Elements of Climate

UNIT 6 WEATHER PHENOMENON

Structure
6.0 Introduction
6.1 Objectives
6.2 Weather: An Introduction
6.3 Introduction to Air Masses
6.3.1 Classification of Air Masses
6.4 Fronts and Temperate Cyclones
6.5 Tropical Cyclones
6.6 Jet Streams
6.7 South-West and North East Monsoons
6.7.1 South-West Monsoon
6.7.2 Classification of Air Masses
6.8 El Nino Southern Oscillation (ENSO)
6.9 Classification of Climate by Koeppen and Thornthwaite
6.10 Let Us Sum Up
6.11 Keywords
6.12 References and Suggested Further Readings
6.13 Answers to Check Your Progress

6.0 INTRODUCTION
In the previous unit of this block, you have been introduced to elements of climate and
their controls. So now you are aware that there is differential heating of different parts
of the earth due to incoming solar radiation. This brings about differences in
temperatures, pressure and rainfall etc and in fact different climatic conditions across
the globe. In this unit, we are concentrating more on weather phenomenon or weather
disturbances. You have already been introduced to weather and climate in the previous
unit. In this unit, you will get a short introduction to weather. To understand different
weather phenomenon we need to get an overview of different air masses. Meeting of
different air masses leads to development of fronts and in fact cyclones. You will also
study about tropical cyclones which are non-frontal cyclones and about jet streams
and monsoons. El Nino is also discussed and is to be studied in connection with
monsoons. Lastly, you will study climatic classification given by Koeppen and how his
scheme was different from that of Thornthwaite’s scheme of climatic classification.

6.1 OBJECTIVES
After studying this unit, you should be able to:
• define weather and air masses and explain various processes in different air
masses that lead to weather disturbances;
• discuss the formation of fronts and temperate cyclones;
• explain the processes related to tropical cyclones;
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Climatology and • describe the origin and mechanism of monsoons;
Meteorology
• analyse the affect of jet streams and El Nino in the outbreak and progression of
monsoons and other weather phenomenon; and
• describe the classification of climates given by Koeppen.

6.2 WEATHER: AN INTRODUCTION

Let us have a quick recap about weather. ‘Weather’ refers to the state of atmosphere
at any given time and denotes the short term variations of atmosphere in terms of
temperature, pressure, wind, moisture, cloudiness, precipitation and visibility. These
are also known as the elements of weather. Weather is highly variable. It varies not
only from day to day but even hour to hour. This is because of the constant change in
the elements of weather which are closely interrelated. Take for example if temperature
rises, pressure becomes low, winds get attracted towards low pressure area, gives
rise to convection currents and results in cloudiness, low visibility and precipitation.
Extremes of these conditions may lead to cyclones or other weather disturbances. It is
important to note that most of the weather phenomena occur in the lowest levels of the
atmosphere, that is, troposphere, just below the stratosphere.

6.3 INTRODUCTION TO AIR MASSES

As mentioned earlier, in order to understand atmospheric disturbances, we need a
clear understanding of air masses. This is because air masses play a major role in
bringing about day-to-day weather changes and in producing different climate types in
the long run. So let us learn about air masses. An air mass is an immense body of air
usually 1600 km or more across and perhaps several kilometers thick, which is
characterised by homogenous physical properties (particular temperature and moisture
content) at any given altitude. Trewartha defines air mass as “an immense body of air
that moves over earth’s land-sea surface as a recognizable entity, with temperature
and humidity characteristics which are relatively uniform in a horizontal direction at
different levels.” Thus, we conclude that air mass is a large body of air whose physical
properties especially temperature, moisture content and lapse rate are more or less
uniform horizontally for hundreds of kilometers.
Let us now study how air masses are formed and what are source regions of air
masses. Areas where air masses form are known as source regions. Nature and
degree of uniformity of air mass are determined by the properties of the source area,
changes introduced in the air mass during its journey away from the source area and
the age of the air mass. When air remains in contact with a large and uniform surface
for a quite some time, let us say a couple of days, its temperature and moisture attain
equilibrium with the surface. So an ideal source region should an extensive and
homogenous earth’s surface so that it may possess uniform temperature and moisture
conditions. There should be divergent air flow so that air may stay over the region for
longer period of time. Thus, anticyclonic conditions characterised by high barometric
pressure and low pressure gradient favours the development of air masses. Also the
atmospheric conditions should be stable for considerably long period of time so that
air may attain the conditions of the surface.
6.3.1 Classification of Air Masses
There are three bases of classification of air masses. These are:
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• Thermal
• Latitudinal Weather
Phenomenon
• Configurational
On the basis of temperature (thermal) air masses can broadly be divided into warm air
mass and Cold air mass. On the basis of latitudinal extent it may be (i) tropical and (ii)
polar air mass and on the basis of configuration they can be (i) Continental air mass
and (ii) maritime air mass.
Again a composite scheme may be given by combining all the 3 above mentioned
schemes. Composite scheme shows that there are two major types, four second order
types and eight 3rd order types. Each one is again divided on the basis of stability.
Altogether, it becomes 16 types of air masses. Air masses however in general are
instable. Instability brings modification in the characteristics of air mass. Hence only 8
types of air masses are considered important. Refer to Figure. 6.1 for understanding
the scheme of classification of air masses.

