Cloud Types
Stratus, Stratocumulus, Orographic, Stratus, Cumulus Humilis, Cumulus Mediocris, Cumulus
Congestus, Pyrocumulus, Cumulonimbus Calvus, Cumulonimbus with Pileus, Cumulonimbus Incus,
Cumulonimbus with Mammatus, Altostratus, Altostratus Undulatus, Altocumulus, Altocumulus
Undulatus, Altocumulus Mackerel Sky, Altocumulus Castellanus, Altocumulus Lenticularis, Cirrus,
Cirrus Uncinus, Cirrus Kelvin-Helmholtz, Cirrostratus, Cirrocumulus, Contrails
Exam Question Tips:
Stratified clouds are formed when a moist, stable layer of air is forced to rise against a mountain
range.
In mid-latitudes, the tops of Cumulus are often limited by temperature inversion e.g. Cumulus
Humilis. Cumulus clouds with little or no vertical development characterized by a generally flat
appearance. Their growth is usually limited by a temperature inversion, which is marked by the
unusually uniform height of the clouds. Also called fair-weather cumulus.
Haboob
It is the name given in the Sudan to sandstorms associated with thunderstorms.
Thunderstorm
Single cell Thunderstorm moves in line with medium level winds (generally 10,000 feet). Active
period is less than one hour.
Developing Stage:
- Updrafts 3-4000 fpm
- Turbulence mixes rising air with environmental cold air and limits its development.
- Moderate to severe turbulence
- No precipitation
- Last for 15-20 mins
Mature stage:
- Updrafts and downdrafts.
- Rain begins to fall.
- Downdraft 3-4000 fpm may reach 6,000 fpm in severe storms.
- First gust
- Severe wind shear under thunderstorm.
- Microburst
- Activity ends by two factors:
a) Mixing of rising air with dry cold environmental air - reducing instability.
b) Downdrafts suppressing the updrafts.
- Mature stage lasts about 30-40 minutes
Dissipating Stage:
- Cessation of continuous rain.
- Beginning of sporadic showers
- Air subsides, vertical currents weaken
- Higher clouds have anvil appearance
- Virga
Gust Fronts
The cold air descending from a thunderstorm can also run ahead of the storm producing a mini
cold front with windshear, turbulence and squally conditions. This is called a gust front. Gust fronts
can extend as far as 15 to 20nm ahead of the thunderstorm and up to 6,000ft. When there is a
line or there are group of thunderstorms gust fronts can extend to twice this distance. Windshear
around a gust front has been measured with speed changes up to 80KT and direction changes of
90 deg.
The Super-Cell Thunderstorm
- Good supply of warm moist air at low level. Held down by a thin stable layer above so that the
energy supply is not dissipated by turbulence or small-scale convection.
- A change of direction and strengthening of the winds aloft which will tilt the towering CB.
- The updraft is no longer on the same axis as the downdraft and the two exist side by side
enabling the convection to continue.
- The increased size and intensity of the storm isolates the central core of the convection from the
cold dry environmental air allowing it to reach high vertical speeds.
- This carries hail aloft and takes the top of the storm through the tropopause.
- Occur most frequently at the boundary between sub-tropical and polar air.
- The main area for super-cells is in the central area of the American Mid-West.
- Can last for several hours in their fully active state.
- Tend to move at an angle to the medium level wind and at a different speed, either 20 deg to the
right and slower or 20 deg to the left and faster. The 20 deg right and slower option being more
common in the Northern Hemisphere.
Thunderstorm Avoidance:
- Flying restriction under the anvil relates to hail rather than to CAT.
The recommended turbulence avoidance limits are:
- In visual flight - Avoid by 10 NM
- With weather radar - Avoid the echoes by:
Between 0 to FL250 by 10 NM
Between FL 250 to 300 by 15 NM
Over FL 300 by 20 NM
Note: Wx radar echoes show the core of the cloud not the edge so the avoidance limits are
greater.
Exam Question Tips
At temperate latitudes; hail may be expected in connection with a CB from ground up to a
maximum of FL 450.
The building stage of a thunderstorm last for approximately 20 mins.
The mature stage of a thunderstorm lasts for approximately 20 to 30 mins.
Airmass thunderstorms are the most difficult to forecast and detect.
If you cannot avoid penetrating a thunderstorm, the best area to penetrate is the "Sides".
Icing is possible in temperatures lower than -23 deg C in a CB with thunderstorm in its mature
stage.
