in size and can create foggy or misty conditions
conditions when evaporation when they evaporate before reaching the ground.
occurs.

Snowfall Precipitation in the form of fine Moisture freezes into hexagonal ice crystals, forming
snowflakes when the temperature is delicate snowflakes that fall to the ground.
below 0°C.

Sleet Frozen raindrops and refrozen Occurs when raindrops leave a warmer air layer and
melted snow water. solidify upon encountering sub-freezing temperatures
near the ground. They reach the ground as small ice
pellets.

Hailstones Solidified raindrops formed into Develop when drops of rain are lifted into colder
small rounded pieces of ice. atmospheric layers, freezing into small, rounded pieces
of ice with concentric layers. This typically occurs within
severe thunderstorms.
 4

Types of Rainfall:
 On the basis of origin, rainfall may be classified into three main types – convectional, orographic or
relief, and cyclonic or frontal.
 Coalescence is the process by which two or more droplets, bubbles or particles merge during contact to form
a single daughter droplet, bubble or particle.
1. Convectional Rainfall:
 The air on being heated, becomes light and rises up in
convection currents. As it rises, it expands and loses heat
and consequently, condensation takes place and cumulus
clouds are formed (Cumulonimbus).
 This process releases latent heat of condensation which
further heats the air and forces the air to go further up.
 Convectional precipitation is heavy but of short duration,
highly localized, and is associated with a minimum amount
of cloudiness.
 Conditions:
 Hot ground surface
 Huge moisture
 It occurs mainly during summer or in the hotter part of the day and is common in the equatorial regions
and interior parts of the continents, particularly in the northern hemisphere.
 Features:
 Daily rainfall in the afternoon
 5

 Short duration (quickly)

 Thick cloud - Cumulonimbus clouds
 Thunder and lightning
 Lush green luxurious vegetation- Evergreen forest (Indonesia, Brazil, Ecuador, D.r of Congo,central
america,mexico)
2. Orographic Rainfall
 When the saturated air mass comes across a
 mountain,it is forced to ascend and as it rises,
 it expands (because of a fall in pressure); the
 temperature falls,and the moisture is condensed.
 This type of precipitation occurs when warm,
 humid air strikes an orographic barrier
 (a mountain range) head-on.Because of the
 initial momentum, the air is forced to rise.
 As the moisture-laden air gains height,
 Condensation Sets in, and soon saturation is
 reached.
 The surplus moisture falls down as orographic
 precipitation along the windward slopes.
 Conditions:
 Mountain barrier across the path of winds
 Sufficient moisture in the winds
 Sea coast near mountains for the presence of moisture
 Sufficient height so that air parcel can rise and condense
 The chief characteristic of this type of rain is that the (Cumulonimbus clouds) windward slopes receive
greater rainfall.
 After giving rain on the windward side, when these winds reach the other slope, they descend, and their
temperature rises.
 Then their capacity to take in moisture increases and hence, these leeward slopes remain rainless and
dry.
 The area situated on the leeward side, which gets less rainfall is known as the (Nimbostratus clouds) Rain-
shadow area (Some arid and semi-arid regions are a direct consequence of the rain-shadow effect.
 It is also known as the relief rain.
 Features:
 Coastal region - more rainfall
 Windward side- Cumulus clouds more rainfall, leeward side stratus clouds less rainfall
 Foothills- more rainfall, plains low rainfall
 6

 Any season - Northwest monsoon in winter, southwest monsoon in summer

 Example: Patagonian desert in Argentina, Eastern slopes of Western Ghats).

 Example: Mahabaleshwar, situated on the Western Ghats, receives more than 600 cm of rainfall, whereas
Pune, lying in the rain shadow area, has only about 70 cm.
3. Frontal Precipitation:
 When two air masses, due to
contrasting temperatures and
densities clash with each other,
condensation and precipitation
occur at the surface of contact.
This surface of contact is
called a ‘front’ or
 ‘frontal surface’.
 Conditions: Presence of air
mass, existence of fronts.
 For instance, in north-west
Europe, cold continental air and
warm oceanic air converge to
produce heavy rainfall in
adjacent areas.
 If a cold air mass drives out a
warm air mass’ it is called a
‘cold front’ and if a warm air
mass replaces the retreating cold
air mass, it is called a ‘warm
front’.
 7

 If the two air masses are drawn simultaneously towards a low-pressure area, the front developed is stationary
and is called a ‘stationary front’.
 Cold front causes intense precipitation in comparatively small areas, while the precipitation due to a
warm front is less intense but is spread over a comparatively larger area.
 Cold fronts move faster than warm fronts and usually overtake them, the frontal surfaces of cold and warm
air sliding against each other. This phenomenon is called ‘occlusion’ and the resulting frontal surface is
called an ‘Occluded front’.
Summary:

Precipitation Convectional Rainfall Orographic Rainfall Frontal Precipitation

Type

Description Air heated, rises, expands, Moist air forced to rise over Collision of contrasting air
condenses, forming cumulus mountains, expands, cools, masses causes condensation
clouds. condenses, and falls as and precipitation along the
precipitation. front.

Conditions Hot ground, high moisture, Mountain barrier, sufficient Presence of air masses,
summer/hotter part of the day, moisture in winds, proximity to existence of fronts.
equatorial regions, and sea coast, height for air to rise
interior continents. and condense.

Features Afternoon rainfall, short Coastal regions receive more Cold front (intense but small
duration, Cumulonimbus rainfall, the windward side gets area), warm front (less
clouds, thunder and lightning, more rain, and the leeward side intense but larger area),
lush green vegetation. (rain-shadow) remains dry. stationary front, occluded
front.

World distribution of rainfall:

 8

 Different places on the earth’s surface receive different amounts of rainfall in a year and that too in different
seasons.
 In general, as we proceed from the equator towards the poles, rainfall goes on decreasing steadily.
 The coastal areas of the world receive greater amounts of rainfall than the interior of the continents.

 The rainfall is more over the oceans than on the landmasses of the world because of being great sources of
water.
 Between the latitudes 35° and 40° North and South of the equator, the rain is heavier on the eastern
coasts and goes on decreasing towards the west.
 But, between 45° and 65° North and South of the equator, due to the westerlies, the rainfall is first
received on the western margins of the continents and it goes on decreasing towards east.
 Wherever mountains run parallel to the coast, the rain is greater on the coastal plain on the windward side
and it decreases towards the leeward side.
 On the basis of the total amount of annual precipitation, major precipitation regimes of the world are
identified as follows:
 The equatorial belt, the windward slopes of the mountains along the western coasts in the cool
temperate zone, and the coastal areas of the monsoon land receive heavy rainfall of over 200 cm per
annum.
 Interior continental areas receive moderate rainfall varying from 100-200 cm per annum. The coastal
areas of the continents receive a moderate amount of rainfall.
 The central parts of the tropical land and the eastern and interior parts of the temperate lands
receive rainfall varying between 50 - 100 cm per annum.
 Areas lying in the rain shadow zone of the interior of the continents and high latitudes receive very
low rainfall - less than 50 cm per annum.
 Seasonal distribution of rainfall provides an important aspect to judge its effectiveness. In some regions
rainfall is distributed evenly throughout the year such as in the equatorial belt and in the western parts of
cool temperate regions.
 9

Latitudes Rainfall Pattern

0-10 degrees Latitude (Doldrum) Mainly convectional rainfall, along with lightening and
thunder.

10-20 degrees Latitude (Influence of Easterlies) Rainfall on eastern part of the continent.

20-30 degrees Latitude (Sub tropical high-pressure Cause minimum rainfall in the region.
belt and descending winds)

30-40 degrees Latitude (Influence of Westerlies) Rainfall on the western margin of the continent.

40-45 degrees Latitude (Mid-latitude region) Both the Westerlies and Temperate cyclones cause
rainfall.

55-65 degrees Latitude (Sub polar low-pressure Frontal rainfall

zone)

65 degrees Latitude and Beyond Minimum rainfall, maximum precipitation in the form of
snowfall.


 1

DAILY
CLASS NOTES
Geography

Lecture - 41
Thunderstorm and Tornado
 2

Thunderstorm and Tornado

Thunderstorm:

❖ Thunderstorms are storms ranging several kilometers in diameter, created by the rapid lifting of moist
and warm air, as a result of which a dense vertical tower of the cloud is created.
❖ It is strong updrafts of air, and it is associated with intense rainfall.
❖ Thunderstorms are associated with strong winds, hail, lightning, tornadoes, thunder, and heavy rain, and
also associated with large, dense, gray Cumulonimbus clouds.
❖ There are many factors that lead to the uplifting of air, like solar heating, low-pressure troughs, meeting
of two different air streams, or when air is forced uphill.
❖ When humid air is lifted, it gets cooled and the moisture in the air condenses to form clouds. Upon further
uplifting, the cloud will extend higher.
 3

❖ Water droplets in the cloud continue to grow in size. As the cloud extends further upward, ice crystals may
form because of the low temperature there.
❖ A cumulonimbus cloud results when it grows to a height of 10 to 20 kilometers. Thunderstorms are
produced by cumulonimbus clouds.
❖ Thunderstorms have several convective cells.
❖ Thunderstorms normally develop in late afternoon hours when surface heating produces the maximum number
of convection currents in the atmosphere.
❖ Worldwide, there are an estimated 15 million thunderstorms each year, and at any given moment, there are
roughly 2,000 thunderstorms in progress.
❖ Thunderstorms mostly occur on the ground where the temperature is high. Thunderstorms are less
frequent on water bodies due to low temperatures.

NOTE:
Conditions of thunderstorms formation:
❖ Strong updraft of air.
❖ Availability of Warm and Moist air.
❖ Atmospheric instability.
❖ Thicker clouds(Dense clouds).

Thunderstorm Life Cycle:

1. Cumulus Stage (Youth Stage):
 4

➢ Thunderstorm is starting at this stage.

➢ Ground is significantly heated due to solar insolation.
➢ A low pressure starts to establish due to intense upliftment of an air parcel (convection).
➢ Air from the surroundings starts to rush in to fill the low pressure.
➢ Intense convection of moist hot air builds up a towering cumulonimbus cloud.
2. Mature Stage:
➢ In this stage Thunderstorms are fully developed.
➢ This stage is characterized by the intense updraft
of rising warm air, which causes the clouds to
grow bigger and rise to a greater height.
➢ In this stage some hailstorm is also there.
➢ Later, downdraft brings down to earth the cool
air and rain.
➢ The incoming thunderstorms are indicated by
violent gusts of wind. This wind is due to the
intense downdraft (Downward movement of the air).
➢ We can say Downward motion of cold air starts.
➢ The updraft and downdraft determine the path of the thunderstorm. Most of the time, the path is erratic.
 5

3. Dissipating Stage:
➢ In this stage
Thunderstorms are
subsiding.
➢ Dissipation stage is
downward motion of
air, and spread over the
surface.
➢ In this stage, suppress
the upward motion of
warm air because the
surface starts cooling.
➢ In this stage the clouds
are Cirrostratus,
Altostratus, and intense downdraft is there.
➢ When the clouds extend to heights where sub-zero temperature prevails, hails are formed and they come
down as hailstorms. As a result, intense precipitation occurs.
➢ In a matter of a few minutes, the storm dissipates and clear weather starts to prevail.

NOTE:
❖ Insolation, the process of solar radiation heating the surface, initiates a rise in surface temperature.
❖ This increase in temperature, in turn, warms the surrounding moist air, resulting in the formation of
powerful updrafts.
❖ These updrafts facilitate the condensation of water vapor, leading to the creation of clouds.
❖ The culmination of this atmospheric process results in the development of thunderstorms, which
encompass various elements such as precipitation (including rain and hail), thunder (characterized by
explosive sounds and intense rumbling), and lightning ( flashes of light and currents flow).
 6

Motion of Thunderstorm:
❖ Motion of thunderstorms is due to interactions of its
updrafts and downdrafts. Path of a thunderstorm is
erratic.
❖ The speed of isolated storms is typically about 20 km (12
miles) per hour, but some storms move much faster.
❖ In extreme circumstances, a supercell storm may move 65
to 80 km (about 40 to 50 miles) per hour.
Downbursts:
❖ Downdrafts are referred to as macrobursts or
microbursts.
❖ Macroburst is more than 4 km in diameter and can
produce winds as high as 60 meters per second, or 215 km
per hour.
❖ A microburst is smaller in dimension but produces winds as high as 75 meters per second, or 270 km/hour
➢ They are hazardous to aircraft, especially during takeoffs and landings.
 7

Types of Thunderstorms:
1. Thermal/Convective Thunderstorm:
➢ It is caused due to intense heating of ground
during the summer season.
➢ They are prominent in the equatorial regions.
➢ It happens in Amazon, Venezuela, Kerala (Mango
shower), Karnataka (Blossom showers).
2. Orographic Thunderstorm:

➢ Forceful upliftment of warm moist air parcels when it passes over a mountain barrier creates a
cumulonimbus cloud causing heavy precipitation on the windward side.
➢ Orographic ‘Cloud bursts’ are common in Jammu and Kashmir, Cherrapunji, and Mawsynram,
Western Ghats.
➢ Lake Maracaibo is also an example where this type of thunderstorm is prominent.
3. Frontal Thunderstorm:
➢ These are thunderstorms occurring along cold fronts
(Due to interaction of air masses).
➢ Example: In the United Kingdom, France.
 8

SUMMARY:

Thunderstorm Type Description Examples

Thermal/Convective ❖ Caused by intense ground heating during the summer Amazon,

Thunderstorm season. Venezuela,
❖ Prominent in equatorial regions. Kerala,
❖ Examples include Amazon, Venezuela, Kerala (Mango Karnataka
showers), and Karnataka (Blossom showers).

Orographic ❖ Caused by the forceful upliftment of warm moist air parcels Jammu and
Thunderstorm when passing over a mountain barrier, leading to Kashmir,
cumulonimbus cloud formation and heavy precipitation on Cherrapunji,
the windward side. Mawsynram,
❖ Orographic 'cloud bursts' are common in Jammu and Kashmir, Western
Cherrapunji, and Mawsynram in the Western Ghats. Ghats, Lake
❖ Lake Maracaibo is another prominent example. Maracaibo

Frontal Thunderstorm ❖ Occur along cold fronts due to the interaction of different air United
masses. Kingdom,
❖ Examples include thunderstorms in the United Kingdom and France
France.

Single-cell Thunderstorm
❖ Single-cell thunderstorms are small, brief, weak storms that grow and die within an hour or so. They are
typically driven by heating on a summer afternoon.
❖ Single-cell storms may produce brief heavy rain and lightning, very common in India during summers,
mostly April, and May.
❖ In Kerala they are called 'Mango Showers' and in Karnataka 'Blossom showers'.
❖ These storms can set up a better environment for stronger storms, but are themselves not often severe.
❖ These storms generally occur in a lower moisture environment where the storms stay more isolated from
each other.
❖ Most of these single-cell storms are slow moving and some can be nearly stationary.
 9

A Multi-cell Thunderstorm
❖ A multi-cell storm is a thunderstorm in which new updrafts form along the leading edge of rain-cooled air
(the gust front).
❖ Individual cells usually last 30 to 60 minutes, while the system as a whole may last for many hours.
❖ Multicell storms may produce hail, strong winds, brief tornadoes, and flooding.
Lightning and Thunder:
❖ As water vapour moves upward,
decreasing temperatures causes it to
condense.
❖ The heat (the latent heat of
condensation) generated in the
process pushes the water molecules
further up.
❖ As they move beyond zero degrees,
water droplets change into small ice
crystals.
❖ As they continue to move up, they
gather mass until they are so heavy that they start to fall.
❖ This leads to a system where smaller ice crystals move up while bigger crystals come down.
❖ The resulting collisions trigger the release of electrons, in a process very similar to the generation of electric
sparks (this is called ionization – an electron in the outer shell is peeled out of the atom and the atom becomes
an ion).
 10

❖ There are two types of ions based on charge – cation and anion.
➢ Cation: A cation is an atom
or a molecule which is
positively charged, i.e. has
more number of protons than
electrons.
➢ Anion: An anion is an atom
or molecule which is
negatively charged, i.e. has
more number of electrons
than protons. The moving
free electrons cause more
collisions and more
electrons, as a chain reaction
ensues.
❖ The process results in a situation
in which the top layer of the cloud gets positively charged (cations) while the middle layer is negatively
(anions) charged.
❖ The electrical potential difference between the two layers is huge, of the order of 109 or 1010 volts.
❖ In a little time, a huge current, of the order of 105 to 106 amperes, starts to flow between the layers.
❖ It produces heat, leading to the heating of the air column between the two layers of the cloud.
❖ It is because of this heat that the air column looks red during lightning.
❖ The heated air column expands and produces shock waves that result in thunder.

NOTE:
❖ Updraft and downdraft cause polarization of particles, and it leads to two sets of charged particles.
❖ Positively and negatively charged things are the creation of potential difference (Voltage).
❖ Voltage causes the current flows, and current is the flow of electrons.