Fig. 6.1: Classification of Air masses

Check your progress 1

Note: a) Write your answer in about 50 words.
b) Check your progress with possible answers given at the end of the unit.
1. What are the characteristics of a good source region of air masses?
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A body of air is modified through thermodynamic and dynamic processes.
Thermodynamic processes involve heating or cooling of air from below or by solar
heating of the ground over which the air is located. Dynamic changes are mechanical
and they involve mixing and pressure changes associated with actual movement of the
air mass. These modifications bring considerable changes in the weather condition of
four major air masses. Let us discuss them briefly.
A. Continental Polar Air Mass
This air mass brings a lot of meteorological changes. It becomes more effective during
winter season in northern USA and Canadian column of air masses and also Siberian 115
Climatology and air mass. Siberian air mass is formed due to the shift of pressure belts and very low
Meteorology temperature prevailing over the region. Sub-polar low pressure belt disappears from
the Siberian plateau and one of the largest columns of air mass is formed in Eurasia
which is called as Siberian air mass. All these air masses promote polar cold winds
which modify the meteorological condition of the area visited by them. Similarly North
American and Canadian air mass is also developed. North-easterly winds towards
Atlantic create weather disturbances in the form of snow, sleet, icy winds and foggy
conditions. The air moving to the south over the continent acquires higher temperatures
and tends to become unstable, but there is little gain in moisture content.
B. Maritime Polar Air Mass
This air mass prevails over the northern part of North Atlantic and Arctic sea region.
During winters both are in continuity and are responsible for the development of cold
front and temperate cyclone as they move southwards. Visiting areas receive very
cold condition but the moving air is also modified while moving towards lower latitudes.
However, due to low temperature sometimes it brings snowfall even in coastal areas
of Western Europe.
C. Continental Tropical Air Mass
These air masses are characterised by moderate temperatures, low moisture and low
humidity and constitute the source of trade winds and westerlies. However, continental
tropical has seasonal changes in regional perspective e.g. central plains of India develops
these air masses only during winters. During summers, this zone becomes a low pressure
area due to which continental tropical high pressure air mass. In southern U.S.A and
southern China similar situation is formed.
D. Maritime Tropical Air Mass
This air mass is principally the sub-tropical high pressure areas of North Pacific and
North Atlantic. Winds blowing from these sources have greater meteorological
consistency as they move on the ocean surface. These air masses also form the source
of trade winds and westerlies. Westerlies are warm and move towards higher latitudes
that is sub-polar low pressure areas. It leads to condensation in lower atmosphere and
fog appears and visibility decreases and this is the reason why English Channel becomes
permanently foggy due to this.
Vertical movement in an air mass is of great significance because nearly all precipitation
is associated with adiabatic cooling and condensation in the rising air. Stability and
instability of air mass are also related to vertical movements of air. This is an important
parameter for weather forecasting. Sometimes two air masses of contrasting
characteristics meet, they do not mix readily but a front or a sort of boundary is
formed between them. Let us study about these in the next section.

