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JET STREAMS
Aeronautics has played a vital role in the discovery and mapping of jet streams. Many credit
bomber pilots flying missions during World War II with much of the knowledge we have today
about the jet streams. They were able to quicken their missions and beat hasty retreats over
the Mediterranean Sea by making the most of the jet streams.
But even before WWII bomber pilots used the jet streams. Wiley Post, an American pilot and
the first to fly solo around the world in 1933, contributed to our knowledge of these forces of
nature. He developed a pressurized suit to fly higher in the atmosphere and noted the
differences in pressure at various levels. This set the stage for the understanding of the jet
stream and pressurized flight.
German meteorologist H. Seilkopf is often credited with coining the phrase "Jet Stream", as he
used in a research paper published in 1939.
Volcanoes have also played a role in understanding of the jet stream. Observers of the 1883
eruption of the Krakatoa volcanic island in Indonesia documented its effect on the sky, and in
the 1920s Japanese meteorologist Wasaburo Oishi used aviator balloons to identify the jet
stream from a site near Mt. Fuji. More recently, many European flights were grounded after the
2009 eruption of Iceland's Eyjafjallajokull volcano - further proof that plumes of volcanic ash
tend to get sucked into the same jet stream that airplanes use for travel.
Definition
Jet streams are narrow bands of strong wind that generally blow from west to east across the
globe. Earth has four primary jet streams: two polar jet streams, near the north and south poles,
and two subtropical jet streams closer to the equator.
Jet streams are like rivers of wind high above in the atmosphere. These slim strips of strong
winds have a huge influence on climate, as they can push air masses around and affect weather
patterns.
Genesis of Jet Streams
Thermal gradient between pole and equator
•
Pressure gradient between pole and equator
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The genesis of the Jet-streams is provided by three kinds of gradients:
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Pressure gradient between surface and subsurface air over the poles.
The jet streams on Earth - other planets have jet streams as well, notably Jupiter and Saturn typically run from west to east, and their width is relatively narrow compared to their length.
Jet streams are typically active at 6,100 meters to 9,144 meters, or about 11 kilometres above
the surface and travel in what is known as the troposphere of Earth’s multi-layered atmosphere.
While they are narrow, they cover a wide latitude running north to south and often travel a very
winding path; at times they can even fade away or break off into smaller “rivers” of air that
merge again “downstream.”
The seasons of the year, location of lowand high-pressure systems and air
temperature all affect when and where a
jet stream travels. Jet streams form a
border between hot and cold air. Because
air temperature influences jet streams,
they are more active in the winter when
there are wider ranges of temperatures
between the competing Arctic and tropic
air masses.
Fig. 1: On Earth there are four main jet streams: two polar jet streams and two subtropical jet
streams. They form in the atmosphere where warm air masses meet cool air masses
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 402 km/h or greater,
but average about 177 km/h.
Both the Northern and Southern hemispheres have jet streams (Fig. 1), although the jet streams
in the north are more forceful. The motion of the air is not directly north and south but is
affected by the momentum the air has as it moves away from the equator. The reason has to
do with momentum and how fast a location on or above the Earth moves relative to the Earth's
axis.
Sub-Tropical Jet Streams are best developed in winter and early spring. Their maximum
speed approaches 300 knots which are associated with the merger with polar-front jets.
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Types of Jet Streams
A subsidence motion accompanies subtropical jets and gives rise to predominantly fair
weather in areas they pass over. Sometimes they drift northward and merge with a polarfront jet.
2.
Tropical Easterly Jet Stream occurs near the tropopause over Southeast Asia, India, and
Africa during summer. This jet implies a deep layer of warm air to the north of the jet and
colder air to the south over the Indian Ocean. The difference in heating and cooling and
the ensuing pressure gradient is what drives this jet.
3.
Polar-Night Jet Stream meanders through the upper stratosphere over the poles. They
are present in the convergence zone above the sub polar low-pressure belt.