Lightning from Cloud to Earth:

❖ Earth is a good conductor of electricity but is electrically neutral.
❖ In comparison to the middle layer of the cloud, however, it becomes positively charged.
❖ As a result, a flow of current (about 20-15%) gets directed towards the Earth as well.
❖ It is this current flow that results in damage to life and property. There is a greater probability of lightning
striking tall objects such as trees, towers or buildings.
❖ Once about 80-100 m from the surface, lightning tends to change course to hit taller objects (guess why very
tall buildings have a vertical pole above).
❖ This is because travelling through the air, which is a bad conductor of electricity, electrons try to find a better
conductor, and also the shortest route to the relatively positively charged Earth's surface.
 11

❖ Several thousand thunderstorms occur over India every year.

❖ Incidents of lightning have been showing an increasing trend over the last 20 years, especially near the foothills
of the Himalayas.
❖ People are rarely hit directly by lightning. But such strikes are almost always fatal. The most common way in
which people are struck by lightning are by 'ground currents'.
❖ The electrical energy, after hitting a tree or any other object, spreads laterally on the ground for some distance,
and people in this area receive electrical shocks. It becomes more dangerous if the ground is wet, or there is
conducting material like metal on it.
Prediction and Precautions:
❖ Predicting a thunderstorm over a very precise location is not possible. Nor is the exact time that it is likely to
strike.
❖ People are advised to move indoors in a storm.
❖ Moving under a tree or lying flat on the ground can increase risks.
❖ Even indoors, electrical fittings, wires, metal and water must be avoided.
The World's Most Electric Place:
❖ The most lightning activity on Earth is seen on the shore of Lake Maracaibo in Venezuela.
❖ At the place where the Catatumbo River falls. into Lake Maracaibo, sees 28 lightning flashes every
minute, a phenomenon referred to as the Beacon of Maracaibo or the Everlasting Storm.
❖ The reason probably lies in the topography of the spot: winds blow across Lake Maracaibo -the largest in
South America (By volume of water, Titicaca is the largest lake in South America).
❖ Lake Maracaibo has a larger surface area, (though some consider it to be a large brackish bay due to its direct
connection with the sea) which is surrounded by swampy plains and connected to the Gulf of
Venezuela/Caribbean Sea by a very narrow strait.
❖ The Maracaibo plain is enclosed on three sides by high mountain sides into which air masses crash.
❖ The heat and moisture picked from the swampy plains creates electrical charges and, as the air is destabilized
at the mountain faces, thunderstorm activity characterized by almost non-stop lightning activity within clouds
results.
Deadly Strikes:
❖ Direct Strike: Occurs most often in open areas.
❖ Side Flash (Or Side Splash): Occurs when lightning strikes a taller object and some current jumps on to
the victim, who ends up acting as a "short circuit" for the energy.
❖ Generally occurs when the victim is within a foot or two of the struck object. Most victims are those sheltering
under a tree in a rainstorm.
❖ Ground Current: When an object is struck, much of the energy travels outward in and along the ground
surface. This is 'ground current', and anyone close can be a victim.
❖ Ground current affects a larger area than other kinds of current and causes the most lightning deaths and
injuries.
❖ Conduction: Lightning can travel long distances in wires or other metal surfaces.
 12

❖ Most indoor lightning casualties and some outdoor casualties are due to conduction.
Features of Lightning:
❖ Positive charge accumulates at both higher and lower altitudes.
❖ Larger and heavier cloud particles charge with a negative polarity.
❖ Smaller and lighter clouds particles charge with a positive polarity.
❖ Roughly two-thirds of all discharges occur within the cloud. The rest are between the cloud and the ground.
Thunder:
❖ Lightning creates plasma (ionized gas medium) [Temperature as high as 30,000 °C].
❖ The channel pressure greatly exceeds the ambient (surrounding) pressure, and the channel expands at a
supersonic rate (speed of sound).
❖ The resultant shock wave decays rapidly with distance and is eventually heard as thunder once it slows to the
speed of sound.
Tornado:
❖ Tornado is a violently rotating wind/air column touching ground and connected with the Thunderstorm.
❖ From severe thunderstorms sometimes spiraling wind descends like the trunk of an elephant with great
force, with very low pressure at the center, causing massive destruction on its way. Such a phenomenon is
called a tornado.
❖ Tornadoes generally occur in middle latitudes. The tornado over the sea is called a water sprout.
❖ A tornado is a violent windstorm characterized by a twisting, funnel-shaped cloud.
❖ In the United States, twister is used as a colloquial term for tornadoes.
❖ A tornado is a rotating column of air that is in contact with both the surface of the earth and a cloud,
which is generally cumulonimbus and occasionally cumulus.
 13

❖ It occurs with rainfall, and Hailstorm, and it is a quick phenomena.

❖ These whirling atmospheric vortices can generate the strongest winds
known on Earth-wind speeds in the range of 500 km (300 miles) per
hour.
❖ They are often referred to as twisters or cyclones.
❖ These violent storms are the manifestation of the atmosphere's
adjustments to varying energy distribution.
❖ The potential and heat energies are converted into kinetic
energy in these storms and the restless atmosphere again returns
to its stable state.
❖ Tornado is a small-diameter column of violently rotating air
developed within a convective cloud and in contact with the
ground.
❖ Tornadoes occur most often in association with
thunderstorms during the spring and summer (Early may
to july) in the mid-latitudes (Mostly in Canada, USA,
Australia, New Zealand) of both the Northern and Southern
Hemispheres.
❖ Tornadoes rarely occur in polar regions.
❖ Roughly 1200 tornadoes hit the USA annually.
How are Tornadoes formed?
❖ First the rotating cloud base lowers.
❖ This lowering becomes a funnel, which continues descending while winds build near the surface, kicking
up dust and other debris.
❖ Finally, the visible funnel extends to the ground, and the tornado begins causing major damage.
Distribution of Tornadoes:
❖ Rare in polar regions and infrequent at latitudes higher than 50° North and 50° South.
❖ The temperate and tropical regions are the most prone to thunderstorms.
❖ Tornadoes have been reported on all continents except Antarctica.
❖ The United States has the most violent tornadoes.
❖ Canada reports the second-largest number of tornadoes.
❖ In the Indian subcontinent, Bangladesh is the most prone country to tornadoes.
❖ At any moment there are approximately 1,800 thunderstorms in progress throughout the world.
 14

How are they detected?

❖ Tornadoes can be detected before or as they occur through the use of Pulse-Doppler radar by recognizing
patterns in velocity and reflectivity data.
What is Fujitsa and Torro Scale?
❖ Fujitsa scale rates tornadoes by damage caused, and has been replaced in some countries by the updated
Enhanced Fujita Scale.
❖ FO or EFO is the weakest tornado, while F5 or EF5 is the strongest tornado.
❖ TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known
tornadoes.


 1

DAILY
CLASS NOTES
Geography

Lecture - 42
Air Masses of the World
 2

Air Masses of the World

Air Masses:
❖ Air mass is a large body (high volume) of air with an area of around 1000 Sq. Km.
❖ It has distinct characteristics with respect to temperature and humidity.
❖ Its height extends from the surface to the lower stratosphere.
❖ There is very little or nil change
(negligible) in the temperature
horizontally.
❖ Homogeneous surfaces from where
air masses originate.
❖ One air mass interacts (front
formation) with another air mass, and
causes climatic phenomena.
❖ When the air remains over a
homogenous area for a sufficiently
longer time, it acquires the
characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains and plateaus.
❖ The air with distinctive characteristics in terms of temperature and humidity is called an Air Mass. It is
a large body of air having little horizontal variation in temperature and moisture.
❖ Air masses form an integral part of the global planetary wind system. Therefore, they are associated with
one or other wind belts.
❖ They extend from surface to lower stratosphere and are spread across thousands of kilometers.
Sources of Air Mass:

❖ The homogeneous surfaces, over which air masses form, are called the source regions.
❖ The main source regions are the high pressure belts in the subtropics (giving rise to tropical air masses) and
around the poles (the source for polar air masses).
❖ Source Region establishes heat and moisture equilibrium with the overlying air mass.
 3

❖ When an air mass moves away from a source region, the upper level maintains the physical characteristics for
a longer period.
➢ This is possible because air masses are stable with stagnant air which does not facilitate convection.
➢ Conduction and radiation in such stagnant air is not effective.
❖ There are five major source regions. These are:
➢ Warm tropical and subtropical oceans;
➢ The subtropical hot deserts;
➢ The relatively cold high latitude oceans;
➢ The very cold snow covered continents in high latitudes;
➢ Permanently ice covered continents in the Arctic and Antarctica.

Conditions for the Formation of Air Masses:

❖ Source region should be large and extensive areas with gentle, divergent air circulation (slightly at high
pressure).
❖ There should be uniformity in meteorological conditions on the surface over which it is present (Uniform
weather conditions, which means weather must not change near the surface).
❖ The air mass should be provided with sufficient time (ample amount of time) to adapt to the
characteristics (Temperature and Humidity) of the surface.
❖ Plain topography will generally assist their formation ( To avoid the change in temperature and pressure).
E.g temperature inversion will not happen.
❖ Areas with high pressure but little (less) pressure difference or pressure gradient are ideal source regions.
❖ Low-Pressure gradient (avoid wind movement within air mass) with gentle high pressure.
❖ There are no major source regions in the mid-latitudes as these regions are dominated by cyclonic and other
disturbances.

Types of Air Mass:

❖ Air masses can be divided into two main
categories:
➢ If the air mass is found over land,
this is continental air mass.
➢ If the air mass is found over the
water, this is maritime air mass.
❖ Continental air masses occur over the
continents, maritime air masses occur
over the water or marine environments.
❖ These categories are represented by a
lowercase 'c' for continental or 'm' for
maritime.

AIR MASS CLASSIFICATION:

 4

1. Based on Temperature:
❖ Cold Air Mass:
➢ It is the air mass whose temperature is lower than that of the areas/regions visited or it passes
over (Temperature of cold air masses is less than temperature of regions).
➢ It will be low temperature, heavy, dry (relative humidity will be low) and creates stability
conditions.
➢ Example: The air mass in the Arctic will be cold and moist (because of the oceanic area), in Canada
and Siberia it will be cold and dry (because of the continental area).
❖ Warm Air Mass:
➢ It is the air mass whose temperature is higher than the temperature of the region/area visited or
the region it passes over ( Temperature of warm air masses is higher than temperature of
regions).
➢ It will be high temperature, lighter and create instability. There will be clouds and precipitation.
➢ Example: In tropical oceans the air mass will be moist and warm like in the Gulf of Mexico, the
air mass in deserts will be warm and dry like Sahara, Kalahari, Thar etc.
2. Based on Geographical Location:
❖ Continental Air Mass:
➢ It occurs on land.
➢ If it occurs in tropics it is known as Continental Tropical Air Mass (cT). Example: Africa.
➢ If it occurs in poles it is known as Continental Polar Air Mass (cP). Example: Antarctica.
❖ Maritime Air Mass:
➢ It is formed on the ocean or water.
➢ If it occurs in the tropics it is known as Maritime Tropical Air Mass (mT). Example: Equatorial
Pacific.
➢ If it occurs in poles it is known as Maritime Polar Air Mass (mP). Example: Polar Sea.
➢ Continental Arctic: If it occurs in the continental Arctic zone it is known as the Continental Arctic
Air Mass (cA).
3. Based on Source Regions:
❖ Accordingly, the following types of air masses are recognised:
➢ Maritime tropical (mT);
➢ Continental tropical (cT);
➢ Maritime polar (mP);
➢ Continental polar (cP);
➢ Continental arctic (cA).
❖ Tropical Air Masses are warm and Polar Air Masses are cold.
❖ The heat transfer processes that warm or cool the air take place slowly.
❖ Arctic air masses occur over arctic regions, like Greenland and Antarctica.
❖ Polar air masses occur slightly farther from the poles, like in Siberia, Canada and the Northern Atlantic and
Pacific Oceans.
❖ Finally, tropical air masses occur in the tropics, so along the equator and over Mexico and the Southwest U.S.
 5

Cold Air Mass:

❖ A cold air mass is one which is colder than the underlying surface and is associated with instability and
atmospheric turbulence.
❖ Cold source regions (Polar Air Masses):
➢ Arctic Ocean - cold and moist
➢ Siberia - cold and dry
➢ Northern Canada - cold and dry
➢ Southern Ocean - cold and moist

Warm Air Mass:

❖ A Warm Air Mass is one which is warmer than the underlying surface and is associated with stable weather
conditions.
❖ Warm source regions (Tropical Air Masses)
➢ Sahara Desert - warm and dry
➢ Tropical Oceans - warm and Moist

Influence of Air Masses on Global Weather:

❖ The properties of an air mass that influence the
accompanying weather are vertical distribution
temperature (indicating its stability and coldness or
warmness) and the moisture content.
❖ The air masses carry atmospheric moisture from
oceans to continents and cause precipitation over land
masses. Air Mass plays a role in the transfer of moisture
from the equator to the poles.
❖ They transport latent heat, thus removing the latitudinal
heat balance. It transfers heat from the equator to the
poles.
❖ Most of the migratory atmospheric disturbances such
as cyclones and storms originate at the contact zone
between different air masses and the weather associated
with these disturbances is determined by the characteristics of the air masses involved.
❖ It plays a role in the front formation and formation of weather phenomena like precipitation,
thunderstorms, cyclones, fog, tornadoes and the formation of planetary winds (Wind system of the world
due to the motion of air masses).

NOTE:
❖ Heat transfer is happening due to the movement of cold air to tropics and subtropics, and warm air to poles.
❖ It causes moisture transport from tropics to poles (cold and dry air) via warm and moist air.
 6

Classification of Air Masses:

❖ Broadly, air masses are classified into polar and tropical air masses.
❖ Both the polar and the continental air masses can be either of maritime or continental types.
➢ Continental Polar Air Masses (cP)
➢ Maritime Polar Air Masses (mP)
➢ Continental Tropical Air Masses (cT)
➢ Maritime Tropical Air Masses (mT)
Continental Polar Air Masses (cP):
❖ Air mass over the land.
❖ Source regions of these air masses are the Arctic basin, North Canada, northern North America, Eurasia and
Antarctica.
❖ These air masses are characterized by dry, cold and stable conditions.
❖ The weather during winter is frigid, clear sky and stable weather conditions.
❖ During summer, the weather is less stable with the lesser prevalence of anticyclonic winds, warmer
landmasses and lesser snow.
❖ They are marked by surface high pressure, cold temperatures, and low dew points.

Maritime Polar Air Masses (mP):

❖ The source region of these air masses are the oceans lying between 40° and 60° latitudes.
❖ These are actually those continental polar air masses which have moved over the warmer oceans, got heated
up and have collected moisture.
❖ The conditions over the source regions are cool, moist and unstable. These are the regions which cannot lie
stagnant for long.
❖ The weather during winters is characterized by high humidity, overcast skies, frontal interaction and
occasional fog and precipitation, thunderstorm, and frontal cyclones.
❖ During summer, the weather is clear, fair and stable.
Continental Tropical Air Masses (cT):
❖ They are part of upper-level westerlies.
❖ The source regions of the air masses include continents (low moisture) of tropical and Subtropical (land)
Deserts of Sahara in Africa, and of West Asia and Australia, Thar, Mohave, Patagonia, and Arab desert
also.
❖ These air masses are dry, hot and stable and do not extend beyond the source.
❖ They are dry throughout the year.
❖ Continental tropical air masses originate in northern Mexico. They are characterized by clear skies and
negligible rainfall, no clouds, and calm conditions.

Maritime Tropical Air Masses (mT):

❖ The source regions of these air masses include the oceans in the tropics and sub-tropics such as the Mexican
Gulf, the tropics and subtropics of the Pacific and the Atlantic Oceans.
❖ These air masses are warm (high temperature), high relative humidity (moist) and unstable.
❖ The weather during winter has mild temperatures, and overcast skies with fog.
 7

❖ During summer, the weather is characterized by high temperatures, high humidity, cumulus clouds and
convectional rainfall, Tornadoes, thunderstorms.

Continental Polar Air Maritime Polar Air Continental Tropical Maritime Tropical Air Mass
Mass (cP) Mass (mP) Air Mass (cT) (mT)

Source regions: It Source regions: It Source regions: It Source regions: These form
occurs in North occurs between 450 – forms over warm and over tropical and subtropical
America, Eurasia, 600 latitude oceans. dry land regions with oceans.
Siberia, and Canada. Example: In the high temperatures. Examples: Indian Ocean, Gulf
Norwegian Sea, North Example: Deserts of of Mexico, Equatorial Pacific.
Sea Tropics and Sub-
Tropics such as Africa,
Arab, Thar etc.

The air mass will be The air mass is moist No clouds or The air mass will be warm and
cold and dry with and cool. precipitation will form. moist with high relative
relatively low It is a part of upper-level humidity.
humidity. westerlies.

It is a stable air mass. It creates unstable It creates stable weather It creates unstable weather
weather conditions. conditions. conditions.

