INDIAN GEOGRAPHY : Indian Climate Part 1

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Climate of India

  • In a country like India which has different geographical structures that varies in region to region we have varied climatic conditions.
  • India has very hot and very cold regions; as well as regions with very heavy rainfall and very scanty rainfall
  • The climate of India has been influenced by its position, size and relief features
  • Monsoon winds are the main factors that determine the climate of India
  • A large part of India has tropical monsoon climate
There are 4 Climatic Seasons in India they are as follows :
  • Winter – December to February
  • Summer – March to May
  • Monsoons or Rainy season – June to September
  • Retreating monsoons – October and November

The South west monsoon controls the agriculture of India, which is the main occupation of the people.  When the monsoons fails, there is drought, and the crops also fail and when the monsoon is heavy, there are floods causing destruction to life and property.

Distribution of Rainfall in India 

  • The rainfall in India is seasonal, uncertain and unevenly distributed
  • Mainly pours down during the South-West Monsoon period
  • On the basis of the quantity of rainfall, we can divide India into five major rainfall regions viz.
Very low rainfall region
  • Less than 30 cms annually
  • Regions → Karakoram Ranges, Northern Kashmir, Western Kutch & Rajasthan Region (Thar Region)
Low rainfall region
  • 30 cms to 60 cms annually
  • Regions → Zaskar range, parts of Punjab and Haryana, Central Rajasthan, Western Gujarat and the rain-shadow areas of the Western Ghats
Moderate rainfall region
  • 60 cms to 100 cms annually
  • Found over greatest part of India
  • Most of the rain is from the South-West Monsoon winds
Heavy rainfall region
  • 100 cms to 200 cms annually
  • Regions → Western coast, Eastern coastal belt, Foothills of the Himalayas & a part of north-east India
Very heavy rainfall
  • Over 200 cms annually
  • Regions → Western side of the Western Ghats, foothills of Himalayas, Meghalaya plateau (Shillong plateau) and Andaman and Nicobar Islands

Major factors affecting Indian Climate

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

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 as the high pressure built in the Indian Ocean pushes the wind masses towards the low pressure formed on land. Temperature Gradient  It’s the temperature variation between the sea and the landmass

Summer Monsoon in India (SW Monsoon)

  • Originates due to Northward shift of ITCZ SE trade winds cross equator  Deflect & enter into India as SW Monsoon
  • Easterly Jet Stream / SE Monsoon / BOB Monsoon Due to differential heating of Tibetian plateau & Himalayan region wrt BOB
  • The southwest monsoon arrives in two branches: the Bay of Bengal branch and the Arabian Sea branch
  • The Bay of Bengal branch, which 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 roughly three times stronger than the Bay of Bengal branch
  • The Arabian Sea branch moves northeast towards the Himalayas

Initiation of Summer Monsoon

  • The southwest monsoon typically breaks over Indian Territory by around 25 May, when it lashes the Andaman and Nicobar Islands in the Bay of Bengal
  • It strikes the Indian mainland around 1 June near the Malabar Coast of Kerala
  • By 9 June, it reaches Mumbai; it appears over Delhi by 29 June
  • By the first week of July, the entire country experiences monsoon rain
  • On average, South India receives more rainfall than North India
  • However, Northeast India receives the most precipitation

Withdrawal of Summer Monsoon

  • Monsoon clouds begin retreating from North India by the end of August; it withdraws from Mumbai by 5 October
  • As India further cools during September, the southwest monsoon weakens. By the end of November, it leaves the country

Arabian Sea Branch

  • Strikes WGs + Move parallel to Aravallis & Strike Himalayas
  • Rainfall at WGs & Coastal Regions + Northern Plains
  • Strikes at Western Ghats; and gives rainfall to the western most regions
  • While rain shadow interiors, the Deccan plateau receive very less rainfall.

Bay of Bengal Branch 

  • Moves parallel with the Eastern Ghats and produce very less rainfall until it strikes at NE.
  • Bifurcate at Meghalaya hills & move parallel to Himalaya
  • One branch provide rainfall to NE India region
  • Another moves westward providing rainfall to northern plains
  • Going westward Rainfall Decreases
  • Rainfall at Northern East plains + Northern Plains

Retreating or NE Monsoon

  • Around September, with the sun fast retreating south, the northern land mass of the Indian subcontinent cool off rapidly
  • With this, air pressure begins to build over northern India, but the Indian Ocean and its surrounding atmosphere still holds its heat
  • 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

  • While travelling 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
  • Cities like Madras, which get less rain from the Southwest Monsoon, receive rain from this Monsoon.
  • About 50% to 60% of the rain received by the state of Tamil Nadu is from the Northeast 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
  • They at altitudes of about 8 to 15 kilometers, located neartropopause
  • The major jet streams on Earth are westerly winds (flowing west to east)
  • Flow at very high speeds 120 kmph in winters and 50 kmph in summers
  • Jet streams are caused by a combination of a planet’srotationon its axis and atmospheric heating
  • Jet streams form near boundaries of adjacent air masses with significant differences intemperature, such as thepolar region and the warmer air towards the equator
  • All year round westerlies flow over north India south of Himalayas but in summers with shift of the sun they flow north of Himalayas & replaced by easterlies

Sub-tropical jet stream 

  • They prevail over the lower latitudes of westerlies.
  • It is produced by the rotation of earth and its spherical shape.
  • The air over equator has the highest velocity (Coriolis effect)
  • As it rises and moves towards north, it has a higher velocity than the air at lower altitude prevailing at same latitude
  • So it begins to flow from west to east around 30º latitude

Sub-Tropical Westerly Jet

  • Winter – entirely south of Himalayas – over north India
  • Major cause of western disturbance
  • STWJ maintain the High pressure over north India
  • Hence no Monsoon in winters
  • During summers it flows to the north of Himalayas
  • Hence low pressure over north India & monsoon

Note  Tropical Easterly Jet Stream which is associated closely with the burst of monsoon. It is also a major reason why there are no cyclones during Monsoon because the presence of an Easterly jet over the Indian landmass in the upper troposphere prevents vertical circulation of air, which is a pre-condition for formation of cyclones.

Mid-latitude or polar front jet stream 

  • It is more variable and is produced by a temperature difference
  • In summers its position shifts towards poles and in winters towards equator

 

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