6.4 FRONTS AND TEMPERATE CYCLONES

As we have mentioned earlier that, air masses with contrasting physical properties like
temperature, pressure, humidity, density etc. come into contact with one another along
a sloping boundary called front. So fronts are defined as the sloping boundary surface
or the transitional zone between two contrasting air masses. Fronts vary in thickness
from 50-100 km. The boundary always slopes upward over the colder and denser air
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exhibiting abrupt temperature discontinues throughout its vertical extent. Since air Weather
masses are three-dimensional, so the boundary separating two different air masses are Phenomenon
also three-dimensional as is having both vertical and horizontal extent, although in
weather charts they are shown as lines.
Systematic evolution of front in mid latitude regions due to convergence of polar air
mass and the warm sub-tropical air mass is known as frontogenesis. Frontogenesis
is well explained by the ‘polar front theory’ of V. Bjerknes, J. Jakob. The general
atmospheric circulation favours divergence of air masses in sub-tropical and Polar
regions, but there is convergence of two air masses of different properties in sub-polar
low pressure region, which gives rise to fronts and temperate cyclones. Let us now
learn about cyclones and fronts in greater detail.
The atmospheric disturbances which involve a closed circulation about a low pressure
centre, anticlockwise in the northern hemisphere and clockwise in the southern
hemisphere are called cyclones. In simple words movement of air at around a vortex
or a depression or low pressure area is called cyclone. They are of 2 types- (a) Extra
tropical cyclones (wave cyclones) and (b) Tropical cyclones. Extra tropical cyclones
are weather disturbance of temperate and high latitude regions. In common usage they
are also called temperate cyclones.
According to the ‘polar front theory’ frontal activities or temperate cyclone passes
through six stages which are explained as follows:
1st stage: In the initial stage, the two contrasting air masses move parallel to the front
in opposite directions maintaining their boundary zone for some time. The boundary at
this stage is known as stationary front.
2nd stage: The second stage is also called as the initial stage which marks the beginning
of cyclonic circulation as the cold and the warm air masses penetrate into the territories
of each other and thus a wave-like front is formed.
3rd stage: This stage is the stage of evolution of warm sector. Now cold and warm
fronts are clearly formed. The meeting point of the 2 fronts becomes the centre of low
pressure around which cold and warm winds begin to circulate. Cold air advances
equatorwards while the warm air advances polewards along the warm front. Now
isobars are almost circular.
4th stage: During the 4th stage cold air overtakes the warm air. Warm sector is further
narrowed as cold air moves faster equatorwards.
5th stage: This is the stage of occlusion. Here cold front overtakes the warm front
pushing the warm air aloft and closing the warm sector. The area of low atmospheric
pressure is thus filled up by the cold air and occlusion occurs. This new frontal surface
is called as occluded front. The depth of the cold air along the occluded front increases
and the entire warm sector is forced to rise until it spreads out laterally over the cold
air mass.
6th Stage: This is the stage of dissipation. The air fails to turn further and the opposite
moving airs emerge between them the stationary front is re-established. Refer to Figure
6.2 to understand better.

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Climatology and
Meteorology

Fig 6.2: Stages of a Temperate Cyclone

In Fig 6.2, you must have noticed there are different types of fronts. So we see that
temperate cyclone involves four types of fronts. They are the cold front, warm front,
occluded front and the stationary front. These four fronts are associated with different
weather conditions. Let us know about them.
Cold Front: Cold front is that front along which cold air becomes active and invades
the warm air territory. The cold air being denser remains on the ground but forcibly
uplifts the warm air. Since air motion is retarded near the ground surface due to friction
while the free air aloft has higher velocity, hence cold front becomes much steeper than
the warm front. Its slope varies from 1:50 to 1:100 (meaning rise of the wedge of cold
air at the rate of 1 km for every 50 or 100 km horizontal distance). Here the clouds are
generally cumulonimbus which brings hailstorm in Europe and U.K. Sometimes
nimbostratus and stratocumulus clouds are formed over U.K. They bring heavy and
light rain respectively. With the passing of the cold front, pressure begins to rise and
temperature decreases. Sky becomes suddenly clear.
Warm Front: It is that gently sloping frontal surface along which warm air becomes
active and rises slowly over cold and dense air. The average slope of warm front in the
middle latitudes varies from 1:100 to 1:200. This gentle slope develops multi-layered
cloud. The topmost is cirrus followed by cirrostratus and altostratus and further down
nimbostratus and stratus clouds. The Sun is obscured. It first drizzles and then rains
covering a large area.
Occluded Front: It is defined as a front formed when cold front completely overtakes
the warm front and displaces it from the ground. Occluded front is of two types having
different weather conditions. Cold front occlusion occurs when the cold air which
overtakes the warm air is colder that the retreating cold air. In case of warm front
occlusion, the retreating cold air mass is colder than the advancing cold air mass.
Stationary Front: As the name suggests, surface position of this front does not move.
Winds on either side of this front appear to be nearly parallel.