While the polar and subtropical jet streams are
the best known and most studied, other jet
streams can form when wind speeds are above
58 mph (93.3 km/h) in the upper atmosphere at
about 9.6 kilometres to 14.5 kilometres above
the surface. The term is often misused, even by
meteorologists giving the weather forecast
who sometimes, for the sake of simplicity, call
all strong upper-atmosphere winds jet streams.
At higher latitudes, the warm air cools and sinks,
Fig. 2: Cross section of the jet stream
drawing more warm air in behind it. The cooled air flows back towards the equator, creating a
loop or convection cell.
Jet Streams and the Weather
Jets streams play 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.
While they are typically used as one of the factors in predicting weather, jet streams don’t
generally follow a straight path - the patterns are called peaks and troughs - so they can shift,
causing some to point at the poor forecasting skills of meteorologists.
Climatologists say that changes in the jet streams are closely tied to global warming, especially
ups and downs become more extreme, bringing different types of weather to areas that are not
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are warming faster than the remainder of the planet. When the jets streams are warmer, their
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the polar jet streams, because there is a great deal of evidence that the North and South poles
accustomed to climate variations. If the jet stream dips south, for example, it takes the colder
air masses with it.
Jet Streams are also influenced by El Nino and La Nina. During El Nino for example, precipitation
usually increases in California because the polar jet stream moves farther south and brings more
storms with it. Equally, during La Nina events, California dries out and precipitation moves into
the Pacific Northwest because the polar jet stream moves more north. In addition, precipitation
often increases in Europe because the jet stream is stronger in the Northern Atlantic and can
push them farther east. It also has a significant impact on the world’s weather patterns and
severe weather events like floods and droughts.
The freezing, powerful winds that whip the top of Mt. Everest, the world's tallest mountain, are
jet streams. Jet streams can be so cold, and so strong, that climbers cannot leave the shelter of
their tents.
Jet Streams and the Monsoons
The Sub-Tropical Jet stream plays a significant role in both hindering the monsoon winds as well
as in quick onset of Indian monsoons. During winter season the upper air westerly jet streams
are positioned in Asia. Over the Indian subcontinent, there are several separate jet streams
whose speed varies from 110 km/h in summer to about 184 km/h in winter. These are
bifurcated in two branches due to Tibet Himalayan obstruction. North branch blows north of
Himalayas and the Tibetan Plateau. Southern branch blows south of the mighty mountains. In
winter the sub-tropical westerly jet streams bring rain to the western part of India, especially
Himachal Pradesh, Haryana and Punjab. While in summer the sub-tropical easterly jet blows
over Peninsular India approximately at 140 N and bring some rain and storm.
During summer season as sun falls vertically over the Tropic of Cancer the polar surface high
pressure is weakened and upper air circum polar whirl shift northward as a result of which the
upper air westerly jet is also withdrawn from southern slopes of the Himalayas. The removal of
jet stream to north of the Tibetan plateau results in reversal of the curvature of How of free air
to the north and north-west of the subcontinent. This event may well be the trigger that sets
off the ‘burst’ of the monsoon. Origin of the Easterly Jet Stream because of the Plateau of Tibet
becoming Hot in summer.
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Jet streams are so fast and powerful that airplanes have difficulty flying against them. Pilots
either fly with the jet stream or above it; they do not attempt to fly against it. Jet streams also
have an impact on air travel and are used to determine flight patterns. An airplane can travel
much faster, and save fuel, by getting “sucked up” in the jet stream. That can also cause a
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Jet Streams and Aviation
bumpy flight, because the jet stream is sometimes unpredictable and can cause sudden
movement, even when the weather looks calm and clear. Into Thin Air
INDIAN MONSOON
Monsoon comes from an Arabic word 'MAUSAM' which means season. Thus, monsoon are
seasonal winds which reverse their direction of flow with the change of season. They flow
from sea to land during the summer and from land to sea during winter.
