UPSC Geography Climate / जलवायु NCERT Extracts - Climate of India

Weather

• Weather is the momentary state of the atmosphere while climate refers to the average of the weather conditions over a longer period of time.
• Weather changes quickly, may be within a day or week but climate changes imperceptively and may be noted after 50 years or even more.

Factors Determining the Climate of India

Latitude

• The Tropic of Cancer passes through me central part of India in east-west direction.
• Thus, northern part of the India lies in sub-tropical and temperate zone and the part lying south of me Tropic of Cancer falls in me tropical zone.
• The tropical zone being nearer to me equator, experiences high temperatures throughout me year with small daily and annual range.
• Area north of me Tropic of Cancer being away from the equator, experiences extreme climate with high daily and annual range of temperature.

The Himalayan Mountains

• The lofty Himalayas in the north along with its extensions act as an effective climatic divide.
• The towering mountain chain provides an invincible shield to protect the subcontinent from the cold northern winds.
• These cold and chilly winds originate near the Arctic circle and blow across central and eastern Asia.
• The Himalayas also trap me monsoon winds, forcing them to shed their moisture within me subcontinent

Distribution of Land and Water

• India is flanked by me Indian Ocean on three sides m me south and girdled by a high and continuous mountain-wall in me norm.
• As compared to me landmass, water heats up or cools down slowly.
• This differential heating of land and sea creates different air pressure zones in different seasons in and around me Indian subcontinent
• Difference in air pressure causes reversal in me direction of monsoon winds.

Distance from the Sea

• With a long coastline, large coastal areas have an equable climate. Areas in the interior of India areas have extremes of climate
• That is why, the people of Mumbai and the Konkan coast have hardly any idea of extremes of temperature and the seasonal rhythm of weather.
• On the other hand, the seasonal contrasts in weather at places in the interior of the country such as Delhi, Kanpur and Amritsar affect me entire sphere of life.

Altitude

• Temperature decreases with height Due to thin air, places in the mountains are cooler than places on the plains.
• For example, Agra and Darling are located on the same latitude, but temperature of January in Agra is 16°C whereas it is only 4°C in Darling

Relief

• The physiography or relief of India also affects the temperature, air pressure, direction and speed of wind and me amount and distribution of rainfall.
• The windward sides of Western Ghats and Assam receive high rainfall during June-September whereas the southern plateau remains dry due to its leeward situation along the Western Ghats.

• Factors Related to Air Pressure and Wind - e.g. Jet Stream, Western Disturbance etc.

Mechanism of Weather in the Winter Season

Surface Pressure and Winds

• In winter months, the weather conditions over India are generally influenced by the distribution of pressure in Central and Western Asia.
• A high pressure centre in the region lying to the north of the Himalayas develops during winter.
• This centre of high pressure gives rise to me flow of air at me low level from the norm towards the Indian subcontinent, south of the mountain range.
• The surface winds blowing out of the high pressure centre over Central Asia reach India in me form of a dry continental ah- mass.
• These continental winds come in contact with trade winds over northwestern India.
• The position of this contact zone is not, however, stable. Occasionally, it may shift its position as far east as the middle Ganga valley with me result mat me whole of the northwestern and northern India up to me middle Ganga valley comes under me influence of dry northwestern winds.

Jet Stream and Upper Air Circulation

• The pattern of ah- circulation discussed above is witnessed only at me lower level of me atmosphere near the surface of me earth.
• Higher up in me lower troposphere, about three km above me surface of me earth, a different pattern of air circulation is observed.
• The variations in me atmospheric pressure closer to me surface of me earth have no role to play in me making of upper air circulation.
• All of Western and Central Asia remains under the influence of westerly winds along me altitude of 9-13 km from west to east
• These winds blow across the Asian continent at latitudes north of the Himalayas roughly parallel to the Tibetan highlands. These are known as jet streams.
• Tibetan highlands act as a barrier in the path of these jet streams. As a result, jet streams get bifurcated.
• One of its branches blows to the north of the Tibetan highlands, while the southern branch blows in an eastward direction, south of the Himalayas.
• It has its mean position at 25°N in February at 200-300 mb level. It is believed that this southern branch of the jet stream exercises an important influence on the winter weather in India.

Western Cyclonic Disturbance and Tropical Cyclones

• The western cyclonic disturbances which enter the Indian subcontinent from the west and the northwest during the winter months/originate over the Mediterranean sea and are brought into India by the westerly jet stream.
• An increase in the prevailing night temperature generally indicates an advance in the arrival of these cyclones disturbances
• Tropical cyclones originate over the Bay of Bengal and the Indian ocean.
• These tropical cyclones have very high wind velocity and heavy rainfall and hit the Tamil Nadu, Andhra Pradesh and Orissa coast.
• Most of these cyclones are very destructive due to high wind velocity and torrential rain that accompanies it.