6.5 TROPICAL CYCLONES

You were taught in the previous section that there are two types of cyclones, temperate
and tropical cyclones. We discussed about fronts and temperate cyclones there. Let
us now study about tropical cyclones in this section. Tropical cyclone emerges due to
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thermal effect. Thermal effect causes the emergence of the centre of low pressure and Weather
warm air of almost similar properties begins to rush to the centre. It emerges invariably Phenomenon
over the surface of oceans between 8o to 15o latitudes. Extensive shallow lows
occasionally formed along the thermal equator, becomes cyclonic once the situation is
beyond 8o latitudes. It cannot emerge in lower latitudes due to weak effect of Coriolis
force. Coriolis force brings turn in the wind and almost circular system of wind circulation
is developed. The centre of the circular wind system is known as the eye of the cyclone.
It is the centre of low pressure and causes the emergence of high velocity violent winds
in the tropical region but it remains calm as the moving winds develop the tendency of
horizontal rise. The warm winds go on rising which also causes condensation, cloud
formation and ultimately rainfall. This tendency does not allow winds to reach the
destination centre of low pressure. This is how the eye of cyclone is meteorologically
a calm region, otherwise cyclonic winds are violent and destructive having a wind
velocity of 120-200 km per hour. The diameter of the eye of cyclone is generally
below 50 km but the diameter of the cyclonic influence is up to 500 km. The eye of the
cyclone develops ideally when the ocean surface is between 25 to 28o C.
Refer to Figure 6.3 to know the mechanism of tropical cyclone. You can also listen to
the following video programmes to know more about cyclones and how their prediction
is done by IMD (India Meteorological Department):
1) Observing Weather: https://www.youtube.com/watch?v=EKWrrkI1xrY
2) Predicting Weather: https://www.youtube.com/watch?v=ZJE2VrdFxDg&t=5s
Tropical cyclones are known by different names in different places of the world. In the
seas off China, Philippines, Japan and other islands of Western Pacific Ocean they are
known as typhoons and off the coast of north-west Australia, they are known as Willy
Willies. Tropical cyclonic storms in North America are known as Hurricane. You also
need to get familiar with some other terms like thunderstorms, tornadoes and
waterspouts. These are violent storms in which enormous amount of energy is
discharged in an extremely short duration of time due to which they have a disastrous
impact. Thunderstorms are defined as storms produced by cumulonimbus clouds
and are always accompanied by lightning and thunder. Like cyclones, tornadoes are
also intense centres of low pressure. They are defined as a violent rotating column of
air attended by a funnel shaped or tubular cloud extending down from the base of a
cumulonimbus cloud. Tornadoes are common in USA. Waterspouts are tornadoes,
the main difference being that it occurs over water instead of land.
Check Your Progress 2
Note: a) Write your answer in about 50 words.
b) Check your progress with possible answers given at the end of the unit.
1. Differentiate between temperate and tropical cyclones.
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Let us also discuss about anticyclones in brief. Anticyclones can be defined as a large
wind system that rotates about a centre of high atmospheric pressure, clockwise in the 119
Climatology and northern hemisphere and counter-clockwise in the southern hemisphere. It denotes an
Meteorology atmospheric system just opposite to cyclones. Since anticyclones are characterized
by clear and fine weather, no serious study has been conducted in this regard. However,
the location and energy of anticyclone is an important parameter of weather forecasting.
In this unit so far, we have studied about atmospheric circulation and related weather
disturbances in the lower troposphere. Let us now study about air circulation of upper
atmosphere which also has a significant impact on surface weather conditions. These
are jet streams which came to the knowledge of meteorologists during Second World
War.

6.6 JET STREAMS

Jet Streams are defined as swift geostrophic air streams in upper troposphere that
meanders in relatively narrow belts. These are thousands of kilometers in length and a
few hundred kilometers in width and having a vertical thickness of two or more. The
mean velocity of jet streams is about 144 km/h. However, at times the mean velocity
of inner core of jet streams may be as high as 480 km/h.
The regions around 30° north and south of equator and 50°-60° north and south of
equator are areas where temperature changes are the greatest. As the difference in
temperature increases between the two locations the strength of the wind increases.
Therefore, the regions around 30° N/S and 50°-60° N/S are also regions where the
wind, in the upper atmosphere, is the strongest. The 50°-60° N/S region is where
the polar jet is located with the subtropical jet is located around 30°N. Jet streams
vary in height of four to eight miles and can reach speeds of more than 442 km/h.
Refer to Figure 2.3 to know better.
Jet streams play a significant role in determining the weather conditions in lower
atmosphere. The path followed by temperate cyclones or mid-latitude weather
disturbances are largely controlled by these upper air circulations. Movement of jet
streams is also important in determining prolonged periods of floods or droughts. So
we see that surface meteorological conditions are highly affected by the path of jet
streams and so a preliminary knowledge of jet streams is necessary for studying weather
phenomenon. Jet streams also affect the regularity of monsoonal winds. We will discuss
it in the next section of this unit.