The highest probability for severe thunderstorms is when there is advection of maritime cold air
over a warm sea surface.
Thunderstorms are often preceded by Altocumulus Castellanus.
A cold front approaching a mountain range in the evening favours the formation of heavy
thunderstorms?
Aircraft struck by lightning may sometimes get considerable damage and at least temporarily the
manoeuvring of the aircraft will be made more difficult. Aircraft made by composite material may
get severely damaged, the crew may be blinded and temporarily lose the hearing.
The aircraft is temporarily part of the lightning trajectory.
In North America tornadoes are most likely to occur in Spring and Summer.
The diameter of a typical tornado is 100-150 meters.
Microburst
- The microburst is an extreme form of windshear generated by the slug of descending air that
comes from a thunderstorm cell.
- Downdrafts average around 3-4000 fpm but have been measured at 6,000 fpm.
- When they hit the ground they can flow out at 50KT in one direction and 50KT in the opposite
direction. This gives a vector change in the surface wind of 100KT over a relatively short distance
- They last for only a few minutes.
- Downdrafts are initiated by precipitation and are often marked by a column of rain. If the air
below the cloudbase is relatively dry the rain can evaporate creating a "dry" microburst (which is
severe than the wet microburst). Then the evaporation will cool the slug of air making it more
dense and increasing its speed.
Exam Question Tips:
The diameter and the life time of a typical microburst are in the order of 4 km and 1-5 minutes.
Tropical Storm
The origin of a Tropical storm is actually a Convective cloud or Cumulonimbus clouds which are
clouds having massive updraft and simultaneously downdrafts at events and grow vertically
instead of growing horizontally. What happens is that in oceans which is a large pool of warm
waters such convection (thunderstorm formations) happens. During this period, more and more
moist air from the surface rises up creating a surface low pressure. If the Sea Surface Temp or
SST is more than 26-28C more latent heat will be released from the cloud. Thus it’s a fuel for
storms.
Once the initial tropical storm builds due to multiple periods of latent heat release, clouds become
bigger and we call Tropical Storm strengthening.
Conditions for formation:
1) Warm SST
2) Good lower convergence
3) Good upper divergence
4) less horizontal wind shear
5) good vertical shears
6) Coriolis effect
7) Moist area or good area of moisture
Tropical Revolving Storms (TRS)
The correct names for the tropical storms in either hemispheres is Tropical Revolving Storms
(TRS). Other names are usually local to an area egg. Hurricane, Spanish, hurakan, god of the
storm, or typhoon from the Chinese dialect, tai fung or big wind. The earth's rotation, the
geostrophic effect, determines the direction of rotation of the TRS and in the northern hemisphere
it is anti-clock wise or left handed and in the southern hemisphere it is clock wise or right handed.
The storms originate generally between 7 and 15 degrees latitude, south or north. They travel
initially in a direction of between west to south-west in the southern hemisphere and west and
north-west in the northern hemisphere. They generally recurve (change course about 90 degrees
to their original course line) at 25 degrees latitude, may be lower in southern hemisphere, and
take a direction of north-east in the northern hemisphere or south-east in the southern
hemisphere. The formation of a TRS occurs over the ocean as a result of the differential heating
between the air and the sea, this causes spiralling thermal currents which gather intensity
resulting in a low pressure system. They always travel away from the equator and therefore never
cross it.
Tropical Cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous
thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water
evaporated from the ocean is released as the saturated air rises, resulting in condensation of
water vapour contained in the moist air. They are fuelled by a different heat mechanism than other
cyclonic windstorms such as nor'easters, European windstorms, and polar lows. The characteristic
that separates tropical cyclones from other cyclonic systems is that at any height in the
atmosphere, the center of a tropical cyclone will be warmer than its surrounds; a phenomenon
called "warm core" storm systems.
The term "tropical" refers both to the geographic origin of these systems, which form almost
exclusively in tropical regions of the globe, and to their formation in maritime tropical air masses.
The term "cyclone" refers to such storms' cyclonic nature, with counter clockwise rotation in the
Northern Hemisphere and clockwise rotation in the Southern Hemisphere. The opposite direction of
spin is a result of the Coriolis force. Depending on its location and strength, a tropical cyclone is
referred to by names such as hurricane, typhoon, tropical storm, cyclonic storm, tropical
depression, and simply cyclone.