Mechanism of Weather in the Summer Season

Surface Pressure and Winds

• As summer sets in and the sun shifts northwards, the wind circulation over the subcontinent undergoes a complete reversal at both, the lower as well as the upper levels.
• By the middle of July, the low pressure belt nearer the surface [termed as Inter Tropical Convergence Zone (ITCZ)] shifts northwards, roughly parallel to the Himalayas between 20°N and 25°N.
• By this time the westerly jet stream withdraws from the Indian region.
• In fact, metebrologists have found an interrelationship between the northward shift f me equatorial trough (ITCZ) and the withdrawal of me westerly jet stream from over the North Indian Plain.
• It is generally believed mat there is a cause and effect relationship between the two.
• The ITCZ being a zone of low pressure, attracts inflow of winds from different
• The maritime tropical airmass (mT) from the southern hemisphere, after crossing the tor rushes to the low pressure area in the general southwesterly direction.
• It is this moist air current which is popularly known as the southwest monsoon.

Jet Streams and Upper Air Circulation

• The pattern of pressure and winds as mentioned above is formed only at the level of the troposphere.
• An easterly jet stream flows over the southern part of the Peninsula in June, and has a maximum speed of 90 km per hour.
• In August, it is confined to 15°N latitude, and in September up to 22°N latitudes. The easterlies normally do not extend to me north of 30°N latitude in the upper atmosphere.

Easterly Jet Stream and tropical Cyclones

• The easterly jet stream steers the tropical depressions into India. These depressions play a significant role in the distribution of monsoon rainfall over the Indian subcontinent.
• The tracks of these depressions are the areas of highest rainfall in India.
• The frequency at which these depressions visit India, their direction and intensity, all go a long way in determining the rainfall pattern during the southwest monsoon period.

Inter Tropical Convergence Zone (ITCZ)

• The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator where trade winds converge, and so, it is a zone where air tends to ascend. 0 In July, the ITCZ is located around 20°N-25°N latitudes (over the Gangetic plain), sometimes called the monsoon trough.
• This monsoon trough encourages the development of thermal low over north and northwest India.
• Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40° and 6Q°E longitudes and start blowing from southwest to northeast due to the Coriolis force.
• It becomes southwest monsoon.
• In winter, the ITCZ moves southward, and so the reversal of winds from northeast to south and southwest, takes place. They are called northeast monsoons.

Onset of the Monsoon

• Towards the end of the nineteenth century, it was believed that the differential heating of land and sea during the summer months is the mechanism which sets the stage for the monsoon winds to drift towards the subcontinent.
• During April and May when the sun shines vertically over the Tropic of Cancer, the large landmass in the north of Indian ocean gets intensely heated.
• This causes the formation of an intense low pressure in the northwestern part of the subcontinent.                                
• Since the pressure in the Indian Ocean in the south of the landmass is high as water gets heated slowly, the low pressure cell attracts the southeast trades across the Equator.
• These conditions help in the northward shift in the position of the ITCZ.
• The southwest monsoon may thus, be seen as a continuation of the southeast trades deflected towards the Indian subcontinent after crossing the Equator.
• These winds cross the Equator between 40°E and 60°E longitudes.
• The shift in the position of the ITCZ is also related to the phenomenon of the withdrawal of the westerly jet stream from its position over the north Indian plain, south of the Himalayas.
• The easterly jet stream sets in along 15°N latitude only after the western jet stream has withdrawn itself from the region.                                
• This easterly jet stream is held responsible for the burst of the monsoon in India.
• Entry of Monsoon into India: The southwest monsoon sets in over the Kerala coast by 1 June and moves swiftly to reach Mumbai and Kolkata between 10 and 13 June.
• By mid-July, southwest monsoon engulfs the entire subcontinent

Rain-bearing Systems and Rainfall Distribution

• There seem to be two rain-bearing systems in India. First originate in the Bay of Bengal causing rainfall over the plains of north India.
• Second is the Arabian Sea current of the southwest monsoon which brings rain to the west coast of India.
• Much of the rainfall along the Western Ghats is orographic as the moist air is obstructed and forced to rise along the Ghats.
• The intensity of rainfall over the west coast of India is, however, related to two factors:
• The offshore meteorological conditions.
• The position of the equatorial jet stream along the eastern coast of Africa.
• The frequency of the tropical depressions originating from the Bay of Bengal varies from year to year.
• Their paths over India are mainly determined by the position of ITCZ which is generally termed as the monsoon trough.
• As the axis of the monsoon trough oscillates, there are fluctuations in the track and direction of these depressions, and the intensity and the amount of rainfall vary from year to year.
• The rain which comes in spells, displays a declining trend from west to east over the west coast, and from the southeast towards the northwest over the North Indian Plain and (he northern part of the Peninsula.