Fig 6.3: Cross section of the two main jet streams by latitude (Public Domain, 2008
(Source: commons.wikimedia.org)
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 Weather
6.7 SOUTH-WEST AND NORTH-EAST MONSOONS Phenomenon

The term monsoon is derived from an Arabic word ‘mausim’ which means ‘season’. It
refers to the seasonal or six monthly reversal of the direction of winds. Monsoons
predominantly occur in South Asia, South-East Asia, parts of China and Japan, other
regions being southern USA, northern Australia, and in western Africa. Monsoon
winds have helped merchants and travellers who used to travel by boats for long
distances in the Indian Ocean. Now let us study about the South-West monsoon and
North-East monsoons which marks the reversal of wind direction and are also known
as summer and winter monsoon.

6.7.1 South-West Monsoon

During summer, continental regions of Eurasia are heated up faster in comparison with
the adjoining oceans. Hence low pressure develops over the continents in the southern
and south-eastern Asia during summer. However, contrary to this, the oceanic areas
are having relatively higher pressure. This pressure difference causes winds to blow
from Indian Ocean to the continent. The south-east trade winds blowing in the southern
hemisphere crosses the equator, and gets deflected to the right due to the Coriolis
effect and becomes south -west winds which are moisture laden onshore winds and
are in fact south-west monsoon rainfall bearing winds. This south-west monsoon gets
split into two branches due to Indian Peninsula, that is, Bay of Bengal branch and
Arabian Sea Branch. These onshore moisture laden winds bring heavy rainfall in the
windward slopes. Take for example, the Arabian Sea branch, which bring heavy rain
along the windward slopes of Western Ghats. After crossing the windward slopes,
they descend over the leeward eastern slopes, where they get warmed up adiabatically.
So these areas are actually the rain shadow areas as the amount of rain received here
is quite low compared to the windward slopes. The branch continues its journey across
the Indian peninsula and some are even diverted towards north and reach Kutch,
Rajasthan and even Jammu and Kashmir. In the absence of any mountain barrier,
these however do not bring any rain in Rajasthan. The Aravallis are also parallel to the
direction of these winds. This is the reason why Rajasthan is a desert.
A current of Bay of Bengal branch moves towards the north-eastern portion of India.
There the windward slopes of eastern Himalayas receive heavy rainfall. A place called
Mausinrum, along the southern slope of Khasi hills receives the highest amount of
rainfall in the world. The winds now turn due to Himalayas and reach the low pressure
centre of north India. Here the monsoonal winds blow from south easterly direction
and reaches up to the western and north western part of India, via the Gangetic plains.
This is also marked by the decrease in the amount of rainfall from east to west.
Retreating Monsoons: By the end of September and early October, the low pressure
centre in the north-western part of the Indian sub-continent weakens and again shifts
to the equatorial region. This marks the beginning of the retreat of monsoons. Due to
this rainfall starts decreasing in the northern plains of India which almost ends up by
October. This retreat is of gradual nature as the southern part of Indian peninsula
receives rain from this retreating monsoon even after October. Karnataka and Tamil
Nadu receive rain in October-November through retreating monsoon. The retreat
continues till December and soon North-East monsoon is about to set in.