In the lower troposphere, the most obvious motion of clouds is toward the center. However
tropical cyclones also develop an upper-level (high-altitude) outward flow of clouds. These
originate from air that has released its moisture and is expelled at high altitude through the
"chimney" of the storm engine. This outflow produces high, cirrus clouds that spiral away from the
center. The clouds thin as they move outwards from the center of the system and are evaporated.
They may be thin enough for the sun to be visible through them. These high cirrus clouds may be
the first signs of an approaching tropical cyclone. As air parcels are lifted within the eye of the
storm the vorticity is reduced, causing the outflow from a tropical cyclone to have anti-cyclonic
motion.
Physical Structure:
- Eye and Center
A strong tropical cyclone will harbour an area of sinking air at the center of circulation. If this area
is strong enough, it can develop into a large "eye". Weather in the eye is normally calm and free of
clouds, although the sea may be extremely violent. The eye is normally circular in shape, and may
range in size from 3 to 370 kilometres in diameter. Intense, mature tropical cyclones can
sometimes exhibit an outward curving of the eyewall's top, making it resemble a football stadium;
this phenomenon is thus sometimes referred to as the stadium effect.
There are other features that either surround the eye, or cover it. The central dense overcast is
the concentrated area of strong thunderstorm activity near the center of a tropical cyclone; in
weaker tropical cyclones, the CDO may cover the center completely. The eyewall is a circle of
strong thunderstorms that surrounds the eye; here is where the greatest wind speeds are found,
where clouds reach the highest, and precipitation is the heaviest. The heaviest wind damage
occurs where a tropical cyclone's eyewall passes over land. Eyewall replacement cycles occur
naturally in intense tropical cyclones. When cyclones reach peak intensity they usually have an
eyewall and radius of maximum winds that contract to a very small size, around 10 to 25
kilometres. Outer rainbands can organize into an outer ring of thunderstorms that slowly moves
inward and robs the inner eyewall of its needed moisture and angular momentum. When the inner
eyewall weakens, the tropical cyclone weakens (in other words, the maximum sustained winds
weaken and the central pressure rises.) The outer eyewall replaces the inner one completely at the
end of the cycle. The storm can be of the same intensity as it was previously or even stronger
after the eyewall replacement cycle finishes. The storm may strengthen again as it builds a new
outer ring for the next eyewall replacement.
- Size
One measure of the size of a tropical cyclone is determined by measuring the distance from its
center of circulation to its outermost closed isobar, also known as its ROCI.
If the radius is less than two degrees of latitude or 222 kilometres, then the cyclone is "very small"
or a "midget".
A radius between 3 and 6 latitude degrees or 333 to 670 kilometres are considered "average-
sized".
"Very large" tropical cyclones have a radius of greater than 8 degrees or 888 kilometres.
Use of this measure has objectively determined that tropical cyclones in the northwest Pacific
Ocean are the largest on earth on average, with Atlantic tropical cyclones roughly half their size.
Movement and track
- Steering winds:
Although tropical cyclones are large systems generating enormous energy, their movements over
the Earth's surface are controlled by large-scale winds—the streams in the Earth's atmosphere.
The path of motion is referred to as a tropical cyclone's track.
Tropical systems, while generally located equatorward of the 20th parallel, are steered primarily
westward by the east-to-west winds on the equatorward side of the subtropical ridge—a persistent
high pressure area over the world's oceans. In the tropical North Atlantic and Northeast Pacific
oceans, trade winds—another name for the westward-moving wind currents—steer tropical waves
westward from the African coast and towards the Caribbean Sea, North America, and ultimately
into the central Pacific ocean before the waves dampen out. These waves are the precursors to
many tropical cyclones within this region.
In the Indian Ocean and Western Pacific (both north and south of the equator), tropical
cyclogenesis is strongly influenced by the seasonal movement of the Intertropical Convergence
Zone and the monsoon trough, rather than by easterly waves. Tropical cyclones can also be
steered by other systems, such as other low pressure systems, high pressure systems, warm
fronts, and cold fronts.
- Coriolis effect:
The Earth's rotation imparts an acceleration known as the Coriolis effect, Coriolis acceleration, or
colloquially, Coriolis force. This acceleration causes cyclonic systems to turn towards the poles in
the absence of strong steering currents. The poleward portion of a tropical cyclone contains
easterly winds, and the Coriolis effect pulls them slightly more poleward. The westerly winds on
the equatorward portion of the cyclone pull slightly towards the equator, but, because the Coriolis
effect weakens toward the equator, the net drag on the cyclone is poleward. Thus, tropical
cyclones in the Northern Hemisphere usually turn north (before being blown east), and tropical
cyclones in the Southern Hemisphere usually turn south (before being blown east) when no other
effects counteract the Coriolis effect. The Coriolis effect also initiates cyclonic rotation, but it is not
the driving force that brings this rotation to high speeds – that force is the heat of condensation.