The weather will be The weather will be The weather conditions The weather conditions will be
clear with a frigid zone. humid with the will be warm and dry warm and formations of clouds,
occurrence of fog and throughout the year. thunderstorms and precipitation
precipitation. will take place.
Fronts will be formed The types of clouds that will
which will lead to form are cumulus and
frontal rains, frontal cumulonimbus.
storms and frontal
cyclones.

   
 1

DAILY
CLASS NOTES
Geography

Lecture - 43
Cyclones
 2

Cyclones
Cyclones:
 These are the low-pressure systems that are surrounded by the isobars of increasing pressure outwards.
 Cyclones are violent storms that originate over the oceans in the tropics as well in temperate regions.
 These are accompanied by heavy rainfall, violent winds, clouds, and thunder and lightning.
 They are also called atmospheric disturbances.

Difference between Tropical and Temperate cyclones:

Tropical Cyclones Temperate Cyclones

 Form in tropical regions (5-25 degrees)  These are extra-tropical cyclones

 Thermal origin due to the warming of the sea  Form in temperate regions (40-65 degrees), these
surface are frontal cyclones originating along the fronts.

 Very violent in nature, and have high-velocity  These are not much destructive and have
winds (Velocity is more than 250 km/hour). relatively low-velocity winds (Velocity is more
than 150 km/hour).

 Low pressure (around 880 mb)  Pressure around 990 mb

 Originate over oceans but die out on land  These are predictable and form along the
(travels from east to west ), causing destruction westerlies (travels from west to east)
in the coastal regions due to its less
predictability.

 They are occurring late summers (August-  They are occurring in winters (causes westerlies
October) disturbances in India)
 3

Tropical Cyclone:
 Tropical cyclone found in between Tropic of cancer and
Tropic Of Capricorn.
 The movement of tropical cyclones is east to west.
 The size/diameter of tropical cyclones are 80 to 300 kms.
 The velocity of tropical cyclones are 180 km/hour and it
will go upto 400 km/hour.
 For example Super cyclone 1999, Odisha (Velocity is
greater than 250 km/hour).
 The origin of Tropical cyclones is the warm water of
Oceans and they die out once they reach land.
 These are not always mobile, it stays in place for hours,
day and then moves.
 Tropical cyclones are violent storms that originate over
oceans in tropical areas and move over to the coastal areas
bringing about large-scale destruction due to violent winds
(squalls), very heavy rainfall (torrential rainfall), and storm
surge.
 They are irregular wind movements involving the closed
circulation of air around a low-pressure center.
 This closed air circulation (whirling motion) is a result of the
rapid upward movement of hot air which is subjected to
Coriolis force. The low pressure at the center is responsible
for the wind speeds.
 The cyclonic wind movements are anti-clockwise in the northern hemisphere and clockwise in the
southern hemisphere (This is due to Coriolis force).
 The wind field of a tropical cyclone may be divided into three regions.
 First is a ring-shaped outer region, typically having an outer radius of about 160 km (100 miles) and
an inner radius of about 30 to 50 km (20 to 30 miles).
 In this region the winds increase uniformly in speed toward the centre.
 Second is the Eyewall where Wind speeds attain their maximum value. This is typically 15 to 30 km
(10 to 20 miles) from the centre of the storm.
 Third is the Eye, the region surrounding the eyewall, where wind speeds decrease rapidly and the air is
often calm.
 4

Additional Information:
 Squall- A sudden violent gust of wind or localized storm, especially one bringing rain, snow, or sleet.
 Torrent- A strong and fast-moving stream of water or other liquid.
 Tropical cyclones are relatively smaller than Temperate cyclones (Big size).

Types based on Intensity of Cyclones:

Tropical depressions:
 These are the low pressure regions near the ITCZ.
 The velocity of wind speed is more than 40 KM/hour.
 The regions generally, Northern Australia and Indonesia affected.
Tropical Disturbances:
 These occur on 5 to 20 degree latitudes.
 Northern Pacific, Caribbean sea affected and easterlies waves are affecting these regions.
Tropical storms:
 Tropical storms average velocity is 150 km/hour.
 These are heavily disastrous, and the Indian ocean, Arabian Sea, Bay of Bengal, Philippines are affected.
Hurricanes/ Typhoons:
 The term Cyclones in India, Hurricanes in North America, Typhoons in South China Sea, Taifu in
Japan, and Willy Willy in Australia.
 These are highly disastrous and velocity is more than 150 km/hour.
Conditions Favorable for Tropical Cyclone Formation:
1. Large sea surface with temperature higher than 27° C,
2. Presence of the Coriolis force enough to create a cyclonic vortex,
3. Small variations in the vertical wind speed,
4. A pre-existing weak low-pressure area or low-level-cyclonic circulation,
5. Upper divergence above the sea level system,
 5

Good Source of Latent Heat/Ocean Surface:

 Ocean waters having temperatures of 27° C or more is the source of moisture that feeds the storm.
 The condensation of moisture releases enough latent heat of condensation to drive the storm (High
temperature means high evaporation and then warm and moist winds, and then it will raise then start
Condensation and form clouds).
 The storms draw energy from the surface waters of the ocean, and as more heat (energy) is stored in these
upper waters, the cyclones have a larger source of energy on which to draw.
Locations:
 The depth of warm water (26-27°C) should extend for 60-70 m from the surface of the ocean/sea, so that
deep convection currents within the water do not churn and mix the cooler water below with the warmer
water near the surface.
 The above condition occurs only in western tropical oceans because of warm ocean currents (easterly trade
winds push ocean waters towards west) that flow from the east towards west forming a thick layer of water
with temperatures greater than 27°C. This supplies enough moisture to the storm.
 The cold currents lower the surface temperatures of the eastern parts of the tropical oceans making
them unfit for the breeding of cyclonic storms.
Timing:
 In the late summers, Whirling motion is enhanced when
the doldrums (region within ITCZ) over oceans are farthest
from the equator.
 Temperature in the late summer is near about 26-27° C.
 Due to the high specific heat of water, and mixing, the
ocean waters in the northern hemisphere attain maximum
temperatures in August. (Continents attain maximum
temperatures in June-July).
Coriolis Force:
 The Coriolis force is zero at the equator (no cyclones at
the equator because of zero Coriolis Force) but it increases
with latitude. Coriolis force at 5° latitude is significant enough to create a storm (cyclonic vortex).
 In the northern hemisphere wind deflect in right and in the Southern hemisphere it will deflect left.
 About 65 percent of cyclonic activity occurs between 10°
and 20° latitude.
Low-level Convergence:
 Low pressure Tropical depression helps in formation of large
storms.
 Low-level disturbance (thunderstorms- they are the seeds of
cyclones) in the form of easterly wave disturbances in the
 6

Inter-Tropical Convergence Zone (ITCZ) should pre--exist for the formation of a cyclone.
Wind Shear:
 Wind Shear is the difference between wind speeds at different heights.
 Tropical cyclones develop when the wind is uniform.
 Because of weak vertical wind shear, cyclone formation processes are limited to the latitude
equatorward of the subtropical jet stream.
 In the temperate regions, wind shear is high due to westerlies and this inhibits convective cyclone
formation.
 5 to 20 degree very low wind shear so there are a good number of cyclones.

Divergence in Upper Troposphere:

 A well-developed divergence in the upper layers of the atmosphere is necessary so that the rising air
currents within the cyclone continue to be pumped out and low pressure is maintained at the center.
Humidity Factor/Moisture:
 High Humidity (around 50 to 60 percent) is
required in the mid-troposphere since the presence
of moist air leads to the formation of a
cumulonimbus cloud.
 Such conditions exist over the equatorial doldrums,
especially in the Western margins of oceans (this is
because of the east-to-west movement of ocean
currents), which have great moisture, and carrying
capacity because the trade winds continuously
replace the saturated air.
 Supply of moisture helps in formation of violent storms.
 7

Types based on Timing:

 Generally tropical cyclones occur in April-May and September-October.
 Cyclones in April-May: in April May is less deadly/weaker (because the sea surface temperature is not high
enough so less strength of cyclones).
 And in September-October are more deadly/strong cyclones (oceans heat slowly or ocean water gets heated
up till that time and sea surface temperature rises to 27 degrees celsius by mid-September). For example:
Cyclone Gaza, 2018, Tamil Nadu in october.
 In June - July - August, generally no cyclones occur or are visible because of the Monsoon, and monsoon
produces wind shear which does not allow to form cyclones.
Summary:
Timing Cyclone Characteristics Examples
 Generally weaker cyclones  Cyclone Gaza, 2018
April-May
 Lower sea surface temperatures  Tamil Nadu in October
 Stronger and more deadly cyclones
September-October
 Warmer sea surface temperatures
 Monsoon season, no visible cyclones
June-July-August
 Presence of wind shear due to monsoon

Types based on location (This Classification for India):

 The temperature of the Bay of Bengal is more than the Arabian sea due to the mixing of rivers which keeps
the surface water warm. Thus, there are more cyclones in the Bay of Bengal than in the Arabian sea.
 8

Origin and Development of Tropical Cyclones:

 Tropical cyclones have a thermal origin, and they develop
over tropical seas during late summers (August to mid-
November).
 At these locations, the Strong Local Convectional Currents
acquire a whirling motion because of the Coriolis force.
 After developing, these cyclones advance till they find a
weak spot in the trade wind belt.
 It originates under favorable conditions, multiple
thunderstorms originate over the oceans.
 These thunderstorms merge and create an Intense Low-
Pressure System (wind is warm and lighter).
Early Stage/Youth stage:
 In the thunderstorm, the air is uplifted as it is warm and
light. At a certain height, due to lapse rate and adiabatic lapse rate, the temperature of the air falls and
moisture in the air undergoes condensation.

 Condensation releases latent heat of condensation making the air warmer. It becomes much lighter and
is further uplifted.
 The space is filled with fresh moisture laden air. Condensation occurs in this air and the cycle is repeated
as long as the moisture is supplied.
 Due to excess moisture over oceans, the thunderstorm intensifies and sucks in air at a much faster rate.
 9

 The air from the surroundings rushes in and undergoes deflection due to Coriolis force creating a cyclonic
vortex (spiraling air column similar to a tornado).
 Due to centripetal acceleration (centripetal force pulling towards the center is countered by an opposing
force called the centrifugal force), the air in the vortex is forced to form a region of calmness called an eye
at the center of the cyclone.
 The inner surface of the vortex forms the eye wall, the most violent region of the cyclone
 All the wind that is carried upwards loses its moisture and becomes cold and dense. It descends to the
surface through the cylindrical eye region and at the edges of the cyclone (Eye is created due to
tangential force acting on wind that is following a curved path).
 Continuous supply of moisture from the sea is the major driving force behind every cyclone. On reaching
the land the moisture supply is cut off and the storm dissipates.
 If the ocean can supply more moisture, the storm will reach a mature stage.
Development stage:
 In this stage, winds are converging and
subjected or encountered to a coriolis force
(In this stage warm winds rise and come under
action of coriolis force).
 These winds are rising and deflection so that
winds create spirals (a lot of spirals are
forming).
 Spiral of winds because new winds kept on
coming and getting deflected.

Mature Stage:
 At this stage, the spiraling winds create
multiple convective cells with successive
calm and violent regions.
 The regions with cumulonimbus cloud (rising
limbs of the convective cell) formation are called Rain Bands below which intense rainfall occurs.
 The ascending air will lose moisture at some point and descend (subsides) back to the surface through the
calm regions (descending limbs of convection cell–subsiding air) that exist between two rain bands.
 Cloud Formation is dense at the center. The cloud size decreases from the center to the periphery.
 Rain Bands are mostly made up of cumulonimbus clouds. The ones at the periphery are made up of
Nimbostratus and Cumulus Clouds.
 10

 The dense overcast at the upper levels of the troposphere is due to cirrus clouds which are mostly made up
of hexagonal ice crystals.
 The dry air flowing along the central dense overcast descends at the periphery and the eye region.


 1

DAILY
CLASS NOTES
Geography

Lecture - 44
Cyclones (Part 02)
 2

Cyclones (Part 02)

Structure of a Tropical Cyclone:

1. Eye:
 The "eye" is a roughly circular area of comparatively light
winds and fair weather found at the center of a severe tropical
cyclone.
 The ‘Eye’ of a cyclone forms when any wind loses its
moisture and turns dry whenever it rises to a particular
height in the atmosphere. The wind gets dry and cold at that
altitude due to the cold surroundings, therefore cold winds
will tend to subside or fall down as a result of the more
weight. Thus, inside the cyclone's eye, the winds subside as
they fall down. The descending winds inside the eye go
further downward. Air packets will get compressed due to
high pressure and experiences high temperatures. So dried,
and warmed winds were experienced inside the eye of the
cyclone.
 3

 The descending winds in the eye occur up to 2 to 3 km from the surface of the earth, so the temperature of
the eye is warmer than the other cyclonic regions.
 Under this condition, there is little or no precipitation (no clouds) and sometimes blue sky (clear sky) or
stars can be seen .
 In the cyclone's "Eye" region, there are no clouds, thunderstorms, or lightning.
 Stability conditions prevail inside the eye of the cyclone and light winds (calm condition) are present.
 The eye is the region of lowest surface pressure and warmest temperatures aloft (in the upper levels) -
The eye temperature may be 10°C warmer or more at an altitude of 12 km than the surrounding environment,
but only 0-2°C warmer at the surface in the tropical cyclone.
 Eyes range in size from 8 km to over 200 km across, but most are approximately 30-60 km in diameter.
2. Eye Wall:
 The eye is surrounded by the "eye wall", the roughly circular
ring of deep convection, which is the area of highest surface
winds in the tropical cyclone.
 Eye Wall is the region where most disturbances, violent winds,
high-velocity winds, and large amounts of instability exist in
this area where thunderstorms, lightning, and rainfall occur.
 The Eye Wall region also sees the maximum sustained winds,
i.e., the fastest winds in a cyclone occur along the eyewall
region.
 The eye is composed of air that is slowly sinking and the eye
wall has a net upward flow as a result of many moderate -
occasionally strong - updrafts and downdrafts. At the same time,
updrafts and downdrafts create friction, and huge thunderstorms
and lightning occur.
 The eye's warm temperatures are due to compressional
warming (adiabatic) of the subsiding air.
 The wind reaches maximum speed (velocity of winds is greater than 250 km/hr) and violent winds in
this region. Torrential rain occurs here.
 This is a region of maximum instability because of thunderstorms due to updrafts and downdrafts in the
region.
 From the eye wall, rain bands may radiate and a mix of cumulus and cumulonimbus clouds may drift into
the outer region.
 Most soundings taken within the eye show a low-level layer, which is relatively moist, with an inversion
above - suggesting that the sinking in the eye typically does not reach the ocean surface, but instead only gets
to around 1-3 km of the surface.
 4

3. Spiral Bands:
 Another feature of tropical cyclones that
probably plays a role in forming and
maintaining the eye is eyewall
convection.
 Spiral bands are the outer section beyond
the eye wall.
 Spiral bands come out of the eye wall.
 Convection in Tropical Cyclones is
organized into long, narrow rain bands that
are oriented in the same direction as the
horizontal wind.
 Because these bands seem to spiral into the center of a tropical cyclone, they are called “Spiral Bands".
 Along these bands, low-level convergence is a maximum, and therefore, upper-level divergence is most
pronounced above.
 Spiral bands cause rainfall and hailstorms.
 This region is comprised of cumulus and cumulonimbus clouds.
Distribution of Tropical Cyclones:
 Atlantic Region: Gulf of Mexico, Antilles, Central American countries.
 Indian Ocean: The Bay of Bengal and the Arabian Sea where they are called Cyclones.
 Around the Southeast African coast and Madagascar-Mozambique islands.
 5

 Pacific Ocean:
 North Pacific: Philippines Sea, China Sea, and Japan where they are called Typhoons.
 South Pacific: Australian coast (Willy Willy)

Why do Tropical Cyclones Occur in the Above Regions?