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Climatology and 6.7.2 North-East Monsoon
Meteorology
During winter the interior of Asia becomes excessively cold, resulting in the development
of a continental anticyclonic condition or high pressure area. At this time temperatures
over the seas to the east and south of Asia, are relatively higher, and consequently
pressure is lower. As a result of this arrangement of the pressure areas, the gradient is
from the continent towards the oceans. Consequently, cold surface winds move out
from Asia toward the surrounding seas. The winds blow from the north-east direction
in southern Asia and hence called the north-east monsoon which brings winter rain to
the areas wherever they are onshore moisture laden winds. The southern part of Indian
peninsula receives rain from north-east monsoons.
The above explanation of the mechanism of monsoon is the Classical Theory or Thermal
Concept of the origin and mechanism of monsoons as it was thermally induced due to
differential heating and cooling of land and oceanic areas in different seasons. This
concept was propounded by Halley in a memoir presented to the Royal Society in
1686.
However, the above concept regarding the monsoons could not explain the erratic
behavior of monsoons as thermal heating and cooling of land and oceanic areas is
consistent. So it suggests that there are some other factors too which affect the onset
and the entire mechanism of monsoons. We need to discuss about them in brief here.
According to Koteswaram, upper air warm anticyclonic conditions developing over
Tibetan plateau is closely related with the burst of monsoons. His observation was
further validated in the Monex (Monsoon Expedition) studies conducted by joint efforts
of India and Soviet Union in 1973. Let us discuss this.
Tibetan Plateau is an enormous block of about 4000 m high plateau which is 2000 km
in length and 600-1000 km wide so summertime heating of Tibetan plateau makes it a
massive heat source due to which warm anticyclonic conditions emerge in the upper
part of the plateau. At this time sub tropical jet stream disappears from the Indian sub-
continent. The upper air anticyclonic conditions in the plateau, produce an easterly jet
stream from its southern portion, with the flow direction from east to west. These are
known as Tropical Easterly Jet. After sweeping the entire sub-continent, these easterly
jet streams descend in the high pressure region of Indian Ocean further intensifying the
high pressure prevailing there. It is from this high pressure oceanic region that winds
start blowing towards the thermally induced low pressure area as monsoon winds. So
the strength of monsoons, is dependent to a great extent on the meteorological conditions
prevailing over Tibet and the strength of tropical easterly jet stream. Besides, Tropical
easterly jet stream, monsoons are also affected by El Nino. Let us study about El Nino
in the next section.

6.8 EL NINO SOUTHERN OSCILLATION (ENSO)

El Nino is a warm ocean current off the Peruvian coast in South America which brings
heavy rain in the adjoining coastal region of South America. Due to El Nino an entire
low pressure region is created across Pacific Ocean and Indian Ocean. This weakens
the high pressure region of Indian Oceanand tropical easterly jets. Thus the source
region of the origin of monsoon winds is weakened and hence it affects the origin and
mechanism of monsoons.
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El Nino and the Southern Oscillation, also known as ENSO is a periodic fluctuation in Weather
sea surface temperature (El Nino) and the air pressure of the overlying atmosphere Phenomenon
(Southern Oscillation) across the equatorial Pacific Ocean. For this sea level barometric
pressure across two observation stations, that is, Darwin in Australia and Tahiti are
measured. It is quantified in the Southern Oscillation Index (SOI), which is
a standardized difference between the two barometric pressures. If the pressure is
lower over Darwin and higher over Tahiti, then air circulation is from east to west,
which also draws warm surface water westwards and brings precipitation to Australia
and the western Pacific regions. Due to El Nino, the pressure difference between the
two stations weaken, and parts of the western Pacific as well as Australia experience
severe drought, while across the ocean, heavy precipitation can bring flooding to the
west coast of equatorial South America.

