Climate of India

by Devender

0 1522

The climatic conditions of India are very varied because of the different relief features in such a vast country. India has regions with very hot temperatures as well as very cold ones too. India has regions with very heavy rainfall and very scanty rainfall.

Climate of India

The climate of India gets influenced by its position, size, and relief features. A large part of India has a tropical monsoon climate. Monsoon winds are the main factors that determine the climate of India.

Climatic Seasons of India

There are 4 types of climatic seasons in India:

Winter - December to February
Summer - March to May
Monsoons or Rainy season - June to September
Retreating monsoons - October and November

Agriculture is the main occupation of people in India and the Southwest monsoon control agriculture. If the monsoon fails, there will be drought and when it is too heavy, it will cause floods leading to the destruction of property and life.

Distribution of Rainfall in India

The rainfall in India is seasonal and uncertain. It is also unevenly distributed and mainly pours down during the South-West Monsoon period.

India can be divided into 5 major rainfall regions on the basis of the quantity of rainfall which are:

1 Very low rainfall region

The rainfall in this region is less than 30 cms annually. The regions that fall in this region are -

Karakoram Ranges, Northern Kashmir, Western Kutch & Rajasthan Region (Thar Region)

2 Low rainfall region

This region has a rainfall of 30 to 60 cms annually. The regions in this group are -

Zaskar range, parts of Punjab and Haryana, Central Rajasthan, Western Gujarat, and the rain-shadow areas of the Western Ghats

3 Moderate rainfall region

This region has a rainfall of 60 to 100 cms annually. This region is found in most parts of India.

Most of the rain happens due to South-West Monsoon winds

4 Heavy rainfall region

This region has a rainfall of 100 to 200 cm annually. The regions in this group are -

The western coast, Eastern coastal belt, Foothills of the Himalayas & a part of north-east India

5 Very heavy rainfall region

This region has a rainfall of over 200 cm annually. The regions in this group are -

The western side of the Western Ghats, foothills of Himalayas, Meghalaya plateau (Shillong plateau), and Andaman and the Nicobar Islands

Mawsynram in the Meghalaya plateau gets the heaviest rainfall in the world

Major factors affecting Indian Climate

The major factors that affect the climate of India are -

Northward shifting of the Westerly Jet
Northward shifting of the ITCZ
S-E trade winds from the S. hemisphere cross the equator and turn right due to Coriolis force
Latitudinal Extent
Southern Seas
Northern Mountains
El – Nino
La – Nina
Westerlies in the Northern part of India from the Mediterranean (in winters)
Easterlies due to Heating of Tibetian Plateau
Jet streams

Indian Monsoon Features
Unique weather phenomenon
Seasonal reversal of winds
Sudden Onset (Sudden rain start)
Gradual Advance
Gradual retreat
Variation – regional and temporal

The monsoon is seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. The southwest monsoon brings rains towards the end of summer because the high pressure built in the Indian Ocean pushes the wind masses towards the low pressure formed on land.

Temperature Gradient is the temperature variation between the sea and the landmass.

Summer Monsoon in India (SW Monsoon)

It originates due to the Northward shift of ITCZ. The SE trade winds cross the equator which deflects & enters India as SW Monsoon.

The Easterly Jet Stream/SE Monsoon/BOB Monsoon happens due to differential heating of the Tibetian plateau and Himalayan region with respect to the Bay Of Bengal. The southwest monsoon arrives in two branches which are:

Bay of Bengal branch
Arabian Sea branch

The BOB branch initially tracks the Coromandal Coast northeast from Cape Comorin to Orissa, swerves to the northwest towards the Indo-Gangetic Plains.

The Arabian sea branch extends toward a low-pressure area over the Thar Desert and is three times stronger than the Bay of Bengal branch. It moves northeast towards the Himalayas.

Initiation of Summer Monsoon

The southeast monsoon basically reaches Indian territory around 25 May. It first hits the Andaman and Nicobar Islands in the Bay of Bengal. It reaches the mainland of India by 1 June when it reaches the Malabar Coast of Kerala.

It reaches Mumbai by 9 June and Delhi by 29 June
The entire country experiences the monsoon rain by the first week of July

Averagely, South India receives more rainfall than North India but North India receives the most precipitation.

Summer Monsoon Withdrawal

The Monsoon clouds start withdrawing from North India by the end of August. It withdraws from Mumbai around 5 October. India cools down during September making the Southwest monsoon weak. So, it leaves the country by the end of November.