 These areas are lying in the tropics, these are regions of high warm temperatures.
 These cyclones occur on the eastern coast of continents because easterlies bring the warm water from east
to west.
 These regions have warm currents like Koroshio, Gulf Stream, Western Australian Current, etc.
Tropical Cyclones:
 Tropical cyclones occur in the range of latitudes 5 degrees to 25 degrees (North and South).
 Due to the southern hemisphere's greater accessibility to ocean water, the northern hemisphere
experiences more cyclones than the southern hemisphere. The essential temperature of 27 degrees
Celsius to cause a cyclone is harder to achieve since the water heats up more slowly.
 High insolation and high heat make it easy to arrive at 27 degrees Celsius temperature in the tropic
region, so tropical cyclones are stricter to latitudes 5 degrees to 25 degrees (North and South).
 Easterlies (Trade winds) bring warm water, and that warm water piles up at the eastern coast of the
continents (latitudes 5 degrees to 25 degrees). So more tropical cyclones occur in this region.
 Warm ocean currents (above 27 degrees Celsius) cause warm waters to exist in the eastern regions.
 Due to the very low Coriolis force below 5 degrees, tropical cyclones only affect areas above 5 degrees
latitude.
 6

Path of Tropical Cyclones:

 These cyclones start with a westward movement, but turn northwards around 20° latitude.
 They turn further north-eastwards around 25° latitude, and then eastwards around 30° latitude. They then
lose energy and subside.
 Tropical Cyclones follow a parabolic path, their axis being parallel to the isobars.
 Coriolis Force or earth’s rotation, easterly and westerly winds influence the path of a Tropical Cyclone.
 Tropical Cyclones die at 30° latitude because of cool ocean waters and increasing wind shear due to
westerlies.
Temperate Cyclones/Extratropical Cyclones/Wave Cyclones/Mid-Latitude Cyclones/Frontal Cyclones:
 These are low-pressure centers existing in temperate regions.
 The systems developing in the mid and high latitude (40°
latitude and 65° latitude in both hemispheres), beyond the
tropics, are called the Middle Latitude Cyclones or Extra-
Tropical Cyclones or Temperate Cyclones or Frontal Cyclones
or Wave Cyclones.
 According to the Polar Front Theory, the warm-humid air
masses (Westerlies) from the tropics converge with the dry-cold
air masses (Polar Easterlies) from the poles, and thus a polar front
is formed as a surface of discontinuity. They form along the front.
 Such conditions occur over subtropical high, sub-polar low-
pressure belts and along the Tropopause.
 The cold air pushes the warm air upwards from underneath. Thus,
a void is created because of the lessening of pressure. Raising air will
cause condensation which causes clouds and precipitation.
 The surrounding air rushes in to occupy this void and coupled with
the earth’s rotation, a cyclone is formed which advances with the
westerlies (Jet Streams).
 These cyclones are formed gradually and hence predictable.
 The temperate cyclones generally have low speeds and are
dynamically induced.
 There is no eye formation in these types of cyclones.
 The size of these cyclones is an average of 1000 km to 2000km in
diameter. The range of these cyclones is up from 150km to 3,000
km.
 The area of this cyclonic storm is 10 lack square km and height is up to 12km.
 The wind velocity of this cyclone is from 32km/hr to 50km/hr.
 These cyclones move in the direction of westerly winds.
 7

 Temperature Distribution:
 Cold Sector: North, North West, and North East.
 Warm Sector: South
 The temperate cyclone shape will be elliptical, inverted V-shaped, and Circular.

Distribution of Temperate Cyclones:

 USA and Canada – It extends over the Sierra
Nevada, Aleutian Islands, Alaska Gulf,
Labrador, North of Atlantic, Baltic region,
Mediterranean Region, Colorado, Eastern
Canadian Rockies, and the Great Lakes region,
the belt extending from Iceland to the Barents
Sea and continuing over Russia and Siberia.
 The Mediterranean basin extends up to Russia
and even up to India in winter (called
Western Disturbances) and the Antarctic
frontal zone.
Formation of Temperate Cyclones:
 Stationary Stage: In the Northern Hemisphere, warm air blows from the south, and cold air from the north
of the front. This is called Cyclogenesis. This is the initial stage when both air masses just interact with each
other.
 8

 Incipient Stage: In the center of this circulation, there is mass convergence. When all that air hits the center,
we have a rising motion because it has nowhere else to go. When the pressure drops along the front, the
warm air moves northwards and the cold air moves towards the south setting in motion an Anticlockwise
Cyclonic Circulation (northern hemisphere). This is due to the Coriolis Force. Cold air mass tries to
penetrate into warm air mass. Warm air mass tries to penetrate cold air mass.
 Mature Stage: The Cyclonic Circulation leads to a well-developed extratropical cyclone, with a warm front
and a cold front. There are pockets of warm air or warm sector wedged between the forward and the rear
cold air or cold sector. Cold air masses dominate and try to lift warm air masses. Then cyclones get fully
developed. The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the
warm front and cause precipitation. The cold front approaches the warm air from behind and pushes the
warm air up. As a result, Cumulus Clouds develop along the cold front.
 Warm Sector Narrows: The cold front moves faster than the warm front, ultimately overtaking the warm
front. The warm air is completely lifted up and the front is Occluded (occluded front) and the Cyclone
Dissipates. The warm sector is narrowed down completely.
 Final Stage: Cold air mass completely overtakes warm air mass and a temperate cyclone forms. Hail is
associated with severe thunderstorms that form along or in front of cold fronts during the spring and summer
months. The processes of wind circulation both at the surface and aloft are closely interlinked. Temperate
cyclones are intense frontogenesis involving mainly occlusion-type fronts.
The Naming of Cyclones:
 WMO (World Meteorological Organization) divided the world's Oceans into Basins and assigned the
responsibility of naming the Cyclones to the respective regional bodies.
 Each regional body has its own rules for naming cyclones. In most regions, predetermined alphabetical lists
of alternating male and female names are used.
 In the northwest Pacific, the majority of names used are not personal names. While there are a few male and
female names, the majority are names of flowers, animals, birds, trees, foods, or descriptive adjectives.
 The WMO/ESCAP Panel on Tropical Cyclones at its twenty-seventh Session held in 2000 in Muscat,)
Sultanate of Oman agreed in principle to assign names to the tropical cyclones in the Bay of Bengal and
Arabian Sea.
 Eight countries India, Pakistan, Bangladesh, Maldives, Myanmar, Oman, Sri Lanka, and Thailand
participated in the panel and came up with a list of 64 names.
 The Panel member names are listed alphabetically country-wise. The names will be used sequentially
column-wise.
 For instance, the first name will start from the first row of column one and continue sequentially to the last
row in column eight. For example, Onil, Hibaru, Pyar, Baaz, and so on.
 9

Names of Indian Ocean Cyclones:

Color Coded Warning System for Cyclone Prone Areas:

 GREEN: The green color signifies everything is smooth and well i.e. "all is well" with no adverse weather
conditions.
 YELLOW: The yellow color code asks the guards to "be updated" to handle the bad weather that can last
for days, with a warning of affecting daily activities.
 ORANGE: The alert for color code orange indicates to "be prepared". It can be a warning of extreme
damage to communication disruptions that can lead to power cuts, and road and railway blockages. The
orange alert is also a sign of evacuation and keeping the basic necessities ready for families.
 RED: The red color is the highest level of warning that notifies the authorities to "take action". This is a
case in which there is a threat to life with the worst weather conditions.


 1

DAILY
CLASS NOTES
Geography

Lecture - 45
Oceanography
 2

Oceanography

Content Overview:
❖ Ocean Relief
➢ Major Ocean Relief Features
➢ Minor Ocean Relief Features
❖ Continental Shelf
➢ Width
➢ Depth
➢ Importance of continent shelves
❖ Continental Slope
❖ Continental Rise
❖ Deep Sea Plain or Abyssal Plain
❖ Oceanic Deeps or Trenches
❖ Mid-Oceanic Ridges or Submarine Ridges
❖ Abyssal Hills
❖ Submarine Canyons
❖ Bank & Shoal
❖ Significance of Study of Oceanic Relief

Ocean Relief:
❖ Ocean relief is largely due to tectonic, volcanic, erosional, and depositional processes and their interactions.
❖ Ocean relief features are categorized into major and minor relief features.
 3

Minor Ocean Relief Features:

 4

Four Major Types of Relief Features:

1. Continental Shelf:
❖ A continental shelf is the edge of a continent that lies under the ocean.
❖ It extends from the coastline of a continent to a drop-off point called the shelf break. From the break, the
shelf descends toward the deep ocean floor in what is called the continental slope.
❖ These extended margins of each continent are occupied by relatively shallow seas and gulfs.
❖ The Continental Shelf of all oceans together covers 7.5-8.5% of the total area of the oceans.
❖ A gradient of the continental shelf is 1°-3° (Degrees). (Chandipur coast in Odisha has a flatbed)
❖ The shelf typically ends at a very steep slope, called the shelf
break.
❖ The continental shelves are covered with variable
thicknesses of sediments brought down by rivers, glaciers,
etc.
❖ Massive sedimentary deposits received over a long time by
the continental shelves, become the source of fossil fuels
[Example - Petroleum].
❖ Examples: Siberian shelf in the Arctic Ocean, the largest in
the world, stretches to 1,500 km in width. Along parts of the
U.S. state of California, the continental shelf extends less
than a kilometer (62 miles).
❖ In this region, the best fishing grounds, ports, etc. are available.
❖ The shelf is formed mainly due to:
➢ Faulting and Submergence: Example: The western Indian coast submerged (100 km width) due to
faulting and the Eastern coast is an emergent coast due to endogenetic forces.
➢ Glaciation and Warming Age: Glaciation led to both erosion and deposition. The rise and fall in sea
level due to the melting of glaciation helped in the formation of the continental shelf.
➢ Deposition by the Rivers: The Fluvial deposits carry silt, sand, etc. which lead to the formation of the
continental shelves. Example: Indus river, Ganga River, etc.
 5

➢ Landforms: Due to erosion and deposition by the waves, various landforms have formed which also
helps in the formation of the continental shelves.
Width of the Continental Shelf:
❖ The average width of continental shelves is between 70–75 km.
❖ Most Wide in the Arctic Sea, Adriatic Sea, and China Sea.
❖ Medium - Found in North America (Approx 96 Kms).
❖ Narrow -The shelves are almost absent or very narrow where Mountains are parallel and close to the coast.
Example - In South America along the Andes -16 km and the West Coast of Sumatra.
❖ The Siberian shelf in the Arctic Ocean is the largest in the world which stretches to 1,500 km in width.
❖ In India - On the West Coast -100 km, while on the East Coast -50 km.

Depth of Continental Shelf:

❖ The depth of the shelves is variable.
❖ Average-600 feet.
❖ It may be as shallow as 30 meters in some areas while in others it is as deep as 600 meters.
Types of Shelves (Based on Sediments): There are various types of shelves based on sediments of terrestrial
origin:
 6

Importance of Continent Shelves:

❖ Marine food comes almost entirely from continental shelves.
❖ They provide the richest fishing grounds. These are the most productive regions (due to the penetration of
sunlight and the development of plankton).
❖ They are potential sites for economic minerals. Examples: Bombay highs, Cambay coast, etc.
❖ Polymetallic Nodules (manganese nodules; concentric layers of iron and manganese hydroxides, etc.) are
good sources of various mineral ores like manganese, iron copper, gold, etc.
❖ These make very good ports and harbors because of their low depth.
2. Continental Slope:
❖ The continental slope connects the continental shelf
and the ocean basins.
❖ It begins where the bottom of the continental shelf
sharply drops off into a steep slope.
❖ It covers 8.5 % of the Total Surface Area.
❖ The gradient of the slope region varies between 2°-
5°.
❖ The depth of the slope region varies between 200 and
3,000 m.
❖ The seaward edge of the continental slope loses
gradient at this depth and gives rise to continental rise.
❖ Canyons and trenches are observed in this region.
➢ Canyons start from the continental shelf and go up to the continental slope and rise.
 7

❖ Reasons for the formation of continental slope:

➢ Deposition: Fluvial and marine deposits by the rivers.
➢ Faulting: A crack in the earth's crust resulting from the displacement of one side with respect to the other.
➢ Erosion: Submarine canyons are an example of both erosion and deposition in the oceans.
3. Continental Rise:
❖ The continental rise is a layer of sediment from the base of the slope that forms a dip between the foot of
the continental slope and the deep ocean floor.
❖ At the base of many continental slopes, the ocean bottom flattens out to a mere gradient of one degree.
❖ The continental slope gradually loses its steepness with depth.
❖ It is a buffer or joint between the continental slope and deep-sea plains.
❖ When the slope reaches a level between 0.5° and 1°, it is referred to as the continental rise.
❖ With increasing depth, the rise becomes virtually flat and merges with the abyssal plain.

4. Deep Sea Plain or Abyssal Plain:

❖ Deep sea plains are gently sloping areas of the ocean
basins.
❖ These are the flattest and smoothest regions of the
world having Pelagic Deposits of Plants, Animals,
and Volcanic Deposits.
❖ It covers nearly 75.5% of the Total Surface Area of
the ocean floor.
❖ The depths vary between 3,000 and 6,000m.
❖ These plains are covered with fine-grained sediments
like clay and silt.
❖ These are generally featureless but Sea Mountain,
Ridges, and Guyots are significant features.
 8

5. Oceanic Deeps or Trenches:

❖ The trenches are relatively steep-sided, narrow basins
depressions.
❖ These areas are the deepest parts of the oceans.
❖ They are of tectonic origin and are formed during
Ocean–Ocean Convergence and Ocean-Continent
Convergence.
❖ They are some 3-5 km deeper than the surrounding
ocean floor.
❖ The trenches lie along the fringes of the deep-sea plain
at the bases of continental slopes and along island arcs.
❖ The trenches run parallel to the bordering fold mountains or the
island chains.
❖ The trenches are very common in the Pacific Ocean and form
an almost continuous ring along the western and eastern
margins of the Pacific.
❖ The Mariana Trench off the Guam Islands in the Pacific
Ocean is the deepest trench with a depth of more than 11
kilometers.
❖ They are associated with active volcanoes and strong
earthquakes (Deep deep-focus earthquakes like in Japan).
➢ This makes them very significant in the study of plate
movements.
❖ As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean
and 6 in the Indian Ocean.
6. Mid-Oceanic Ridges or Submarine Ridges:
❖ A mid-oceanic ridge is composed of two chains of
mountains separated by a large depression.
[Divergent Boundary]
❖ The mountain ranges can have peaks as high as
2,500 m and some even reach above the ocean’s
surface.
❖ Running for a total length of 75,000 km, these
ridges form the largest mountain systems on earth.
 9

❖ These ridges are either broad, like a plateau, gently sloping, or in the form of steep-sided narrow mountains.
❖ These oceanic ridge systems are of tectonic
origin and provide evidence in support of
the theory of Plate Tectonics.
❖ Iceland, a part of the mid-Atlantic Ridge,
is an example.
7. Abyssal Hills:
❖ Seamount: It is a mountain with pointed
summits, rising from the seafloor that does
not reach the surface of the ocean.
➢ Seamounts are volcanic in origin.
➢ These can be 3,000-4,500 m tall.
➢ The Emperor Seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is an example.
❖ Guyots: The flat-topped mountains (seamounts) are known as guyots. They show evidence of gradual
subsidence through stages to become flat-topped submerged mountains.
❖ Seamounts and guyots are very common in the Pacific Ocean where they are estimated to number around
10,000.
8. Submarine Canyons:
❖ Canyons: It is a deep gorge, especially one with
a river flowing through it. The Hudson Canyon
is the best-known submarine canyon in the
world.
❖ There are various theories on the formation of
submarine canyons:
➢ Endogenetic forces (Diastrophic
Theory): Tensile forces lead to cracks
which get enlarged and form submarine
canyons.
➢ Erosion and Submergence (Subaerial
Erosion Theory): Erosion by the rivers
and submergence due to faulting. Examples: Indus Canyon, Godavari Canyon, etc.
❖ Gorge: A steep, narrow valley or ravine.
❖ Valley: A low area between hills or mountains or depression, typically with a river or stream flowing through
it.
❖ These are deep valleys, some comparable to the Grand Canyon of the Colorado River.
❖ They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths
of large rivers.
 10

Ocean Bank and Shoal:

Ocean Bank:
❖ These marine features are formed as a result of erosional and
depositional activity.
❖ A bank is a flat-topped elevation located in the continental margins.
❖ The depth of water here is shallow but enough for navigational purposes.
❖ The Dogger Bank in the North Sea and the Grand Bank in the north-
western Atlantic, Newfoundland are famous examples.
❖ The Ocean banks are sites of some of the most productive fisheries in the
world.
❖ These are the most productive fishing grounds.
Shoal:
❖ A shoal is a detached elevation with shallow depths.
❖ Since they project out of the water with moderate heights, they are
dangerous for navigation.
Significance of Study of Oceanic Relief:
❖ It is important to understand the marine life of plants and animals and
the effect of geography. Examples: Fish, precious metals, etc.
❖ Ocean relief controls the motion of seawater.
❖ It helps in resource exploration for economic development
manufacturing, trade, etc.
❖ The climatic phenomenon is also affected by the oceans (temperature,
salinity, etc.) like El Nino, etc. Since the relief feature affects the
characteristics of the oceans, thus, understanding these provides helpful information to the researchers. Thus,
the oceanic movement in the form of currents, in turn, causes many variations in both oceans and in the
atmosphere.
❖ The bottom relief of oceans also influences navigation and fishing.
❖ It is important for the futuristic studies of marine fisheries, temperature, etc.