6.9 CLASSIFICATION OF CLIMATE BY

KOEPPEN AND THORNTHWAITE
Climatic classification helps in comprehending the multiplicity of atmospheric conditions
in meaningfully organized simple and general terms in a way that large and diverse
information is grouped into similar entities and are different from other groups by
specific attributes. Different approaches have been adopted by geographers for climatic
classification. These are empirical, genetic and applied. An empirical classification is
based on the observable elements of climate which may be considered singly or in
combination to frame criteria for climatic types. A classification based on genetic or
causative factors of climate is known as a genetic classification. In applied classification
of climate the climatic regions are delineated in terms of effects of climate on other
phenomena. The most common elements which have been used for climatic classification
are temperature, precipitation and vegetation. The most widely known schemes of
climatic classification are given by Koeppen and Thornthwaite.
Check Your Progress 3
Note: a) Write your answer in about 50 words.
b) Check your progress with possible answers given at the end of the unit.
1. What is the purpose of a climatic classification?
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Koeppen recognised five major categories of climate and each category is designated
by a capital letter as listed below:
A – Tropical Rainy Climates
B – Dry Climates
C – Humid Mesothermal Climates or Warm Temperate Rainy Climates
D – Humid Microthermal Climates or Cold Forest Climates
E – Polar Climates
Koeppen further identified finer details and further subdivided these five major 123
Climatology and categories into second and third order types which are mentioned in the Table 6.1
Meteorology given below:
Table 6.1: Koeppen’s System of Climatic Classification
Letter Symbol Precipitation and temperature
1st 2nd 3rd
A Average temperature of the coldest month is 180C or
above.
f Moist; Every month has 6 cm of precipitation or more;
no dry season
m Short dry season in monsoon type of precipitation;
driest month < than 6 cm but equal to or more than 10-
R/25 (R is annual precipitation in cm)
w Well-defined winter dry season; precipitation in driest
month < than 10-R/25.
s Well-defined summer dry season (rare distribution)
B Potential evaporation exceeds precipitation. It is
identified on the basis of average annual precipitation
(R), average annual temperature (T) and seasonal
distribution of precipitation in following manner:
(i) R<2T+28, when 70 % or more of precipitation
occurs in warmer 6 months.
(ii) R<2T, when 70 % or more of precipitation occurs in
cooler 6 months.
(iii) R<2T+14, when neither half year has 70 % or more
of precipitation.
S When R is more than ½ of the upper limit for B.
W When R is less than ½ of the upper limit for B.
h Average annual temperature is 180C or above.
k Average annual temperature is less than 180C.
C Average temperature of the coldest month is < than
180C and above -30C; average temperature of the
warmest month is greater than 100C.
w At least ten times as much rain in the wettest month as in
the driest winter month; precipitation in driest summer
month < than 4 cm.
s At least three times as much rain in wettest winter month
as in the driest summer months; precipitation in driest
summer month < than 3 cm.
f Precipitation throughout the year and no dry season.
Difference between the rainiest and driest months is less
than that for w and s and the driest month of summer
receives > 3 cm rainfall.
a Hot summer; average temperature of the warmest
month > 220C; at least four months above 100C.
b Cool summer; average temperature of the warmest
month < 220C; at least four months above 100C.
c Cool short summer; average temperature of the
warmest month < 220C; at least one to three months
124 above 100C.
 D Average temperature of coldest month is – 30C or Weather
below; average temperature of warmest month is Phenomenon
greater than 100C.
s,w,f Same as under C.
a,b,c,d Same as under C.
E Average temperature of the coldest month is – 380C or
below. Average temperature of warmest month is below
100C.
T Average temperature of warmest month is between 00C
and 100C.
F Average temperature of warmest month is 00C or
below.
As Koeppen’s climatic classification is based on vegetation classification of A. de
Candolle the climatic boundaries match with vegetation boundaries. As it is based on
shorthand code of letters for the climate types the repetition of descriptive terms is
easily avoided. The use of symbols and representation of climatic types on map further
increases its utility for meteorologists. However, his climatic classification also has
some limitations. As it is based on mean monthly temperature and precipitation values
it fails to highlight the variations over time in one locality or region and makes comparison
of one locality with another difficult. Further, Koeppen ignored the role of weather
elements such as winds, atmospheric pressure, cloudiness, air masses and cyclones
etc. This way it is empirical but not genetic classification as causative factors were
ignored totally
Thornthwaite in his climatic classification (1931) used the criteria of thermal efficiency
and precipitation effectiveness. Keeping in view the significance of temperature in
vegetation growth, Thornthwaite introduced an index of thermal efficiency. It is
expressed by the positive departure of monthly mean temperature from the freezing
point. Precipitation Effectiveness is the amount of precipitation which is actually available
for the growth of natural vegetation. It is also known as precipitation efficiency. It is a
function of precipitation and evaporation. In 1948, Thornthwaite proposed a new
classification of climates with a number of modifications in criteria. In this classification
he developed and introduced the concept of potential evapotranspiration. Potential
evapotranspiration is an index of thermal efficiency and water loss. It represents the
amount of moisture that would be transferred to the atmosphere by evaporation of
liquid or solid water plus transpiration from plants.
Terminal Questions
1. Define air masses and classify them. Briefly discuss four major air masses.
2. What is frontogenesis and temperate cyclone? Explain the different stages of
progression of a temperate cyclone.
3. Describe the origin and mechanism of Indian monsoon. How are monsoons
affected by jet streams and El Nino? Explain.