- Interaction with the mid-latitude westerlies
When a tropical cyclone crosses the subtropical ridge axis, its general track around the high-
pressure area is deflected significantly by winds moving towards the general low-pressure area to
its north. When the cyclone track becomes strongly poleward with an easterly component, the
cyclone has begun recurvature. A typhoon moving through the Pacific Ocean towards Asia, for
example, will recurve offshore of Japan to the north, and then to the northeast, if the typhoon
encounters southwesterly winds (blowing northeastward) around a low-pressure system passing
over China or Siberia. Many tropical cyclones are eventually forced toward the northeast by
extratropical cyclones in this manner, which move from west to east to the north of the subtropical
ridge.
- Landfall
Officially, landfall is when a storm's center (the center of its circulation, not its edge) crosses the
coastline. Storm conditions may be experienced on the coast and inland hours before landfall; in
fact, a tropical cyclone can launch its strongest winds over land, yet not make landfall; if this
occurs, then it is said that the storm made a direct hit on the coast. As a result of the narrowness
of this definition, the landfall area experiences half of a land-bound storm by the time the actual
landfall occurs. For emergency preparedness, actions should be timed from when a certain wind
speed or intensity of rainfall will reach land, not from when landfall will occur.
- Multiple storm interaction
When two cyclones approach one another, their centres will begin orbiting cyclonically about a
point between the two systems. The two vortices will be attracted to each other, and eventually
spiral into the center point and merge. When the two vortices are of unequal size, the larger
vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. This
phenomenon is called the Fujiwhara effect, after Sakuhei Fujiwhara.
Dissipation:
A tropical cyclone can cease to have tropical characteristics in several different ways. One such
way is if it moves over land, thus depriving it of the warm water it needs to power itself, quickly
losing strength. Most strong storms lose their strength very rapidly after landfall and become
disorganized areas of low pressure within a day or two, or evolve into extratropical cyclones. There
is a chance a tropical cyclone could regenerate if it managed to get back over open warm water,
such as with Hurricane Ivan. If it remains over mountains for even a short time, weakening will
accelerate. Additionally, dissipation can occur if a storm remains in the same area of ocean for too
long, mixing the upper 60 metres (200 ft) of water, dropping sea surface temperatures more than
5 °C (9 °F). Without warm surface water, the storm cannot survive. A tropical cyclone can
dissipate when it moves over waters significantly below 26.5 °C (79.7 °F). This will cause the
storm to lose its tropical characteristics (i.e. thunderstorms near the center and warm core) and
become a remnant low pressure area, which can persist for several days. This is the main
dissipation mechanism in the Northeast Pacific ocean.
Intensity Classifications
Tropical cyclones are classified into three main groups, based on intensity: tropical depressions,
tropical storms, and a third group of more intense storms, whose name depends on the region. For
example, if a tropical storm in the Northwestern Pacific reaches hurricane-strength winds on the
Beaufort scale, it is referred to as a typhoon; if a tropical storm passes the same benchmark in the
Northeast Pacific Basin, or in the Atlantic, it is called a hurricane.
Neither "hurricane" nor "typhoon" is used in either the Southern Hemisphere or the Indian Ocean.
In these basins, storms of tropical nature are referred to simply as "cyclones".
- Tropical Depression
A tropical depression is an organized system of clouds and thunderstorms with a defined, closed
surface circulation and maximum sustained winds of less than 33 knots. It has no eye and does
not typically have the organization or the spiral shape of more powerful storms. However, it is
already a low-pressure system, hence the name "depression".
- Tropical Storm
A tropical storm is an organized system of strong thunderstorms with a defined surface circulation
and maximum sustained winds between 33 knots and 62 knots. At this point, the distinctive
cyclonic shape starts to develop, although an eye is not usually present.
- Hurricane or Typhoon
A hurricane or typhoon (sometimes simply referred to as a tropical cyclone, as opposed to a
depression or storm) is a system with sustained winds of at least 64 knots. A cyclone of this
intensity tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at
the center of circulation. The eye is often visible in satellite images as a small, circular, cloud-free
spot. Surrounding the eye is the eyewall, an area about 16 to 80 kilometres wide in which the
strongest thunderstorms and winds circulate around the storm's center. Maximum sustained winds
in the strongest tropical cyclones have been estimated at about 165 knots.