The most dominating factor of the Indian climate is the 'Monsoon winds' as a result of which
it is often called the Monsoon Climate. The south-west summer monsoons from the Arabian
sea and the Bay of Bengal bring rainfall to the entire country. The north-eastern winter
monsoons travel from land to sea and do not cause much rainfall except along the Coromandel
coast after getting moisture from the Bay of Bengal.
The theories regarding the monsoons are generally divided into following two broad
categories:
Classical theory: Halley
explained the monsoon
as resulting from thermal
contrasts
between
continents and oceans
due to their differential
heating. In summer the
sun shines vertically over
the tropic of cancer
resulting
in
temperature
and
high
low
pressure in central Asia
while the pressure is still
sufficiently
high
over
Arabian Sea and Bay of
Ben- gal. This induces air
brings heavy rainfall to
India
and
her
neighbouring countries.
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flow from sea to land and
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In winter the sun shines vertically over the tropic of Capricorn. The north western part of
India grows colder than Arabian Sea and Bay of Bengal and the flow of the monsoon is
reversed. It was lacking in the physical ingredient of the effect of rotation of the earth.
Modern theory:
a) Role of ITCZ; according of FEOHN monsoon is only the normal seasonal migration of
planetary winds following the sun. According to him the existence of Asian monsoon
is not due to contrast between land and sea but mainly due to the annual migration
of thermally produced planetary winds and pressure belts under continental
influence. The southeast trade winds of the southern hemisphere cross the equator
and start flowing from southwest to northeast direction under the effect of Coriolis
force. These displaced trade winds are called south west monsoon and bring monsoon
to the region.
b) Role of Jet Streams; M.T.Yin had given this concept stating that the burst of monsoon
depends upon the upper air circulation. Two prominent jet streams effect the
monsoon winds.
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The sub-tropical westerly jet stream, this jet stream dominates in wintertime in upper
troposphere circulation of the northern latitudes. It has a global extent between
latitudes 25-32 ° N and can be located over south Asia at an elevation of about 12 km.
the jet stream is split owing to the 0presence of Himalayan mountain system in its
path.
•
The winds tend to descend over north- western part of India resulting in atmospheric
stability.
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Equatorial easterly jet stream, this jet is a prominent feature of the upper air
circulation during the Indian monsoon season appearing as a band of strong easterlies
extending from south East Asia across the Indian Ocean and Africa to the Atlantic.
•
The western and eastern jet streams flow in the north and south of the Himalayas
respectively. The eastern jet becomes powerful and stationed and this results in more
active south west monsoon.
c) Role of Tibetan plateau, the Tibetan plateau is located more than 4500 km above sea
level with a length of 2000 km and with a width of 600 km in the west and 1000 in the
east.
This plateau is one of the key factors in the development of monsoon. The Tibetan
plateau exerts its influence as a mechanical barrier as well as high heat plateau. An
anticyclone appears in upper troposphere due to latent heating over the Tibetan
plateau. It generates an area of rising air, during its ascent the air spreads outwards
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2.
and gradually sinks over the equatorial part of the Indian Ocean. It picks up moisture
from the Indian Ocean and causes rainfall in India and adjoining countries.
Characteristics of Monsoonal Rainfall
i.
Rainfall from the southwest monsoons is seasonal in character, which occurs between
June and September.
ii.
Monsoonal rainfall is largely governed by relief or topography. For instance, the
windward side of the Western Ghats registers a rainfall of over 250 cm. The heavy rainfall
in the north- eastern states can be attributed to their hill ranges and the Eastern
Himalayas.
iii.
The monsoon rainfall has a declining trend with increasing distance from the sea. Kolkata
receives 119 cm during the southwest monsoon period, Patna 105 cm, Allahabad 76 cm,
Delhi 56 cm and Bikaner 24 cm.
The above given Figure shows that, normally the trade winds and strong equatorial currents
flow towards the west. At the same time, an intense Peruvian current causes upwelling of
cold water along the west coast of South America
iv.