 1

DAILY
CLASS NOTES
Geography

Lecture - 46
Temperature Distribution in
Oceans
 2

Temperature Distribution in Oceans

Ocean Temperature:
❖ Temperature: It is the degree of hotness/coldness of any material/body such as solid, liquid, or gas. Ocean
temperature is measured by a Thermograph.
❖ Ocean Temperature: Ocean waters get heated up by solar energy just as land. The process of heating and
cooling of oceanic water is slower than land.
❖ It plays many important roles inside the ocean. Thus, it is very important to understand ocean temperatures. It
affects the:
➢ Marine Biodiversity: Plant and animal species require optimum temperature for survival. Thus, Ocean
temperature affects marine resources like fish, crabs, turtles, etc.
➢ Pressure Systems: Low-pressure and high-pressure systems are governed by ocean temperature. It also
leads to wind formation.
➢ Ocean Currents: It is governed by the ocean temperature and distributes heat throughout the globe.
➢ Climatic Phenomenon: These are also governed by the ocean temperature. Examples: El Nino, La Nina,
etc.
➢ Economy: Ocean temperature is a critical factor for the growth of the shipping industry.
Average Temperature of Oceans:

Oceans Average Temperature

1. Average Temperature of the Ocean 26.5°C

2. Indian Ocean 25°C

3. North West Pacific 32.2°C

4. Red Sea 37°C

5. Persian Gulf 34.5°C

6. Scotland Ocean 33.3°C

7. The Equatorial Pacific 26°C

The difference in temperature in different oceans is due to Insolation. There is high temperature in the Tropics
where the Sun is overhead, whereas there is low temperature in the Poles where the sun rays are oblique.
Diurnal Range of Temperature:
❖ It is the difference between the day and night temperature of a region.
 3

❖ It depends on many factors, such as large bodies of water, soil type and cover, wind, cloud cover/water
vapor, and moisture on the ground.
❖ The diurnal range is high when there is a clear sky (when there are no clouds).
❖ The density of the water of the ocean: If the density of water is high, then there will be a slow temperature
change. If the density of water is low, the temperature change will be fast.
Annual Range of Temperature:
❖ It is the maximum to minimum temperature of the ocean on a yearly basis.
❖ There are various factors affecting the annual range of temperature:
1. Size of the Sea: If there is a large volume of water, it will heat slowly and vice versa. Example:
Temperature range of Atlantic and Pacific Oceans.
2. Enclosed Sea surrounded by land has a higher range of temperature than open sea due to less interaction
with oceans/sea (low distribution in the heat). Examples: Mediterranean sea, Red sea, etc.
Factors Affecting the Distribution of Temperature of Ocean Water:

❖ Latitude: The temperature of surface water decreases from the equator towards the poles because the amount
of insolation decreases poleward. At the tropics, the temperature is maximum and at the poles, the temperature
is minimum due to a reduction in the insolation.
❖ Unequal Distribution of Land and Water: The oceans in the Northern Hemisphere receive more heat due
to their contact with a larger extent of land than the oceans in the Southern Hemisphere.
❖ Nature of the Sea: The enclosed seas in the low latitudes record relatively higher temperatures than the
open seas; whereas the enclosed seas in the high latitudes have lower temperatures than the open seas.
❖ Prevailing Wind: The winds blowing from the land towards the oceans drive warm surface water away from
the coast resulting in the upwelling of cold water from below. It results in the longitudinal variation in the
ocean temperature. Contrary to this, the onshore winds pile up warm water near the coast and this raises the
temperature.
➢ Trade winds push the wind towards the east coast or the west of the oceans. It pushes the warm waters
away from the west coast. This phenomenon of Easterlies is observed in the 5°-25° North and South
hemispheres.
➢ Westerlies push the warm water towards the west coast or the east of the oceans. It pulls warm waters
away from the east coast. This phenomenon of Easterlies is observed in the 35°-60° mid-latitudes.
➢ Ocean currents: Warm ocean currents raise the temperature in cold areas while cold currents decrease
the temperature in warm ocean areas. The Gulf Stream (warm current) raises the temperature near the
 4

eastern coast of North America and the West Coast of Europe while the Labrador Current (cold current)
lowers the temperature near the north-east coast of North America.
➢ Other factors can be cloudiness, rainfall, fog, etc. which affect the temperature.
❖ All these factors influence the temperature of the ocean currents locally.
Horizontal Distribution of Temperature:
❖ The average temperature of surface water of
the oceans is about 27°C and it gradually
decreases from the equator towards the poles.
❖ The rate of decrease of temperature with
increasing latitude is generally 0.5°C per latitude.
❖ The average temperature is around 22°C at 20°
latitudes, 14°C at 40° latitudes and 0°C near
poles.
❖ The oceans in the northern hemisphere record
relatively higher temperatures than in the southern
hemisphere.
❖ The highest temperature is not recorded at the
equator but slightly towards the north of it around 5° N latitude in places like the Western Pacific at 32.2°
C.
❖ The temperature of the northern hemisphere is higher than the southern hemisphere because more land
is available in the northern hemisphere.
❖ The average annual temperatures for the northern and southern hemispheres are around 19° C and 16°
C respectively.
❖ This variation is due to the unequal distribution of land and water in the northern and southern
hemispheres.
❖ In the northern hemisphere, in the North Atlantic
Ocean, there is a dominance of warm currents thus,
the temperature reduction is slow. Whereas in the
Southern Atlantic Ocean, the rate of temperature
reduction is relatively fast due to less dominance of
warm currents.
❖ On the sea surface temperature, there is a higher
temperature due to more exposure to insulation.
❖ The Coldest temperature is found in the polar
regions, that is 80 degrees in the northern
hemisphere and 75 degrees in the southern hemisphere.
 5

Vertical Distribution of Temperature:

❖ The temperature-depth profile for the
ocean water shows how the temperature
decreases with increasing depth.
❖ The profile shows a boundary region
between the surface waters of the ocean and
the deeper layers.
❖ The boundary usually begins around 100-400
m below the sea surface and extends several
hundred meters downward.
❖ This boundary region, from where there is a
rapid decrease in temperature, is called the
thermocline.
❖ About 90% of the total volume of water is
found below the thermocline in the deep
ocean. In this zone, temperatures approach 0° C.
❖ The photic zone extends up to 200 m where sunlight is visible and photosynthesis is possible.
❖ The temperature structure of oceans over middle and low latitudes can be described as a three-layer system
from the surface to the bottom.
➢ The first layer represents the top layer of warm oceanic water and it is about 500 m thick with
temperatures ranging between 20° and 25° C.
✓ This layer, within the tropical region,
is present throughout the year but in
mid-latitudes, it develops only during
the summer season.
➢ The second layer called the thermocline
layer lies below the first layer and is
characterized by a rapid decrease in
temperature with increasing depth. The
thermocline is 500 -1,000 m thick.
➢ The third layer is very cold and extends up
to the deep ocean floor.
❖ In the Arctic and Antarctic circles, the surface water temperatures are close to 0° C so the temperature change
with the depth is very slight.
❖ Here, only one layer of cold water exists, which extends from the surface to the deep ocean floor.
❖ It is a well-known fact that the maximum temperature of the oceans is always at their surfaces because they
directly receive the heat from the sun and the heat is transmitted to the lower sections of the oceans through
the process of convection.
 6

❖ It results in a decrease in temperature with increasing depth, but the rate of decrease is not uniform throughout.
❖ The temperature falls very rapidly up to the depth of 200 m and thereafter, the rate of decrease of temperature
is slowed down.
❖ Temperature decreases with an increase in ocean depth.
❖ The temperature reduction at the surface and depth is different.
➢ On the surface (from the equator to the poles) the temperature reduction is fast and at depth, it is slow.
➢ The rate of reduction at the equator is fast.
➢ The rate of reduction at poles is slow.
➢ On the eastern margin (tropics) of the continents, there is a sharp reduction.
➢ On the western side (tropics) of continents, there is a slow reduction.
➢ In the temperate zones, Westerlies pile up warm water on the west coast or west margin so there is a sharp
reduction in the temperature. On the eastern coast due to the availability of cold water, there will be a
slow reduction.

Different Zones of Oceans Characteristics

(Vertically)

Epilimnion ❖ It starts from the ocean surface up to 500 meters depth.

❖ It is the top layer which has a temperature of around 20-25 degrees Celsius.
❖ This is the photic zone where the sunlight penetration is maximum.
❖ Life is possible in this layer and algae perform photosynthesis in this zone.
❖ Temperature change is very slow or negligible in this zone.

Metalimnion ❖ It lies between 500 meters to 1000 meters.

❖ Rapid temperature reduction is seen here. (19-20 degrees Celsius)
❖ It is also known as the thermocline.

Hypolimnion ❖ It lies below 1000 meters.

❖ Availability of cold waters.
❖ It is predominant in the polar region.

  
 1

DAILY
CLASS NOTES
GEOGRAPHY

Lecture – 47
Climatic Phenomena
 2

Climatic Phenomena

Normal Conditions in the Pacific Ocean:

❖ In a normal year, a surface low pressure
develops in the region of northern
Australia and Indonesia, and a high-
pressure system over the coast of Peru.
❖ As a result, the trade winds over the
Pacific Ocean move strongly from east
to west.
❖ The easterly flow of the trade winds
carries warm surface waters westward,
bringing convective storms.
❖ Cold nutrient-rich water wells up to the
surface to replace the warm water.
❖ The eastern coast of the continent has
warm water and the western coast of the
continent has cold water.
Walker Cell:
❖ The Walker Circulation is an atmospheric system of air flow in the equatorial Pacific Ocean.
❖ The trade winds across the tropical Pacific flow from east to west: air rises above the warm waters of the
western Pacific, flows eastward at high altitudes, and descends over the eastern Pacific.
❖ The easterly flow of the trade winds carries warm surface waters westward and brings clouds and precipitation
to Indonesia and
coastal Australia.
❖ The winds that move
towards the east
become dry, cold and
heavy and descend
along the coast of
South America.
❖ One section of the
rising winds move
towards the west.
These winds become
cold. The heavy winds
descend along the Mascarene region. The winds rising from the Tibetan plateau and the Tropical Easterly jet
descends here. They create a region of high pressure and contribute to the South-West monsoon.
 3

Upwelling:
❖ Winds blowing across the ocean surface push water away. Water then rises up from beneath the surface to
replace the water that was pushed away. This process is known as “upwelling.”
❖ Upwelling occurs in the open ocean and along coastlines.
❖ Water that rises to the surface as a result of upwelling is typically colder and is rich in nutrients. These nutrients
“fertilize” surface waters, meaning that these surface waters often have high biological productivity. Therefore,
good fishing grounds typically are found where upwelling is common.
❖ Along the coast of Peru, cold bottom cold nutrient rich water wells up to the surface to replace the warm
water that is pulled to the west.
❖ The Walker cell is indirectly related to upwelling off the coasts of Peru and Ecuador. This brings nutrient-rich
cold water to the surface, increasing fishing stocks.
 4

El Nino:
Normal Years:
❖ In normal years, the coastal waters of the Pacific Ocean near the South American region along Peru and
Ecuador (Eastern Pacific) remain cold, while the other side of the Ocean (Western Pacific) near Australia
remains relatively warm.
❖ It creates a normal atmospheric condition in which ground-level winds move from east to west (also called
easterly trade winds) taking warmer surface water to the western coast of the Pacific, thus keeping the Peruvian
coast colder.
❖ It then rises upwards near Australia due to a warmer surface, causing precipitation there and mostly diverting
back to the eastern Pacific side in the upper atmosphere. It’s technically called ‘Walker Circulation’. In this
condition, the monsoon in India does not get affected negatively.
El Nino Year:
❖ In El Nino year, the trade winds get weakened due to reasons not fully known yet.
❖ They are not able to move the warmer surface water of the Peruvian coast to the western Pacific side, due to
which the surface water of the eastern Pacific Ocean gets warmer. This creates low Pressure in the eastern
Pacific which contributes to rainfall and flooding in this region and dry conditions in Australia, Indonesia
and Papua New Guinea.
❖ This affects the Walker Circulation, which in turn adversely affects the southwest monsoon winds in India.

Thus, the entire phenomenon is known as ENSO (El Nino Southern Oscillation) where El Nino is the piling up
of warm waters at eastern equatorial Pacific whereas Southern Oscillation is the creation of low pressure at South
American coast and high pressure along the Australian coast.
Effects of El Nino:
❖ The warmer waters had a devastating effect on marine life existing off the coast of Peru and Ecuador.
❖ Fish catches off the coast of South America were lower than in the normal year (Because there is no upwelling).
 5

❖ Severe droughts occur in Australia, Indonesia, India and southern Africa.

❖ Heavy rains in California, Ecuador, and the Gulf of Mexico.
❖ The warm water which piles up along the coast of South America moves further towards the African continent
and raises the temperature of the Indian Ocean compared to the Pacific Ocean.
Reason of El Nino:
❖ General circulation reverses every 4-5 years.
❖ Due to the pacific ring of fire, heat piles up and that region becomes completely warm.
Impact of El Nino winds on Indian Monsoon:

❖ The warming causes changes in atmospheric patterns, leading to a weakening of the monsoon circulation over
the Indian subcontinent.
❖ El Nino has been generally known to suppress monsoon rainfall in India, since it is the easterly wind that
forms water bearing clouds to Indian monsoon rainfall.
❖ The winds from Tropical easterly jets are descending whereas the other section of winds that diverge from the
Australian continent are ascending which creates a negative impact on the Indian monsoon.
❖ As a result, the Indian monsoon tends to be weaker and less reliable during El Nino years.
La Nina:
❖ After an El Niño event weather conditions usually return back to normal.
❖ However, in some years the trade winds can become extremely strong and an abnormal accumulation of
cold water can occur in the central and eastern Pacific. This event is called a La Niña.
❖ The Walker cell becomes strong which leads to a heavy rainfall and flooding in Australia, Indonesia, Papua
New Guinea. Hence, it is good for the Indian Monsoon.
❖ Large production of fishes in the South American Coast which contributes to a crash in the market.
 6

Indian Ocean Dipole (IOD):

❖ It is an Indian Nino occurring
only in the Indian Ocean.
❖ It is the difference in the sea
surface temperature of
Western and Eastern Indian
ocean.
❖ The Indian Ocean Dipole
(IOD) is defined by the
difference in sea surface
temperature between two
areas (or poles, hence a
dipole) – a western pole in
the Arabian Sea (western
Indian Ocean) and an
eastern pole in the eastern
Indian Ocean south of Indonesia.
There are two types of IOD:
❖ Positive IOD:
➢ The winds over the Indian Ocean blow from east to west (from Bay of Bengal towards Arabian Sea).
➢ This results in the Arabian Sea (western Indian Ocean near the African Coast) being much warmer and
eastern Indian Ocean around Indonesia becoming colder and dry.
➢ There is less rainfall over southern Australia .
 7

➢ Warm waters in the Western Indian Ocean and cold waters in the Eastern Indian Ocean. It is good for
rainfall in India.
❖ Negative IOD:
➢ In the negative dipole year, reverse happens, i.e,
warm waters in the Eastern Indian Ocean
and cold waters in the Western Indian
Ocean.
Madden Julian Oscillations:
❖ They are low pressure bands in 300N and 300S
traveling from West to East. These are the
disturbances causing rising winds, clouds and
precipitation.
❖ It consists of two phases:
➢ Enhanced Convective Phase: It generates low
pressure which leads to good rainfall.
➢ Suppressed Convective Phase: It generates
high pressure which leads to bad rainfall.
❖ If there is an Enhanced Convective Phase in the
Indian Ocean there will be a Suppressed Convective
Phase in the Pacific Ocean at that time.
   