6.10 LET US SUM UP

Let us recapitulate what we have learnt so far:
• ‘Weather’ refers to the state of atmosphere at any given time and denotes the 125
Climatology and short term variations of atmosphere in terms of temperature, pressure, wind,
Meteorology moisture, cloudiness, precipitation and visibility which are known as the elements
of weather.
• An air mass is an immense body of air usually 1600 km or more across and
perhaps several kilometers thick, which is characterised by homogenous physical
properties (particular temperature and moisture content) at any given altitude. Air
masses play a major role in bringing about day-to-day weather changes and in
producing different climate types in the long run. There are three bases of
classification of air masses. These are thermal, latitudinal and Configurational.
• Fronts are defined as the sloping boundary surface or the transitional zone between
two contrasting air masses. Convergence of two air masses of different properties
in sub-polar low pressure region gives rise to fronts and temperate cyclones.
According to the ‘polar front theory’ frontal activities or temperate cyclone passes
through six stages. Temperate cyclone involves four types of fronts, viz, cold
front, warm front, occluded front and stationary front.
• Tropical cyclone emerges due to thermal effect invariably over the surface of oceans
between 8o to 15o latitudes. . Thermal effect causes the emergence of the centre of
low pressure and warm air of almost similar properties begins to rush to the centre.
Coriolis force brings turn in the wind and almost circular system of wind circulation
is developed, the centre of which is known as the eye of the cyclone which is
meteorologically calm region contrary to the surrounding cyclonic winds which
are violent and destructive.
• Monsoon is derived from an Arabic word ‘mausim’ which means ‘season’. It
refers to the seasonal or six monthly reversal of the direction of winds which leads
to South-West and North-East monsoons. Though monsoons are originated due
to thermal effect but jet streams and El Nino also affect the mechanism of monsoons.
• Climatic classification helps in comprehending the multiplicity of atmospheric
conditions in meaningfully organized simple and general terms in a way that large
and diverse information is grouped into similar entities and are different from other
groups by specific attributes.

6.11 KEYWORDS
Airmass : An air mass is an immense body of air usually
1600 km or more across and perhaps several
kilometers thick, which is characterised by
homogenous physical properties (particular
temperature and moisture content) at any given
altitude.
Cyclone : The atmospheric disturbances which involve a
closed circulation about a low pressure centre,
anticlockwise in the northern hemisphere and
clockwise in the southern hemisphere are called
cyclones.
El Nino : El Nino is a warm ocean current off the Peruvian
126 coast in South America which brings heavy rain
 in the adjoining coastal region of South America Weather
due to which an entire low-pressure region is Phenomenon
created across Pacific Ocean and Indian Ocean.
Jet Stream : Jet Streams are defined as swift geostrophic air
streams in upper troposphere that meanders in
relatively narrow belts. These are thousands of
kilometers in length and a few hundred kilometers
in width and having a vertical thickness of two or
more.
Potential : It is an index of thermal efficiency and water loss.
evapotranspiration It represents the amount of moisture that would
be transferred to the atmosphere by evaporation
of liquid or solid water plus transpiration from
plants.

6.12 REFERENCES AND SUGGESTED FURTHER

READINGS
• Critchfield,H., (1981), General Climatology, Prentice Hall, New York.
• Lal D.S., (2009), Climatology and Oceanography, Sharda Pustak Bhavan,
Allahabad.
• https://www.ncdc.noaa.gov/teleconnections/enso/enso-tech.php

6.13 ANSWERS TO CHECK YOUR PROGRESS

Answers to Check Your Progress 1
Your answers should include the following points:
1. An ideal source region should have the following characteristics:
• Should be extensive;
• Should be homogenous;
• There should be divergent air flow or anticyclonic conditions, so that air may
stay for a long duration; and
• Conditions should be stable for a long period of time.
Answers to Check Your Progress 2
Your answers should include the following points:
2. The main differences between tropical and temperate cyclones are as follows:
• Tropical cyclones originate in tropical oceanic areas between 8-20o latitudes
north and south of equator, while temperate cyclone can originate both in land
or oceanic areas between 35-60o latitudes north and south of equator.
• Tropical cyclones are thermal in origin, while temperate cyclones are dynamic
in origin and involve two different air masses and formation of fronts between
127
them.
Climatology and • Tropical cyclones have a meteorologically calm are which is the central low
Meteorology pressure area and is called the eye of cyclone. In temperate cyclone there is
no area which is calm and having no rains.
• Tropical cyclone localized and is for a shorter duration while temperate cyclone
persists for a longer duration and is covers a larger area.
Answers to Check Your Progress 3
Your answers should include the following points:
3. The basic purpose of climatic classification is to bring order to large and diverse
information by organising similar entities together into groups that are different
from other groups by specific attributes. In this way, an order is generated out of
complexity. Climatic classification helps in comprehending the multiplicity of
atmospheric conditions in meaningfully organised simple and general terms.
Terminal Questions
1. First define air masses. Classify them on the basis of thermal, latitudinal and
configurational bases. Then describe the four major air masses as given in Sec.
2.3.
2. Define frontogenesis. Explain how fronts are related to the phenomenon of temperate
cyclones. With the help of a figure explain different stages of a temperate cyclone
as given in Sec. 2.4.
3. Start you answer by defining monsoons. Discuss the process of origin and
mechanism of monsoons in Indian context with the help of a figure. Then explain
the effect of jet streams and El Nino in the outbreak and progression of monsoons.
Refer to Sec. 2.7 and Sec 2.8.

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