- Because the Coriolis Force is zero at the equator tropical revolving storms do not form at less
than about 5 deg Latitude. When they do form (at latitude 10 to 20 deg) the low latitude results in
strong winds as the low deepens. To qualify as a full-scale TRS the windspeed has to reach 64KT
sustained.
- The real diameter can vary between 300km and 1500km.
- The low-level convergence and convection inside the storm leads to an outflow at height. The
extensive Cu and Cb cloud formations tend to become arranged in bands more or less concentric
with the centre or eye of the storm. Subsidence takes place between these cloud bands in the
outer part of the storm as well as in the eye. The central subsidence produces a clear area of light
winds in the eye of the storm which can be from a few kilometres to 100km in diameter.
- High cirrus, heavy sea swell and continuously falling pressure are classic symptoms of the
approach of a TRS.
- There are never any TRSs in the South Atlantic or in the Pacific off South America because the
sea there is too cold.
- The normal TRS season is the autumn of the hemisphere. about August to October in the North
and February to April in the South.
- In the Typhoon region, the season can start earlier, in June.
- In the Bay of Bengal and the Arabian Sea the northward passage of the ITCZ in April-May and its
southward passage in October-November, triggers TRSs out of the normal timescale, extending
the season from May to November.
Monsoon of Indian Subcontinent
Observed initially by sailors in the Arabian sea traveling between Africa, India and South-East Asia,
Monsoon is a major weather phenomenon in India (and the subcontinent) for the influence it casts
on the lives of its inhabitants since centuries. Monsoon in India can be categorized into two
branches based on their spatial spread over the sub-continent:
- Arabian Sea Branch
- Bay of Bengal Branch
Alternatively, it can be categorized into two segments based on the direction of rain bearing
winds:
- South-West Monsoon (SW Monsoon)
- North-East Monsoon (NE Monsoon)
Based on the time of the year that these winds bring rain to India, they can also be categorised in
two rain periods called:
- The Summer monsoon (June to September)
- The Winter monsoon (October to December)
Mechanism of Monsoon
Monsoon is a tropical phenomenon. Indian subcontinent, lying northwards of the equator up to the
Himalayas and Hindukush, lies primarily in the tropical zone of the Northern Hemisphere. It
involves winds blowing from the south-west direction (known as South-West Monsoon) from the
Indian Ocean onto the Indian landmass during the months of June through September. These are
generally rain-bearing winds, blowing from sea to land, and bring rains to most parts of the
subcontinent. They split into two branches, the Arabian Sea Branch and the Bay of Bengal Branch
near the southern-most end of the Indian Peninsula. They are eagerly awaited in most parts of
India for their agricultural and economic importance.
Subsequently later in the year, around October, these winds reverse direction and start blowing
from north-east direction. Given their land to sea flow, from subcontinent onto the Indian Ocean,
they have less moisture and bring rain to only limited parts of India like Andhra Pradesh and Tamil
Nadu. This is known as the North-East Monsoon. However, this rain is responsible for the rice
bowls of South India. This mechanism completes the annual Monsoon cycle of the Indian
subcontinent.
Although the SW and NE Monsoon winds are seasonally reversible, they do not cause precipitation
on their own. Two factors for rains are essential for rain formation: Moisture-laden winds and
Droplet formation.
Additionally, one of the causes of rain need to happen, which in this case is primarily Orographic
due to presence of highlands right across the paths of the winds. Orographic barriers in the path of
a wind force it to rise. Consequently, precipitation occurs on the windward side of highlands due to
adiabatic cooling and condensation of the rising motion of the moist air.
For all the above scenarios to fulfill simultaneously, the unique geographic relief features of Indian
subcontinent come into play. The notable features of Indian sub-continent, required in explanation
of the Monsoon mechanism, are as follows:
1) First is the presence of abundant water bodies around the subcontinent - Arabian Sea, Bay of
Bengal and Indian Ocean. These help in accumulation of moisture in the winds during the hot
season.