 1

DAILY
CLASS NOTES
Geography

Lecture - 48
Salinity of Oceans
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Salinity of Oceans

Ocean Salinity:
❖ In general terms, it is the amount of salt dissolved in
water bodies (lakes, seas, etc.).
❖ Salinity is the term used to define the total content of
dissolved salts in seawater.
❖ It is defined as the ratio between the weight of the
dissolved solids (salt in grams) to the weight of the
sample seawater (liter).
❖ It is calculated as the amount of salt (in gm) dissolved in
1,000 gm (1 kg) of seawater.
❖ It is usually expressed as parts per thousand (o/oo) or
ppt.
❖ All natural waters, whether rainwater or ocean water,
contain dissolved mineral salts.
❖ The amount of salt in seawater is gradually increasing
because it is brought from land every year.
❖ Rivers containing Calcium Sulphate, while Sodium Chloride are the most important constituents of sea
salt.
❖ Salts in the ocean are Sodium Chloride, Magnesium Sulphate, Calcium Sulphate, Magnesium Chloride.
❖ Besides salt, silver, Gold, Platinum, Radium and Polymetallic Nodules too occur in ocean water.
❖ Salinity is an important property of seawater.
❖ Highest salinity in water bodies: Lake Van in Turkey (330), Dead Sea (238), Great Salt Lake (220), etc.
❖ According to a famous geographer, Joly, if all the salts are spread only over the lands, these will form a
150-meter-thick layer. If the same is uniformly distributed all over the world it will be 45 meters thick.
Importance of Ocean Salinity:
❖ It affects marine biodiversity (flora and fauna) and human life due to their dependence on the ocean.
❖ It also affects the physical properties of the ocean like density, temperature, pressure, etc.
❖ Ocean dynamics such as winds, ocean currents, etc. are also influenced by ocean salinity.
❖ It also has a huge impact on climate dynamics. Example- El Nino, Rainfall, etc.
❖ It affects the (if more salts are there the freezing point reduces, and the boiling point also changes) rate of
evaporation and condensation of water vapor which has a huge impact on rainfall patterns.
Factors Affecting Ocean Salinity:
❖ The salinity of water in the surface layer of oceans depends
mainly on evaporation and precipitation.
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❖ Evaporation: There is a direct relation between salinity and evaporation. Greater evaporation is due to
greater salinity.
➢ Salinity is higher near the tropics than at the equator because both areas record a higher rate of
evaporation because of more heat supply.
➢ Example - At 5°N in the Equatorial region there is a great amount of insolation, insolation leads to
cloud formation which again leads to frequent rainfall (here salinity is 34.67‰), beyond Equator, at
20°N in the Tropical region there is an existence of trade wind zones with high insolation and
evaporation, clouds are less compared to Equatorial region and the great amount of heat as well as
evaporation leads to less water content and same salt content. In the tropics region there is high
salinity of about 37- 38‰.
➢ NOTE: Subtropical High Pressure Belt and Trade Winds zone experiences very high salinity due
to high evaporation.
❖ Precipitation: Precipitation is inversely related to salinity. Higher the precipitation, the lower the salinity.
Example- Low salinity at the Equator is due to higher amounts of rainfall.
❖ Freshwater Inflow from Rivers: Surface salinity is greatly influenced in coastal regions by the freshwater
flow from rivers and in polar regions by the processes of freezing and thawing of ice.
➢ Voluminous rivers bring fresh water, thus, there is low salinity at the mouth of the river. Example: Low
salinity at the mouth of Ganga, Congo, Nizer, Gulf of Bothnia, Amazon and St. Lawrence. The
salinity of Black Sea-reduced with constant flow of fresh water from Danube, Dnieper, Dniester.
➢ In polar regions, the salinity is low due to freshwater influx because of the melting of glaciers. The
salinity in subtropical high-pressure regions
is more than the salinity of polar regions.
❖ Wind Movement: Wind also influences the
salinity of an area by transferring water from one
area to another.
➢ Wind Direction - Wind helps in the
redistribution of the seawater as winds drive
away saline water to less saline areas.
➢ In areas of upwelling, more saline water is
replaced by less saline water. Example - Trade
winds drive away saline water from the western
coasts of continents (or eastern margins of
oceans) and pile them up near the eastern coasts
of continents.
❖ Atmospheric Conditions: Anticyclonic conditions
with stable air and high temperature increase the
salinity of the surface water. Example - Subtropical
high-pressure belts represent such conditions.
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❖ Circulation of Ocean Currents: Ocean currents

affect the spatial distribution of salinity by mixing
sea waters.
➢ Warm Ocean Currents lead to high
evaporation, which again results in high
salinity.
➢ Cold Ocean Currents lead to low evaporation,
and it further results in decreasing salinity.
➢ Example - Equatorial currents drive away salts from western coasts to eastern coasts. Further, North
Atlantic Drift increases the salinity along the Northwest coasts of Europe.
➢ The ocean currents contribute to the salinity variations. Salinity, temperature, and density of water are
interrelated. Hence, any change in the temperature or density influences the salinity of water in an area.

Horizontal / Latitudinal Distribution of Salinity:

❖ On average, Salinity decreases from the Equator to the Poles.
❖ Marginal areas of the oceans bordering the continent record lower salinity because freshwater is added to
the marginal areas through rivers.
❖ The salinity of the partially enclosed seas is seldom decided by latitudes; rather it depends on the influx of
the water. For example - the Baltic Sea records lower salinity than the North Sea both having the same
latitudinal extent.

Latitudinal Zone Salinity

Equator Zone ❖ Low salinity due to excessive rainfall

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Tropical Zone ❖ The highest salinity is not recorded at the Equator but from 20-40 Degree
North because this zone is characterized by High Temperature, low Rainfall
& High Evaporation.

Temperate Zone ❖ The zone of 40-60 degrees North & South records lower salinity due to
freshwater influx.

Polar and Subpolar Zone ❖ Salinity further decreases at poles because of the Influx of meltwater.
(presence of icecaps)

Hemisphere Salinity

Northern Hemisphere ❖ Low evaporation due to more landmass.

❖ Freshwater influx by rivers

Southern Hemisphere ❖ High evaporation

❖ Low or negligible influx of fresh water.

Note: Salinity of the Northern Hemisphere is low in comparison with the southern hemisphere.
Regional Distribution of Salinity:
❖ Region of High Salinity: Where salinity is
above normal (35-40‰), for example Red
Sea, Persian Gulf, Mediterranean Sea, etc.
(Due to high evaporation, closed sea and less
interaction)
❖ Region of Medium Salinity: Where salinity
is more or less normal (34-35‰), for
example Gulf of Mexico, Caribbean sea,
etc.
❖ Region of Low Salinity: Where salinity is
below normal (20-34‰): Arctic sea,
Okhotsk sea, Bering sea, Gulf of
Bothnia(5%) etc. (Due to influx of melt water)
❖ Inland Seas and Lakes: Very high salinity (200-300 PPT): Lake Van (330%, Turkey), Lake Urmia,
Dead Sea (240%), Great Salt Lake (220%), and Caspian Sea (low salinity due to River Volga and River
Ural) etc.
Vertical Distribution of Salinity:
❖ At Higher Latitudes, Salinity increases with depth due to denser water below.
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❖ In the middle latitudes, it increases up to 35 meters and then it decreases. At the equator, surface salinity is
lower. The cold water with high density is very heavy and settles down at the bottom of oceans.
❖ At Lower Latitudes, Salinity decreases with depth in low latitudes.
❖ Salinity at the surface increases by the loss of water to ice or evaporation or decreases by the input of fresh
waters, such as from the rivers.
❖ Salinity at depth is very much fixed because there is no way that water is ‘lost’, or the salt is ‘added.’ There
is a marked difference in the salinity between the surface zones and the deep zones of the oceans.
❖ The lower salinity water rests above the higher salinity dense water (high latitudes).
❖ Salinity, generally, increases with depth and there is a distinct zone called the halocline (compare this with
thermocline), where salinity increases sharply..
❖ Other factors being constant, increasing the salinity of seawater causes its density to increase. High-salinity
seawater, generally, sinks below the lower-salinity water. This leads to stratification by salinity.

Tropical Regions ❖ The region experiences high evaporation and more salt content.
❖ Surface water has high salinity, whereas the deep water has comparatively low
salt content.
❖ In tropics as depth increases from Surface water to Deep water salinity
decreases.
Temperate and ❖ This region experiences low evaporation.
Polar Regions ❖ Surface water is light and has low salinity, whereas Deep water is heavy and
dense which increases the salinity in the deep water.
❖ In the Temperate and Polar zone, the water is dense as it is cold and saline,
where salt is settled down due to being heavy in nature. Therefore, in temperate
and Polar regions salinity increases as depth increases.

Salinity in the Indian Ocean:

❖ The average salinity is 35%.
❖ The salinity of the Bay of Bengal region is less than the salinity of the Arabian sea due to freshwater influx
from the rivers.
❖ The Arabian Sea has high salinity due to high evaporation and low freshwater influx due to the lack of any
major river.
❖ The Persian Gulf and the Red Sea regions are enclosed and have less interaction with fresh water. Thus,
these regions have high salinity.
❖ Western Australia has dry weather due to west wind drift and high salinity.
❖ The western margin of the Indian Ocean has high salinity.
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Salinity in the Atlantic Ocean:

❖ The Atlantic Ocean, on average, maintains a salinity of approximately 35.67%.
❖ Within this vast expanse, the tropics experience the highest salinity levels due to limited rainfall and
intense evaporation.
❖ In particular, the western part of the Atlantic Ocean, characterized by warm waters, exhibits both high
salinity and significant evaporation rates.
❖ Furthermore, the North Atlantic Drift, a powerful ocean current, also contributes to elevated salinity levels
due to enhanced evaporation.
❖ These factors collectively shape the salinity distribution in the Atlantic Ocean, impacting its ecological
dynamics and climate patterns.
❖ Understanding the salinity patterns in the Atlantic Ocean is crucial as they influence the ocean's ability to
support marine life, regulate oceanic circulation, and interact with global climate systems.
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 1

DAILY
CLASS NOTES
Geography

Lecture - 49
Corals and Coral Reefs
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Corals and Coral Reefs

Coral Reefs:
❖ Coral reefs are built by and made up of thousands of tiny animals, coral
“polyps”, that are related to anemones and jellyfish.
❖ Polyps are shallow water organisms that have a soft body covered by a
calcareous skeleton. The polyps extract calcium salts from seawater to
form these hard skeletons.
❖ The polyps thrive in colonies attached to the rocky sea floor, while the
tubular skeletons extend upward and outward, forming a fused, calcareous
rock formation known collectively as corals.
❖ These are the living organisms situated in the submarine zones. They are
formed due to the accumulation and compaction of lime secreting coral
polyps and dead corals.
❖ There is a symbiotic relationship with the coral polyps. These are
located in tropical zones- 25 degrees north to 25 degrees south.
❖ Some of the Examples: Andman and Nicobar, Gulf of Munnar,
Lakshadweep, Maldives, Chagos Island, Marshall Island, French
polynesia, Seychelles.
❖ These are submarine animals which cannot tolerate a high range of temperatures.
❖ After the coral polyps die, they shed off their skeleton, known as coral, upon which fresh polyps then develop.
❖ The cycle is repeated for over millions of years leading to the accumulation of layers of corals [Shallow rock
created by these depositions is called reef].
❖ These layers at different stages give rise to various marine landforms. One such important landform is called
Coral Reef.
❖ Coral Reefs, over a period of time, transform or evolve into coral islands (As seen in Lakshadweep).
❖ The corals occur in different forms and colors, depending upon the nature of the salts or constituents they are
made of.
❖ Small marine plants (algae) also deposit calcium carbonate contributing to coral growth.

Corals They are a combination of Algae in soft tissue covered by a hard skeletal structure.

Coral Reefs These are structures formed due to the deposition of layers after layers of coral polyps.
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Corals and Zooxanthellae (Symbiotic Relationship):

❖ Many invertebrates, vertebrates, and plants live in
close association with corals, with tight resource
coupling and recycling, allowing coral reefs to
have extremely high productivity and
biodiversity, such that they are referred to as ‘the
Tropical Rainforests of the Oceans’.
❖ Corals receive their nutrients and energy
resources in two ways.
➢ They used to capture tiny planktonic
organisms with their tentacles.
➢ Having a symbiotic relationship with single-
cell algae known as Zooxanthellae.
❖ Zooxanthellae are autotrophic [prepare their
own food] micro-algae.
❖ Zooxanthellae live symbiotically within the coral
polyp tissues and assist the coral in nutrient
production through its photosynthetic activities.
❖ These activities provide the coral with fixed
carbon compounds for energy, enhance
calcification, and mediate elemental nutrient flux.
❖ The host coral polyp in return provides its
zooxanthellae with a protected environment to
live within, and a steady supply of carbon dioxide
for its photosynthetic processes.
❖ The symbiotic relationship allows the slow-growing corals to compete with the faster-growing
multicellular algae. The corals can feed by day through photosynthesis and by night through predation.
❖ The tissues of corals themselves are actually not the beautiful colors of the coral reef but are instead clear. The
corals receive their coloration from the Zooxanthellae living within their tissues.
Ideal Conditions for Coral Growth:
❖ Stable Climatic Conditions: Corals are extremely vulnerable to rapid changes of climatic conditions and
thrive in areas where the climate remains remarkably stable over longer periods.
❖ Perpetually Warm Waters: Corals thrive in tropical waters [30°North and 30°South latitudes, the
temperature of the water is around 20-21°C] where diurnal and annual temperature ranges are very narrow
(cannot tolerate high/low temperatures).
❖ Abundant Plankton Availability: An adequate supply of oxygen and tiny marine organisms known as
plankton, specifically phytoplankton, is crucial for their growth and development. Since plankton is more
plentiful on the seaward side, corals experience accelerated growth in the seaward side.
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❖ Salinity: The favorable salinity condition for corals is around 31-33%. It cannot sustain in high salinity
conditions. Salts are utilized by the corals' polyps.
❖ Depth: The favorable depth for the survival of corals is 60-77 meters or 200-300 feet. In higher depths, corals
die due to a lack of sunlight and lack of oxygen.
❖ Minimal to Zero Pollution: Corals are
exceptionally delicate and susceptible to the
effects of climate change and pollution. Even a
slight rise in marine pollution levels can have
devastating consequences for them.
➢ If the turbidity is high, there is no sunlight
resulting in clogging of the mouth of coral
polyps.
➢ There should not be any oil spillage,
industrial discharge, and chemical
infiltration.
➢ Corals cannot thrive in front of the river
mouth, because fresh water is injurious to
the health of corals due to less salinity and deposition of sediments.
❖ Ocean Currents: These are very crucial for maintaining conducive conditions like temperature, salinity etc.
❖ Submarine Platform: There should be extensive submarine platforms to form
coral colonies. It also helps in growing the variety of marine organisms which
provides a conducive environment for corals.
➢ With the use of Biorock technology, the government is providing a
conducive platform for the development of coral reefs.
Development Of Major Coral Reef Types:
❖ The basic coral reef classification scheme was first
proposed by Charles Darwin and is still widely
used today.
❖ Darwin postulated that fringing reefs
commenced their growth close to the coastlines
of emerging islands, taking advantage of favorable
ecological conditions for hard coral development.
❖ Subsequently, as the island slowly subsided into
the ocean, the coral continued to grow at a
proportional rate, albeit further from the shoreline,
transforming into a barrier reef.
❖ Ultimately, when the island submerged entirely beneath the sea's surface, only the circular coral structure
surrounding the central lagoon remained—a formation known as an atoll.
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Steps in the Development of Corals:

❖ Step 1: A fringing reef forms first, and starts growing in the shallow waters
close to a tropical island.
❖ Step 2: Over time, the island subsides and the reef grows outwards, and the
distance between the land and the reef increases.
➢ The fringing reef develops into a barrier reef.
❖ Step 3: If the island completely subsides, all that is left is the reef.
➢ The reef retains the approximate shape of the island it grew around, forming a
ring enclosing a lagoon.
Coral Reef Relief Features and Types:
❖ Fringing Reef, Barrier Reef, and Atoll (coral islands are formed on atolls) are the most important relief
features.

Fringing Reefs (Shore Reefs):

❖ Fringing reefs are reefs that grow directly from a
shore. Example: Sakau Island.
❖ These reefs are situated in close proximity to the land,
frequently creating a shallow lagoon that lies
between the beach and the primary expanse of the
reef.
❖ A fringing reef extends in the form of a narrow strip,
typically spanning a width of 1 to 2 kilometers.
❖ They originate from the depths of the ocean floor, with
the seaward side exhibiting a steep incline into the deep sea.
❖ These may/may not have lagoons.
❖ Coral Polyps do not extend outwards because of a sudden and large increase
in depth.
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❖ Among the three primary coral reef types, the fringing reef stands out as the most widespread, with numerous
instances found in all major coral reef development regions.
❖ Examples of fringing reefs can be observed in locations such as the New Hebrides Society islands near
Australia and along the southern coast of Florida.
Lagoon:
❖ A lagoon refers to a comparatively wide band of water that lies between the shore and the main area of reef
development.
Barrier Reefs:
❖ Barrier reefs are extensive linear reef complexes that
lie parallel to a shore, and are separated from it by the
lagoon.
❖ This is the largest (in size, not distribution) of the three
reefs, runs for hundreds of kilometers, and is several
kilometers wide.
❖ A barrier reef takes the form of a fragmented, uneven
circle encircling a coastline or an island, following a
nearly parallel course to it.
❖ While barrier reefs are less prevalent compared to fringing reefs or atolls, instances of them can be located
in both the tropical Atlantic and Pacific regions.
❖ The 1200-mile long Great Barrier Reef off the Northeastern coast of Australia is the world's largest
example of this reef type.
❖ The Great Barrier Reef is not actually a single reef as the name implies, but rather a very large complex
consisting of many reefs.
❖ Sometimes barrier reefs divide in the form of small patches which are called patch reefs.
Atolls:
❖ An atoll is a predominantly circular (annular) coral
reef system in the ocean, encompassing a sizable
central lagoon, often of considerable depth.
❖ The lagoon has a depth of 80-150 meters (240-250
feet) and may be joined with sea water through a
number of channels cutting across the reef.
❖ Atolls typically exhibit a circular or oval shape,
featuring a central lagoon. Portions of the reef
platform may rise above the water's surface as one or
more islands, while openings or breaks in the reef
create pathways to access the central lagoon.
❖ Atolls are far more common in the Pacific than in any other ocean.
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❖ The Fiji atoll and the Funafuti atoll in Ellice/Island are well-known examples of atolls. A large number of
atolls also occur in the Lakshadweep Islands.
❖ In the South Pacific, most atolls occur in mid-ocean.
❖ Instances of this type of reef are frequently found in regions such as French Polynesia, the Caroline and
Marshall Islands, Micronesia, and the Cook Islands. In addition, the Indian Ocean hosts a multitude of atoll
formations.
❖ Examples are found in the Maldives and Chagos island groups, Seychelles, and in the Cocos Island group.
Distribution in India:
❖ India's primary coral reef formations are concentrated in
the Gulf of Mannar, Palk Bay, Gulf of Kutch,
Andaman and Nicobar Islands, and the Lakshadweep
Islands.
❖ Among these, the Lakshadweep reefs take the form of
atolls, while the rest are characterized as fringing
reefs.
❖ Additionally, scattered coral patches can be found in the
intertidal regions along the central west coast of the
country.
Distribution Across the World:
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Coral Bleaching:
❖ Coral reefs are susceptible to disturbances caused
by both anthropogenic (human activities) and
natural occurrences.
❖ The recent rapid decline in coral reefs is primarily
attributed to human-induced factors, including
overexploitation, overfishing, heightened
sedimentation, and excessive nutrient input.
❖ In 1919, Alfred Mayor, noticed that around 70% of the
reefs were undergoing the effects of bleaching. It is
visible near Lakshadweep, Andamans, Kenya, etc.
❖ Coral reefs can be negatively affected by various
natural disturbances, including severe storms, floods,
extreme temperature fluctuations (both high and
low), El Niño Southern Oscillation (ENSO) events, exposure during low tides, outbreaks of predators,
and epizootics.
❖ Many of these disturbances can trigger a common stress response in corals known as coral reef bleaching.
❖ Bleaching occurs when the densities of Zooxanthellae decline and/or the concentration of photosynthetic
pigments within the zooxanthellae falls.[it is no more use for the coral and the coral will bleach it]
❖ When corals bleach they commonly lose 60-90% of their zooxanthellae and each zooxanthellae may lose 50-
80% of its photosynthetic pigments.
❖ When coral experiences bleaching due to stress, it can often recover if the stress is not too severe and
diminishes over time. In such cases, the affected corals typically reestablish their symbiotic algae within
several weeks or a few months.
❖ However, if the loss of zooxanthellae persists for an extended period, meaning that the stress continues and
the depleted zooxanthellae populations do not rebound, the coral host ultimately succumbs and dies.
Causes of Coral Bleaching:
❖ Temperature:- Coral species are adapted to a relatively limited
temperature range, and extreme deviations in sea temperatures—
both unusually low and high (for instance, due to cold currents
along the west coast of tropical temperate continents)—can
trigger coral bleaching.
➢ Bleaching events occur during sudden temperature drops
accompanying intense upwelling episodes [ In 1983, 1998,
1987 El-Nino], and seasonal cold-air outbreaks. While most
reefs recovered, with low levels of coral deaths, the damage has been severe in many places.
➢ This is an instance of coral reefs' susceptibility to increased water temperatures combined with Ocean
Acidification.
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➢ The rise in temperatures has led to more frequent and severe