1 Arabian Sea Branch

It stikes the Western Ghats and gives rainfall to the westernmost regions. It moves parallel to Aravallis & Strike Himalayas.

It provides rainfall to WGs, Coastal Regions, and Northern Plains

While the rain shadow interiors, the Deccan plateau receives very little rainfall.

2 Bay of Bengal Branch

It moves parallel to the Eastern Ghats and produces very little rainfall until it strikes NE. It also bifurcates at Meghalaya hills & moves parallel to Himalaya.

One branch of it provide rainfall to the NE India region
The other branch moves westward providing rainfall to the northern plains
When it moves westward, the rainfall decreases

It provides rainfall at Northern East plains and Northern Plains.

Retreating or NE Monsoon

Around September, when the sun retreats south, northern India cools off rapidly. Due to this, the air pressure builds over northern India but the Indian ocean still holds its heat. The surrounding atmosphere still holds its heat Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsula. This is known as the Northeast Monsoon or Retreating Monsoon.

Winter Rainfall in South India

When these NE monsoon winds move towards the Indian Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and pours it over peninsular India and parts of Sri Lanka.

The cities such as Madras gets less rainfall from the Southwest monsoon but receive rainfall from this monsoon
The state of Tamil Nadu receives 50% to 60% of its rain from this monsoon

In Southern Asia, the northeastern monsoons take place from December to early March when the surface high-pressure system is strongest.

Jet Streams

Jet Streams are currents of air high above the Earth's surface nearly 8 to 15 km. They are near the tropopause.

Major jet streams on Earth are westerly winds
They flow from west to east
They flow at very high speed, 120 kmph in winters and 50 kmph in summers
Jet streams are caused by the combined action of Earth's rotation on its axis and atmospheric heating

Jet streams form near boundaries of adjacent air masses with significant differences in temperature, such as the polar region and the warmer air towards the equator. Westerlies flow over north India south of the Himalayas all the year but in summers with a shift of the sun, they flow north of the Himalayas and are replaced by easterlies.

Sub-tropical jet stream

They prevail over the lower latitudes of westerlies and are produced by the rotation of the earth and its spherical shape.

The air over the equator has the highest velocity (Coriolis effect)
It has a higher velocity than the air at lower altitude prevailing at the same latitude

It begins to flow from west to east around 30º latitude.

Sub-Tropical Westerly Jet

In winters, it flows south of the Himalayas – over north India. It is the major cause of the western disturbance.

This jet stream maintain the high pressure over north India
Therefore, there is no monsoon in winters

During the summers, it flows over the north of the Himalayas. Therefore, there is low pressure over North India and hence, monsoon comes in summers.

Note:

This stream is also a chief reason why there are no cyclones during monsoon because it prevents the vertical circulation of air which is required for the formation of cyclones.

Mid-latitude or polar front jet stream

It is produced by a temperature difference and it is more variable. In summer, it shifts towards the poles while in winters, it shifts towards the equator.

Tropical Cyclones in India

The sea surface temperature of the Bay of Bengal and the Arabian Sea increases in late summer. So, the possibility of a Tropical cyclone prevails. The retreating SW monsoon branch drags them towards the Eastern coast.

Tropical cyclones move from east to West
They are secondary circulations and maintain the larger direction of the planetary winds

Therefore, any cyclones that form in the Arabian sea are likely to affect India, however, the Delta region of the eastern coast is frequently struck by a cyclone.

Tropical Cyclone

A tropical cyclone is a storm system characterized by a large low-pressure centre and it is marked by numerous thunderstorms that produce strong winds & heavy rain.

The winds spiral at a very high speed of around 120 kmph
It develops over the sea where surface temperature is above 27º C
It acquires energy from the latent heat of condensation of water vapour
It quickly dissipates over land as their moisture supply is cut off

It forms between 8-20 degrees north and south of the equator and never between 0-8 degrees north and south of the equator because of weak Coriolis force. It causes heavy rainfall which is short-lived with thunderstorms. These are the most violent and destructible types of storms.

Western Disturbances

These are the temperate cyclones or extra-tropical storms originating in the Mediterranean. It brings sudden winter rain and snow to the northwestern parts of India.

This is a non-monsoonal precipitation pattern driven by the Westerlies
The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean

They travel from place to place due to differences in presence. The high pressure in the northwestern Indian subcontinent favors its travel. During the winters, the low-pressure system originates over the Mediterranean Sea and western Asia. So, it moves into India along with the westerly flow.