El-Nine and the Indian Monsoon

• EI-Nino is a complex weather system that appears once every three to seven years, bringing drought, floods and other weather extremes to different parts of the world.
• The system involves oceanic and atmospheric phenomena with the appearance of warm currents off the coast of Peru in the Eastern Pacific and affects weather in many places including India.
• EI-Nino is merely an extension of the warm equatorial current which gets replaced temporarily by cold Peruvian current or Humbolt current.
• This current increases the temperature of water on the Peruvian coast by 10°C. This results in:
• the distortion of equatorial atmospheric circulation;
• irregularities in the evaporation of sea water; reduction in the amount of planktons which further reduces the number of fish in the sea.
• The word EI-Nino means 'Child Christ' because this current appears around Christmas in December. December is a summer month in Peru (Southern Hemisphere).
• EI-Nino is used in India for forecasting long range monsoon rainfall. In 1990-91, there was a wild EI-Nino event and the onset of southwest monsoon was delayed over most parts of the country ranging from five to twelve days.

Break in the Monsoon

• During the south-west monsoon period after having rains for a few days, if rain fails to occur for one or more weeks, it is known as break in the monsoon.
• These dry spells are quite common during the rainy season. These breaks in the different regions are due to different reasons:
• In northern India rains are likely to fail if the rain-bearing storms are not very frequent along the monsoon trough or the ITCZ over this region.
• Over the west coast the dry spells are associated with days when winds blow parallel to the coast.

The Cold Weather Season

• There are three main reasons for the excessive cold in north India during this season :
• States like Punjab, Haryana and Rajasthan being far away from the moderating influence of sea experience continental climate.
• The snowfall in the nearby Himalayan ranges creates cold wave situation; and
• Around February, the cold winds coming from the Caspian Sea and Turkmenistan bring cold wave along with frost and fog over the northwestern parts of India.
• The Peninsular region of India, however, does not have any well-defined cold weather season. There is hardly any seasonal change in the distribution pattern of the temperature in coastal areas because of moderating influence of the sea and the proximity to equator.

Pressure and Winds:

• By the end of December (22 December), the sun shines vertically over the Tropic of Capricorn in the southern hemisphere.
• The weather in this season is characterised by feeble high pressure conditions over the northern plain.
• In south India, the air pressure is slightly lower. The isobars of 1019 mb and 1013 mb pass through northwest India and far south, respectively.
• As a result, winds start blowing from northwestern high pressure zone to the low air pressure zone over the Indian Ocean in the south.
• Due to low pressure gradient, the light winds with a low velocity of about 3-5 km per hour begin to blow outwards.
• By and large, the topography of the region influences the wind direction. They are westerly or northwesterly down the Ganga Valley.
• They become northerly in the Ganga-Brahmaputra delta. Free from the influence of topography, they are clearly northeasterly over the Bay of Bengal.
• During the winters, the weather in India is pleasant.
• The pleasant weather conditions, however, at intervals, get disturbed by shallow cyclonic depressions originating over the east Mediterranean Sea and travelling eastwards across West Asia, Iran, Afghanistan and Pakistan before they reach the northwestern parts of India.                                
• On their way, the moisture content gets augmented from the Caspian Sea in the north and the Persian Gulf in the south,                                

Rainfall                                

Winter monsoons do not cause rainfall as they move from land to the sea.

It is because firstly, they have little humidity; and secondly, due to anti cyclonic circulation on land, the possibility of rainfall from them reduces.

So, most parts of India do not have rainfall in the winter season. However, there are some exceptions to it:                                                        

In northwestern India, some weak temperate cyclones from the Mediterranean sea cause rainfall in Punjab, Haryana, Delhi and western Uttar Pradesh. Although the amount is meagre, it is highly beneficial for rabi crops. The precipitation is in the form of snowfall in the lower Himalayas. It is this snow that sustains the flow of water in the Himalayan rivers during the summer months. The precipitation goes on decreasing from west to east in the plains and from north to south in the mountains.

Central parts of India and northern parts of southern Peninsula also get winter rainfall occasionally.

During October and November, northeast monsoon while crossing over the Bay of Bengal, picks up moisture and causes torrential rainfall over the Tamil Nadu coast, southern Andhra Pradesh, southeast Kamataka and southeast Kerala.

Some Famous Local Storms of Hot Weather Season

• Mango Shower: Towards the end of summer, there are pre-monsoon showers which are a common phenomena in Kerala and coastal areas of Kamataka. Locally, they are known as mango showers since they help in the early ripening of mangoes.
• Blossom Shower: With this shower, coffee flowers blossom in Kerala and nearby areas.
• Nor Westers: These are dreaded evening thunderstorms in Bengal and Assam. Their notorious nature can be understood from the local nomenclature of 'Kalbaisakhi', a calamity of the month of Baisakh. These showers are useful for tea, jute and rice cultivation. In Assam, these storms are known as "Bardoli Chheerha".
• Loo: Hot, dry and oppressing winds blowing in the Northern plains from Punjab to Bihar with higher intensity between Delhi and Patna.