The monsoon rains are characterized by 'Breaks'. These breaks in rainfall are related to
the cyclonic depressions formed at the head of the Bay of Bengal, and their crossing into
the mainland. The frequency of such de- pressions is 2 to 4 per months, from June to
September. Besides the frequency and intensity of these depressions, the passage
The summer rainfall comes in a heavy downpour leading to considerable run off and soil
erosion.
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followed by them determines the spatial distribution of rainfall.
vi.
Monsoon is the pivot of the agrarian economy of India because over three- fourths of the
total rain in the country is received during the southwest monsoon season.
vii.
Its spatial distribution is quite uneven ranging from 12 cm in western Rajasthan to more
than 400 cm in Meghalaya.
viii.
The beginning of the rains sometimes is considerably delayed over the whole or a part of
the country. The rains sometimes end considerably earlier than usual, causing great
damage to be standing crops and making the sowing of winter crops difficult. Therefore,
monsoons are extremely unpredictive and uncertain.
Rainfall Distribution
1.
Areas of very high rainfall (annual
rainfall of 200cm and above)
These include the west coast from
Thiruvananthapuram in the South to
Mumbai in the North (Avg. annual rainfall
200 - 400 cm). Almost the whole of Assam,
Nagaland, Meghalaya, Mizoram, Arunachal
Pradesh, Sikkim, parts of Manipur, Tripura
and north-eastern tip of West Bengal also
receive 200cm or more, with isolated
pockets receiving over 400 cm. Meghalaya
(The abode of clouds) is the wettest part of
the
country
with
Mawsynram
and
Cherrapunji getting 1221cm and 1102 cm
of annual rainfall respectively.
2.
Areas of High rainfall (100-200cm annual rainfall)
These include eastern slopes of the Western Ghats, major part of the northern plain, Orissa,
M.P. Andhra Pradesh, and Tamil Nadu.
3.
Areas of Low rainfall (50 - 100 cm annual rainfall)
Include large parts of Gujarat, Maharashtra, Western M.P., Andhra Pradesh, Karnataka,
Areas of very Low rainfall (Less than 50 cm of annual rainfall)
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eastern Rajasthan, Punjab, Haryana and parts of Uttar Pradesh.
These are desert and semi-deserts areas. They include large parts of Western Rajasthan,
Kuchchh, and most of Ladakh region of Jammu and Kashmir.
WESTERN DISTURBANCES
With the southward shift of the polar front in winter, the tracks of middle latitude cyclones
pass across the northern portion of the Indian subcontinent during the period October to
June. In the other months the tracks shift far to pole ward and do not usually affect the Indian
region.
The temperate cyclones originate in west ern Asia and Mediterranean Sea and reach the
Indian area in the course of their eastward passage from west India and refer to as western
disturbances. These western disturbances causes,
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Snowfall in higher reaches of Himalayas
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Rainfall in north west plains which are beneficial for rabi crop
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The sudden cold wave that decreases the temperature of that area and hail also takes
place.
LOCAL WINDS
During summers the atmospheric pressure is low all over the country. In the months of May
and June high temperature in North West India builds up steep pressure gradient and under
such conditions dust laden and strong winds blows.
LOO: it is hot dust laden winds that usually starts in the morning and reaches its peak at
afternoon. This increases the temperature of the area and causes high humidity.
AANDHIS: these are basically thunder- storms which move like a solid wall of dust and sand.
The winds velocity is high, and visibility reduces to few meters only. Such dust storms are
common in Rajasthan, Haryana, Delhi, Uttar Pradesh etc.
In West Bengal and adjoining areas of Jharkhand, Orissa the direction of storm is mainly from
the North West and are called NORWESTERS.
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and human beings. Maximum occurs in month of March and April
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KAL BAISAKHIS: these are violent storms causes heavy damage to standing crops, livestock