bleaching events, while acidification has diminished the corals'
capacity for calcification.
➢ Small temperature increases over many weeks or a large increase
(3-4 °C) over a few days will result in coral dysfunction.
➢ Coral bleaching predominantly occurs in summer or towards the end
of a protracted warming period. These events are often associated
with conditions characterized by low wind speeds, clear skies,
tranquil seas, and reduced turbidity.
➢ The conditions favor localized heating and High Ultraviolet (UV) Radiation.
➢ Ultraviolet radiation readily penetrates clear sea waters. The corals actually contain UV-absorbing
compounds that can block potentially damaging UV radiation. But rising temperatures mean a reduction
in the concentration of these UV-absorbing compounds in corals.
❖ Sub aerial Exposure:- Sudden exposure of reef flat corals to the atmosphere during events such as extreme
low tides, ENSO-related sea level drops or tectonic uplift can potentially induce bleaching.
➢ As a result, exposure to extreme temperatures, heightened solar radiation, desiccation, and the dilution of
seawater due to heavy rainfall could contribute to the loss of zooxanthellae, but they could also
potentially result in the death of the coral itself.
❖ Fresh Water Dilution:- Rapid dilution of reef waters due to precipitation and runoff generated by storms has
been proven to trigger coral reef bleaching. Typically, these bleaching incidents are infrequent and limited to
relatively small, coastal regions.
❖ Destructive Fishing: Due to the use of big trollers for deep fishing.
❖ Mining of Corals for commercial, laboratory, and industrial purposes. It leads to the destruction of the
colonies.
❖ Inorganic Nutrients: Rather than causing coral reef bleaching, an increase in ambient elemental nutrient
concentrations (e.g. ammonia and nitrate) actually increases zooxanthellae densities 2-3 times.
➢ Although eutrophication is not directly involved in zooxanthellae loss, it could cause secondary adverse
effects such as lowering of coral resistance and greater susceptibility to diseases.
❖ Epizootics: Pathogen-induced bleaching is different from other sorts of bleaching.
➢ Most coral diseases cause patchy or whole colony death and sloughing of soft tissues, resulting in a white
skeleton (not to be confused with bleached corals).
➢ Some common diseases are Coral Plague, Black Band Disease, White Band disease.
Wetland Destruction : Wetland destruction contributes to coral bleaching, amplifying the detrimental impact on
delicate ecosystems, as the loss of wetlands disrupts the natural balance and resilience of coastal areas.


 1

DAILY
CLASS NOTES
Geography

Lecture - 50
Ocean Tides
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Ocean Tides
Ocean Tides:
Vertical Motion of the Ocean:
 It refers to the periodic rise and fall of water in oceans and
seas.
 Attributed to the gravitational attraction of the sun and the
moon.
 Centrifugal force acts as a reactionary force.

Tidal Waves and Surges:

 Tidal waves: They are waves generated due to tides.
 Surges: These are movements of water caused by
meteorological effects like winds and atmospheric pressure
changes.
 They are irregular and not on a fixed schedule like
tides.

Tide:
 Tides: They are the periodic rise and fall of sea levels, occurring once or twice a day.
 Mainly influenced by the gravitational pull of the sun and moon.

Types of Tide:
 High tide: It refers to ocean water moving towards the
coast, forming a high tide line.
 Low tide: It is when ocean water recedes from the
coast, creating a low tide line.
 Tidal range: It is the difference between high and low
tide lines.

Factors Influencing Tide Variation:

 Depth and volume of ocean water.
 Shape, size, and configuration of ocean relief.
 Nature of the sea, whether it is open or closed.

Tidal Bulge:
 It is a gravitational bulge of the ocean's water
surface.
 It occurs simultaneously at two places:
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 One bulge towards the moon (lunar or solar bulge) and

 Another away from the moon on the opposite side of the
Earth (lunar or solar bulge).
 Tide-generating force results from the difference between
these two forces.
 Generation of Tidal Bulges:
 Near the moon, gravitational attraction exceeds the
centrifugal force, causing a bulge towards the moon.
 On the opposite side of the Earth, the centrifugal force
dominates as it's farther from the moon, creating a second
bulge away from the moon.
 Horizontal tide-generating forces are more significant
than vertical forces in generating tidal bulges.
 Effect of Geography and Ocean Features on Tidal Bulge:
 Tidal bulges on wide continental shelves have greater height.
 Mid-oceanic islands experience lower tidal bulges.
 The shape of bays and estuaries can amplify tidal intensity, especially in funnel-shaped bays.

Tidal Currents:
 It occur when tides are channeled between islands or into bays and estuaries.
 Result from the movement of tidal waters in these constrained areas.

Moon's Tidal Lock:

 The moon is tidally locked with the Earth, causing only one side to be visible (the dark side is not visible).

Types of Tides Based on Sun, Moon, and Earth

Positions:
Spring Tides:
 Tide height is influenced by the relative positions of
the Sun, Moon, and Earth.
 It occurs when the Sun, Moon, and Earth align in a
straight line (syzygy).
 It happens twice a month during full and new moon
phases.
 High tides are about 20% higher than usual.
Neap Tides:
 Occur approximately seven days after spring tides.
 Sun and Moon are at right angles to each other,
counteracting each other's gravitational forces.
 Result in lower high tides and higher low tides.
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 Happen during the first and third quarters of the moon.

Quadrature and Fortnightly Cycle:

 Quadrature: It occurs when the Sun, Earth, and Moon form a right angle.
 Marks the transition between spring and neap tides.
 A fortnight (15-day period) includes one quadrature position, leading to tidal variations.

Equatorial Tides or Tropical Tides:

 It result from the northward and southward alignment of the moon at the Tropic of Cancer and Tropic of
Capricorn.
 When the moon is vertically above the Tropic of Cancer, high tides are higher than usual, and low tides are
higher.
 Conversely, at the Tropic of Capricorn, high tides are lower than normal, and low tides are lower.

Apogee and Perigee Tides:

 Linked to the moon's proximity to Earth.
 Perigee Tide: Occurs when the moon's orbit is closest to Earth, leading to unusually high high tides and low
low tides.
 Greater tidal range during perigee.
 Apogee Tide: Occurs when the moon is farthest from Earth, resulting in slightly lower high tides and slightly
higher low tides.
 Happens two weeks after perigee.

The perihelion and Aphelion Tides:

 These are associated with Earth's proximity to the Sun.
 Perihelion: It occurs when Earth is closest to the Sun (around 3rd January annually), resulting in greater tidal
ranges with unusually high and low tides.
 Aphelion: It occurs when Earth is farthest from the Sun (around 4th July annually), leading to reduced tidal
ranges with tides less than average.
 5

Classification Description
Spring Tide  Occurs during a full moon and new moon.
 The sun, moon, and Earth align in a straight line, leading to higher high tides
and lower low tides.

Neap Tide  Occurs during the first and third quarters of the moon.
 The sun and moon are at right angles to each other, resulting in lower high tides
and higher low tides.

Tropical or Equatorial  It is caused by the northward and southward alignment of the moon at the
Tide Tropics (e.g., Tropic of Cancer).
 High tides are higher than normal at one Tropic and lower at the other, depending
on the moon's position.
Apogee and Perigee  Apogee Tide: It occurs when the moon is farthest from Earth, leading to lower
Tide high tides and higher low tides.
 Perigee Tide: It occurs when the moon is closest to Earth, resulting in higher
high tides and lower low tides.

Classification of tides:

Diurnal Tide:
 It is due to the effect of the day and night.
 There is only one high tide and one low tide during each day.
 The successive high and low tides are approximately of the same height.

Additional Information:
 Mixed Tide: Tides having variations in height are known as mixed tides. These tides generally occur along
the west coast of North America and on many islands of the Pacific Ocean.
 Semi-diurnal Tide: It is the most common tidal
pattern, featuring two high tides and two low tides each
day. The successive high or low tides are
approximately of the same height.
 Tidal Bore: The steep wall of water created by
incoming high tides in the constricted estuaries of
coastal rivers or in bays having narrow and constricted
mouths is called a tidal bore.
 Tides of the Bay of Fundy, Canada: The highest tides
in the world occur in the Bay of Fundy in Nova Scotia,
Canada.
 6

 The tidal bulge is 15 - 16 m. Since there are two high tides and two low tides every day (roughly a 24-
hour period), a tide must come in within about a six-hour period.
 As a rough estimate, the tide rises about 240 cm an hour (1,440 cm divided by 6 hours).
 Tides at Chandipur Coast (Balasore, Odisha): Two times a day, seawater recedes back. People can walk
on the coast for up to 5 kilometers (the only place in India). This is possible due to the flat ocean bed.


 1

DAILY
CLASS NOTES
Geography

Lecture - 51
Ocean Currents
 2

Ocean Currents

Ocean Currents:
❖ Dynamic Nature of Ocean Water: The ocean's physical attributes, including temperature, salinity, density,
and external factors like the sun, moon, and winds, collectively influence the dynamics of ocean water.
❖ Understanding Ocean Currents: Ocean currents represent extensive surface water movements within the
world's oceans. These currents exhibit regular, directional patterns and are primarily driven by gravitational
forces, wind patterns (including the Coriolis Effect), and variations in water density.
❖ Distinguishing Ocean Currents from Ocean Waves: While ocean currents denote the continuous, ordered
flow of vast water masses, it's essential to differentiate them from waves, which signify horizontal water
motion.
❖ Significance of Ocean Current: Mobilize warm water from the equatorial region to the polar region and
cold water from the polar region to the equatorial region.
❖ Strength/Speed of Current: Measured in knots (1.852 km/hr).
❖ Depth and Speed Variations: Ocean currents exhibit diverse characteristics regarding depth and speed.
These currents can extend from depths of 500-5000 feet.
➢ Speed is typically measured in knots, with surface currents often exceeding five knots. Deeper
currents tend to be slower, typically below 0.5 knots.
❖ Current Strength and Behavior: Current strength is directly related to speed, with fast currents considered
strong. Additionally, currents tend to be strongest near the ocean's surface and gradually weaken with depth.
The majority of currents maintain speeds of five knots or less.
Types of Ocean Movements:
 3

❖ Drifts: It refers to the movement of surface water propelled by prevailing winds. Notable examples include
the North Atlantic Drift and West Wind Drift, characterized by speeds of 3-4 miles per hour and depths
ranging from 200-300 feet.
❖ Streams: They are faster-moving than drifts or general ocean currents, with speeds ranging from 10-12
miles per hour. Prominent examples include the Gulf Stream.
❖ Layered Movement: Ocean currents mimic flowing streams, capable of transporting surface water over
extensive distances. These currents initiate layer-by-layer movement, with the uppermost layer being the
most exposed and moving at the swiftest pace. Inter-layer friction acts as a resistance force impeding the
movement between layers.
 4

Classification of the Ocean Currents:

Classification of Ocean Currents by Temperature:
❖ Cold Currents: Cold currents transport cold waters into warmer regions. They typically flow along the west
coasts of continents in low and middle latitudes, and on the east coasts in higher latitudes in the Northern
Hemisphere. These currents can have temperatures ranging from 0 to 2 degrees Celsius.
❖ Warm Currents: Warm currents carry warm water into colder areas. They are commonly found along the
east coasts of continents in low and middle latitudes and on the west coasts of continents in high latitudes in
the Northern Hemisphere. These currents can have temperatures ranging from 27 to 29 degrees Celsius.
Classification of Ocean Currents by Depth:
❖ Surface Currents: Surface currents make up approximately 10 percent of all ocean water and encompass
the upper 400 meters of the ocean. They tend to move swiftly compared to deep currents.
❖ Deep Water Currents: Deep water currents constitute the remaining 90 percent of ocean water. These
currents circulate within ocean basins due to variations in water density and gravitational forces. Deep waters
sink into the ocean's depths at high latitudes, where the cold temperatures increase water density. These
currents have lower speeds due to increased resistance.
Classification of Ocean Currents by Nature:
❖ Ocean currents can also be categorized based on their nature, including their speed and other characteristics.
❖ On the basis of the nature of the current (high speed or low speed etc.)

Drift ❖ Low speed

❖ 2-3 miles/hr
❖ On the surface
❖ Example: West wind drift, North Atlantic Drift, etc.

Current ❖ Medium Speed

❖ 5-8 miles/hr
❖ Example: Labrador Current
 5

Streams ❖ High speed

❖ Large mass
❖ 10-12 miles/hr
❖ Example: Gulf stream

Factors that Affect the Flow of the Currents:

❖ Earth-Related Factors:
➢ Gravitation: Gravity exerts a pull on water, creating variations in gradient. Lighter water remains on
the surface, while denser water moves downward.
➢ Coriolis Force: In the northern hemisphere, the Coriolis force deflects water to the right, while in the
southern hemisphere, it deflects water to the left. This leads to the formation of large circular currents
known as Gyres.
➢ Friction: Friction with the ocean floor can slow down or modify the direction of ocean currents.
➢ Due to Wind: Wind patterns at the ocean's surface play a significant role in driving currents. Prevailing
winds, such as the Trade Winds and Westerlies, can push water in specific directions, creating surface
currents.
❖ Oceanic Factors:
➢ Temperature: Warm currents, reaching colder regions, maintaining higher temperatures in winter.
Equatorial heating causes water to expand, leading to surface flow from the equator to the poles, and
cold, dense, subsurface flow from the poles to the equator.
➢ Salinity: Water with high salinity is denser, causing it to sink, while less saline water rises. For
instance, the Mediterranean Sea's high salinity leads to subsurface flow into the Atlantic Ocean.
➢ Density: Differences in water density, influenced by temperature, salinity, and freshwater influx, affect
vertical mobility. Dense water sinks, initiating subsurface flow, as seen in the Greenland current.
❖ Atmosphere-Related Factors:
➢ Wind Systems: Wind patterns drive water movement. Friction between wind and the water's surface
affects its course.
➢ Planetary Winds: Trade winds move equatorial water polewards and westwards, impacting currents
like the Gulf Stream.
➢ Westerlies: In temperate latitudes, westerly winds influence currents like the North Atlantic Drift.
➢ Pressure Conditions: Falling winds create high pressure, compressing water while rising winds result
in low pressure and water expansion. Water flows from low-pressure to high-pressure areas.
➢ Precipitation: High precipitation leads to increased freshwater influx and lower salinity (light water),
while low precipitation results in higher salinity (heavy water that sinks).
❖ Physical Factors:
➢ The configuration, size, and shape of continents can act as barriers, diverting and guiding currents
parallel to coastlines.
 6

Impact of Ocean Currents:

❖ Heat Redistribution:
➢ Cold water from polar regions moves towards the equator, while warm water from the equator moves
towards polar regions.
❖ Moderate Weather:
➢ North Atlantic Drift: Warms European coasts and melts ice caps.
➢ Canary Current: Brings
cooling effect to Spain and
Portugal.
❖ Climatic Phenomena:
➢ Warm currents along east
coasts result in warm, rainy
climates.
➢ Fog forms at warm and cold
current intersections, e.g.,
Japan and the North Atlantic.
❖ Desert Formation:
➢ Cold currents cause aridity and
desert formation on western
continental sides, e.g., Canary and Humboldt/Peru currents.
❖ Precipitation:
➢ Wind over warm currents brings moisture and increased precipitation, e.g., North Atlantic Drift and
Kuroshio Current.
 7

❖ Impact on Marine Life:

➢ Mixing of warm and cold currents supports
plankton growth, crucial for fish
populations.
➢ Nutrient-rich upwelling zones nurture the
world's best fishing grounds.
❖ Navigational Aid:
➢ Sailors use ocean currents for navigation, e.g., Indian Ocean Arab travelers.
❖ Ocean currents have a profound impact on climate, ecosystems, and human activities, shaping the world's
weather patterns, marine life, and even historical navigation routes.