These are important for the development of the Rabi crop in the northern subcontinent especially wheat
They shower rain over Pakistan, India, and Nepal

Extra-tropical storms are global phenomena with moisture usually carried in the upper atmosphere, unlike tropical storms where it is carried in the lower atmosphere.

The Winter rainfall occurs due to:

North-East Monsoon
Western Disturbances
Tropical Cyclones

Factors Responsible for Regional Variability of Rainfall over India

The Windward regions (Such as the Western Ghats) get more rainfall than interiors. The regions obstructing monsoonal branches like that perpendicular to the Arabian Sea branch get rainfall such as Himalayan foothill states.

The Aravallis parallel to the Arabian Sea branch remains dry
The Southeastern region parallel to the Bay of Bengal branch remains dry
As the winds near the sea bear more moisture, the regions near the seas get more rainfall
The regions of the confluence of the 2 major branches also receive more rainfall

Southern Oscillations

There exists a see-saw pattern of meteorological changes between the Indian Ocean and Pacific ocean. Whenever pressure is higher over the Indian Ocean, low pressure prevails over the Pacific Ocean and vice versa.

The scale that is used for this pattern is SOI (Southern Oscillation Index) which is:

(Tahiti – Darwin) Pressure

Tahiti = Pacific Ocean, Darwin = Indian Ocean

When SOI > 0, India gets a good monsoon which is the La Nina Condition
When SOI < 0, India gets a bad monsoon which is the Al Nino Condition

Normal Year – Walker cell Condition at south Pacific:

LP – Northern Australia and HP – South America (Peru).

The South equatorial current pile up water in northern Australia that increases SST and is known as West Pacific Pool.

It brings rainfall in Northern Australia
The diverging air above Australia moves towards the Peruvian coast

They descend at Peruvian coast = HP – desiccating effect to Atacama desert. This completes the Walker cell.

Normal Conditions:

As the south equatorial current takes water from east to west, it led to water from the bottom coming up and take the space. This Up-welling at the Peruvian coast makes rich fishing ground.

El Nino:

In this, the direction of the walker cell reverses. The South equatorial current weakens and strong counter-current activates which weakens the piling up of water in Northern Australia and brings a weakening of the west Pacific Pool.

The Ocean water moves towards the Peruvian coast creating an LP system over there and rainfall at the Atacama Desert. The rising and diverging wind above Peru descends over Australia = HP condition – drought in Northern Australia.

The submarine cycle is also altered because of the reversal in wind direction
The down-welling at the Peruvian coast brings loss in the fishing business

It is a warm current that appears off the coast of Peru in December (3 – 36º S of Equator), also known as child Christ as it appears around Christmas. It is the temporary replacement of cold Peruvian/Humbolt current which normally flows against the coast & appears once in 3 – 7 years.

It is responsible for widespread floods & droughts in various tropical regions of the world
The warming of tropical pacific water affects the global pattern of pressure and wind system, including monsoon winds in the Indian Ocean

The high pressure of the Indian Ocean & low pressure at said area of the Pacific Ocean shifts some of the monsoon winds to the Pacific Ocean side which results in scarcity of rainfall in India. It brings drought conditions in Indonesia as well – forest fire.

La Nina:

It means The Girl in Spanish. After the El Nino when weather conditions return to normal, trade winds become strong. Therefore, they cause abnormal accumulation of cold water in the central and eastern pacific regions.

It creates a high-pressure region in the Pacific Ocean as compared to the Indian Ocean
It brings Heavy rainfall and flood condition in Northern Australia but good monsoon in India
It also leads to very good fishing business on the Peruvian coast which leads to a price crash
It brings drought to the Atacama

It brings heavy monsoon showers in India due to the Northeast monsoon along with monsoon laden Pacific winds from the tropical Pacific Ocean although it marks an active hurricane season in Peru.

Some basic Terms Related to Pre Monsoon

Mango Showers

These are the pre-monsoon showers that happen in Karnataka and Kerala that help in the ripening of mangoes. They are also known as April rains or Summer showers.

They are the result of thunderstorms over the Bay of Bengal
These summer rains normally come in the second half of the month of April

These showers are very crucial for the mango cultivators of South India as it prevents the mangoes from dropping prematurely from trees.

Cherry Blossom

It is the local thunderstorms in the state of Karnataka and the associated region. These are Pre-monsoon Showers that occur in the month of April & May caused because of the meeting of humid sea winds and hot dry local wind.