2) Second is the presence of abundant highlands like the Western ghats and the Himalayas right
across the path of the SW Monsoon winds. These are the main cause of the substantial orographic
precipitation all over the Indian subcontinent.
a) The Western Ghats are the first highlands of India that the SW Monsoon winds encounter. The
Western Ghats rise very abruptly from the Western Coastal Plains of the subcontinent making
effective orographic barriers for the Monsoon winds.
b) The Himalayas play more than the role of just the orographic barriers for Monsoon. They help in
its confinement onto the subcontinent. Without it, the SW Monsoon winds would blow right over
the Indian subcontinent into China, Afghanistan and Russia without causing any rain.
c) For NE Monsoon, the highlands of Eastern Ghats play the role of orographic barrier.
Traditional Theory for Mechanism of Monsoon
Also known as the thermal theory or the Differential Heating of Sea and Land Theory, it portrays
the Monsoon as a large-scale sea breeze. It states that during the hot sub-tropical summers, the
massive landmass of Indian Peninsula heats up at a different rate than the surrounding seas
resulting in a pressure gradient from South to North. This causes flow of moisture laden winds
from sea to land. On reaching the land these winds rise up due to the geographical relief, cooling
adiabatically and leading to orographic rains. This is the southwest monsoon. Reverse happens
during winter when the landmass is colder than the sea establishing a pressure gradient from land
to sea. This causes the winds to blow over Indian landmass towards Indian Ocean in a north-
easterly direction causing the northeast monsoon. Since the SW monsoon is from sea to land, it
has more moisture (therefore causing more rain) than the NE monsoon. Only a part of the NE
monsoon passing over Bay of Bengal picks up moisture causing rain in Andhra Pradesh and Tamil
Nadu during the winter months.
However many meteorologists argue that the Monsoon is not a local phenomenon as explained by
the traditional theory but a general weather phenomenon along the entire tropical zone of earth.
This criticism, does not deny the role of differential heating of sea and land in generating monsoon
winds but merely restricts it to one of the several factors rather than the only one.
Dynamic Theory for Mechanism of Monsoon
The dynamic theory of Monsoon explains monsoon on the basis of the annual shifts in the position
of global belts of pressure and winds. According to it, Monsoon is the result of the shift of the Inter
Tropical Convergence Zone (ITCZ) under the influence of the vertical sun. Though the mean
position of the ITCZ is taken as the equator it keeps shifting northwards and southwards with the
migration of the vertical sun towards the tropics (Tropic of Cancer and Tropic of Capricorn) during
the summer of the respective hemispheres (Northern and Southern Hemisphere). As such, the
theory states that during the northern Summer (months of May and June), the ITCZ moves
northwards, along with the vertical sun, towards the Tropic of Cancer. The ITCZ being the zone of
lowest pressure in the tropical region, is the target destination for the Trade winds of both the
hemispheres. Consequentially, with ITCZ at the Tropic of cancer, the South East Trade winds of
the Southern Hemisphere have to cross the equator to reach the ITCZ. However, due to Coriolis
effect, these South East are deflected eastwards in the Northern Hemisphere transforming into
South West trades. These pick up the moisture while traveling from sea to land and cause
orographic rain once they hit the highlands of the Indian Peninsula. This results in the South-West
Monsoon.
During the northern winter, the ITCZ shifts southwards towards the Tropic of Capricorn. Indian
Peninsula comes under the effect of North-East Trade Winds (which are now shifting southwards)
blowing over India as NE monsoon.
The dynamic theory provides the explanation of the system of Monsoon as a circum-global weather
phenomenon rather than just a local one. And when coupled with the Traditional Theory (based on
heating of Sea and Land) it enhances the explanation of the differential intensity of precipitation
impact of Monsoon along the coastal regions with orographic barriers.
Jet Stream Theory for Mechanism of Monsoon
Over India, a subtropical westerly jet develops in the winter season which is replaced by the
tropical easterly jet in the summer season. The high temperature over the Tibetan Plateau, as well
as over Central Asia in general, during the summer is believed to be the critical factor leading to
the formation of the tropical easterly jet over India in summer. The mechanism affecting monsoon
is that the westerly jet causes high pressure over northern parts of the subcontinent during the
winter. This results in the north to south flow of the winds in the form of the NE Monsoon. With the
northwards shift of the vertical sun, this jet shifts northwards too. The intense heat over the
Tibetan Plateau, coupled with associated terrain features of high altitude of the plateau, etc.
generate the tropical easterly jet over central India. This jet creates a low pressure zone over the
northern Indian plains influencing the wind flow towards these plains, assisting the establishment
of the SW Monsoon.