 1

DAILY
CLASS NOTES
Geography

Lecture – 52
Ocean Currents (Part - 2)
 2

Ocean Currents (Part - 2)

Pacific Ocean Currents:

North Equatorial Current:

❖ It originates off the coast of Mexico and reaches to the Philippines (east to west direction).
❖ It originates due to the California current and North-East Monsoon.
1. One Branch emerges near Taiwan and flows northwards to join Kuroshio.
2. Southwards Branch turns Eastwards to form the Counter Equatorial Current.
South Equatorial Current:
❖ It is characterized by South East Trade winds.
❖ It is stronger than the North Equatorial Current.
❖ Numerous branches join this current from the
left and it increases the volume of water
westwards.
❖ The Current bifurcates into North and South
branches near Guinea.
❖ The Northern Branch turns eastward and joins
to form the Counter Equatorial Current.
❖ The Southern Branch moves towards the North
and North - East Coasts of Australia.
 3

Equatorial Pacific Ocean Current:

❖ Under the influence of prevailing trade winds
[tropical easterlies], the north equatorial current
and the south equatorial current start from the
eastern pacific (west coast of Central America) and
traverses a distance of 14,500 km moving from the
east to west.
Counter Equatorial Current:
❖ This raises the level of the western pacific (near
Indonesia and Australia) ocean by a few centimeters.
❖ It creates a counter-equatorial current which flows
between the north equatorial current and the south
equatorial current in the west-east direction.
❖ Three factors aid the formation of a Counter-
Equatorial current.
➢ Piling up of water in the western pacific due to
trade winds.
➢ The presence of doldrums (Equatorial Low-
Pressure Belt) in between the North Equatorial
Current and the South Equatorial Current.
[Doldrums are narrow regions with calm
atmospheric conditions. Such conditions aid
the backward movement of piled up western
pacific waters.]
➢ Piling of water in the western part of oceans due to the rotation of the earth.
Kuroshio System:
❖ Runs from Taiwan to the Bering Strait.
❖ Consists of Several Branches:
1. Kuroshio Current: North Equatorial
currents turned by the Philippines
generate this Current. The average
temperature is 8 degrees, Salinity is
35 ppt. It is a warm Current.
2. Kuroshio Extension: It turns
Eastwards under the influence of
 4

Westerlies. One Branch moves in the East direction and the second branch moves in the Northeast
direction to join Oyashio coming from the North.
3. North Pacific Drift: The Kuroshio current, under influence of westerlies, reaches the western coast of
North America. It bifurcates into two branches:
✓ North Branch became Aleutian Current (Alaska current).
✓ South Branch becomes Cold California Current. It gets mixed with the counter equatorial current.
4. Tsushima Current: Near 30 degrees latitude, one
branch leaves Kuroshio and enters the west coast of
Japan and becomes Tsushima current.
Kuroshio Current:
❖ The North Equatorial Current turns northward off the coast
of the Philippines to form the Kuroshio current.
❖ Most of it lies in the sub-tropical high-pressure belt and its
northern part is under the influence of westerlies.
Oyashio Current and Okhotsk Current:
❖ There are two more cold currents in the northern Pacific.
❖ Oyashio flows across the east coast of the Kamchatka
Peninsula to merge with the warmer waters of Kuroshio.
❖ Okhotsk Current flows past Sakhalin Islands to merge with
the Oyashio current off Hokkaido (Northern Japanese
Island).
California Current:
❖ It is the eastward extension of the North Pacific Drift.
❖ This current is generated in order to compensate for the loss
of water which is caused due to large-scale transport of water off the coast of Mexico under the influence of
trade winds in the form of North East Currents.
❖ This, after reaching the Mexican coast, turns westwards and mixes with the North Equatorial Current.
North-Pacific Current:
❖ From the South-east Coast of Japan, under the influence of prevailing westerlies, the Kuroshio Current turns
eastwards and moves as the North-Pacific Current, reaches the West Coast of North America, and bifurcates
into two.
Alaska and California Current:
❖ The northern branch flows anti-clockwise along the coast of British Columbia and Alaska and is known as
the Alaska Current.
 5

❖ The water of this current is relatively warm as compared to

the surrounding waters in this zone.
❖ The southern branch of the current moves as a cold current
along the west coast of the USA and is known as
Californian Current.
❖ The Californian Current joins the north equatorial current to
complete the circuit.
East Australian Current:
❖ Following the pattern in the Northern Hemisphere, the South
Equatorial Current Flows from east to west and turns southwards as
the East Australian current.
❖ It meets the South pacific current near Tasmania which flows from
west to east.
Southern Pacific Current:
❖ Current becomes much stronger due to the high volume of water
mass and high-velocity winds called Roaring Forties.
Peru or Humboldt Current:
❖ It flows from South to North near the Western coast of
South America.
❖ It causes a desiccating effect in the Atacama desert.
❖ The Northwards deflection of West Wind Drift is due
to the Southern tip of South America.
❖ Reaching the South-western coast of South America, it
turns northward as the Peru current.
❖ It is a cold current, which finally feeds the South
Equatorial Current, thus completing the great circuit.
❖ The zone where the cold Peru Current meets the Warm
Equatorial Ocean waters is an important fishing zone.
❖ West Wind Drift:
❖ It flows Westeastern under the influence of Westerlies between Tasmania and South American Coast at 40 -
50 Degrees Latitudes.
❖ Current becomes much stronger due to the high volume of water mass and high velocity winds called
Roaring Forties.
 6

Atlantic Ocean Currents:

North Equatorial Currents:
❖ It is formed between the equator and 10 degrees
latitude.
❖ It is generated because of the upwelling of cold water
near the African Coast.
❖ This current, after being obstructed by a land barrier,
is divided into two branches:
➢ Antilles current: It helps in the formation of the
Sargasso Sea.
➢ Caribbean Current: It enters the Gulf of Mexico.
Equatorial Atlantic Ocean Currents:
❖ Under the influence of prevailing trade winds [easterly
trade winds], the north equatorial current and the south equatorial current start from the eastern Atlantic
(west coast of Africa), moving from east to west.
❖ This raises the level of the western Atlantic Ocean by a few centimeters.
❖ This creates a counter-equatorial current that flows between the north equatorial current and the south
equatorial current in a west-east direction.
South Equatorial Current:
1. It flows from East to West.
2. Between -Equator to 20 Degrees South.
3. It is more constant, stronger and of greater extent.
4. Its two Branches:
✓ Northwards Branch: It merges with the North Equatorial Current.
✓ Southern Branch: It turns southwards and merges with
Brazilian Current.
Antilles Current:
❖ The South Equatorial Current bifurcates into two branches
near Cape de Sao Roque (Brazil).
❖ A part of the current enters the Caribbean Sea along with
North Equatorial Current into the Mexican Gulf, while the
remainder passes along the eastern side of the West Indies as
the Antilles current.
 7

❖ There is a rise in water level in the Mexican Gulf because of large amounts of water brought by the
Mississippi river and branches of North and South Equatorial Currents.
Counter Equatorial Current:
❖ It has a West-East flow between the North and
South Equatorial Current.
❖ This current originated due to:
➢ Influence of Equatorial Westerlies which
blow in the calm zone of Doldrums.
(Convergence zone of trades).
➢ Piling up of the immense volume of water
because of the convergence of two
Equatorial Warm currents. Piling up raises
the water level and water flows eastwards
as compensation current.
Gulf Stream and North Atlantic Drift:
❖ Antilles current creates a current that flows out through the Strait of Florida as Florida Current, which mixes
with Antilles Current from the south.
❖ This combined current moves along the east coast of the USA and is known as the Florida current up to the
Cape Hatteras and as the Gulf Stream beyond that.
❖ It is the strongest, of High energy, and High-density current.
❖ It originates in the Gulf of Mexico and move in the N.W Direction.
❖ It moves in 3 directions:
➢ One branch (East): Britain Coast
➢ 2nd branch (North): Arctic
➢ 3rd branch (South): Iberian Peninsula as Canaries Current.
❖ Near the Grand Banks, the Gulf Stream mixes with cold Labrador and East Greenland currents and flows
eastward across the Atlantic as the North Atlantic Drift.
❖ Here, the westerly movement of North Atlantic Drift is due to the influence of westerlies.
Norwegian Current:
❖ The North Atlantic Current breaks up into two branches on reaching the eastern part of the ocean.
❖ The main current, continuing as the North Atlantic Drift, reaches the British Isles from where it flows along
the coast of Norway as the Norwegian current and enters the Arctic Ocean.
❖ Norwegian current is very important as it keeps the ocean to the North of Norway partly free from ice and
also moderates the extremes of climate.
 8

❖ It is because of this current, Russia is able to move cargo in summer through the Arctic ocean (Barents Sea).
❖ The southerly branch flows between Spain and Azores as the cold Canary current.
❖ This current finally joins the North Equatorial Current completing the circuit in the North Atlantic.
❖ The Sargasso Sea, lying within this circuit, is full of large quantities of seaweed and is an important
geographical feature.
Canaries Current:
❖ It flows along the western coast of North Africa.
❖ It is an extension of the North Atlantic Drift that turns Southwards near the Spanish coast and flows South
along the coast of Canaries Island.
❖ It brings cold water from high latitudes to warm water from low latitudes.
❖ It causes a desiccating effect on the Saharan Desert.
Labrador Current:
❖ It originates in the Baffin Bay and Davis Strait.
❖ After flowing through the coastal waters of Newfoundland and Grand Bank, it merges with the Gulf Stream
at around 50 degrees Longitude.
❖ It brings Icebergs to Newfoundland which present hindrances in oceanic navigation.
❖ Dense fogs are produced due to the meeting of Labrador and Gulf Stream near Newfoundland.
Sargasso Sea:
❖ Anticyclonic Circulation of ocean currents formed by North East Current, Gulf Stream and Canary Curren t
in the North Atlantic.
❖ Here, Water is confined in a gyre and is calm and motionless.
❖ Sargassum here refers to Seaweed.
❖ Its extent is from 20 to 40 degrees North Latitudes, 35-75 degrees West.
❖ Annual Temperature is 28 degrees.
❖ It is characterised by high salinity, high temperature and evaporation.
Origin of Sargasso Sea-Attributed to Several Factors:
❖ Maximum part of the North Atlantic is confined between the Gulf Stream, the North equatorial current, and
the Canary current. It has confined waters and has no connection with other waters.
❖ Sargasso sea is located in the transition zone of Easterlies and Westerlies which is a zone of calmness.
❖ This zone is a zone of subsidence of air resulting in high pressure and Anticyclonic circulation, thus, leading
to stability, Calm Winds with little mixing.
❖ Confined waters become calm due to the higher velocity of the North Equatorial Current and Gulf Stream.
 9

❖ The Sargasso Sea is a region in the gyre in the

middle of the North Atlantic Ocean.
❖ It is the only sea on Earth that has no coastline.
❖ It is bounded on the:
➢ West by the Gulf Stream;
➢ North, by the North Atlantic Current;
➢ East, by the Canary Current; and
➢ South, by the North Atlantic Equatorial Current.
❖ This system of ocean currents forms the North
Atlantic Gyre.
❖ All the currents deposit the marine plants and refuse
they carry into this sea.

Grand Banks- Richest Fishing Grounds on Earth:

❖ The two cold currents— East Greenland Current and the Labrador Current —flow from the Arctic Ocean
into the Atlantic Ocean.
❖ The Labrador Current flows along part of the east coast of Canada and meets the warm Gulf Stream.
❖ The confluence of these two currents, one hot and the other cold, produce the famous fogs around
Newfoundland.
❖ As a result of the mixing of cold and warm waters, one of the world's most important fishing grounds is
created here.

Brazil Current:
❖ It is characterized by high temperature and high salinity.
❖ It is generated due to the bifurcation of the South Equatorial
current due to obstruction at the Brazilian Coast at Sun
Rock.
❖ It flows southwards along the Eastern Coast of South
America and flows Eastwards due to the deflective force due
to rotation and the influence of Westerlies.
❖ Further, it meets with Falkland current coming from the
South.
❖ In the South Atlantic Ocean, the south equatorial current,
flowing from east to west, splits into two branches near
Cape de Sao Roque (Brazil).
 10

❖ The northern branch joins the north equatorial current (a part of it flows in Anatilles Current and other into
the Gulf of Mexico), whereas the southern branch turns southward and flows along the South American coast
as the warm Brazil current.
❖ The south-flowing Brazil current swings eastward at about latitude 35°S (due to westerlies) to join the West
Wind Drift flowing from west to east.
❖ A current results from the movement of water from the West Wind Drift as it rounds Cape Horn called
Falkland Current (also known as Malvinas Current).
❖ It mixes with the warm Brazil Current at the southern tip of Brazil.
Benguela Current:
❖ A branch of the South Atlantic splits at the southern tip of Africa and
flows along the west coast of South Africa as the Cold Benguela
Current, which joins the south equatorial current to complete the
circuit.
❖ Flows from South-North along the western coast of South Africa.
❖ By South Atlantic Drift diverting towards the North due to diversion
from the Southern tip of Africa.
Indian Ocean Currents:
❖ The Indian ocean is half an ocean, hence the behavior of the
North Indian Ocean Currents is different from that of Atlantic
Ocean Currents or the Pacific Ocean Currents.
❖ Also, monsoon winds in the Northern Indian ocean are
peculiar to the region, which directly influence ocean surface
water movement [North Indian Ocean Currents].
❖ The currents in the Northern Portion of the Indian Ocean
change their direction from season to season in response to
the seasonal rhythm of the monsoons.
❖ The effect of winds is comparatively more pronounced in the
Indian Ocean.
Direction of Indian Ocean Currents:
❖ The ocean currents of the Indian Ocean are slightly different
from the Pacific and Atlantic Ocean.
❖ Land and monsoon winds have a major impact on the Indian
Ocean current.
❖ The direction of current flow changes twice in the Northern Indian Ocean due to North -East and South-West
monsoon winds whereas Southern Indian ocean currents have a definite pattern.
 11

Winter Circulation:
❖ Under the influence of Prevailing Trade Winds [easterly trade winds], the North Equatorial Current and the
South Equatorial Current start from the south of Indonesian islands, moving from east to west.
❖ This raises the level of the western Indian (southeast of the horn of Africa) Ocean by a few centimeters and
this creates a counter-equatorial current that flows between the North Equatorial Current and the South
Equatorial Current in West-east Direction.
❖ The North-East Monsoons drive the water along the coast of the Bay of Bengal to circulate in an anti-
clockwise direction.
❖ Similarly, the water along the coast of the Arabian Sea also circulates in an anti-clockwise circulation.
Summer Circulation:
❖ In summer, due to the effects of the strong south-west monsoon and the absence of the north-east trades,
a strong current flows from west to east, which completely obliterates the north equatorial current. Hence,
there is no counter-equatorial current as well.
❖ Thus, the circulation of water in the northern part of the ocean is clockwise during this season.
❖ The general pattern of circulation in the southern part of the Indian Ocean is quite similar to that of the
southern Atlantic and Pacific oceans.
❖ It is less marked by seasonal changes.
❖ The South Equatorial Current, partly led by the corresponding current of the Pacific Ocean, flows from east
to west.
❖ It splits into two branches, one flowing to the east of Madagascar known as Agulhas current, and the other
between Mozambique and the Western Madagascar coast known as Mozambique Current.
❖ At the southern tip of Madagascar, these two branches mix and are commonly called the Agulhas current. It
still continues to be a warm current, till it merges with the West Wind Drift.
❖ The West Wind Drift, flowing across the ocean in the higher latitudes from west to east, reaches the southern
tip of the west coast of Australia.
❖ One of the branches of this cold current turns northwards along the west coast of Australia. This current,
known as the West Australian Current, flows northward to feed the south equatorial current.

   