• India is a tropical monsoon country, indicating the impact of its location in tropical belt and the monsoon winds.
  • Although a sizeable part of the country lying north of tropic of cancer falls in the northern temperate zone but the shutting effects of the Himalayas and the existence of the Indian ocean have largely given India a distinct tropical climatic characteristics.

Weather Conditions

The year is conveniently divided by Indian Meteorological Department into following four seasons:

I. The Cold Weather Season 

  • It begins in early December and continues upto February.
  • During winter season there is general increase of temperature from North to South. Isotherms run in east-west direction almost parallel to the lati-tudes.
  • The 21°C isotherm runs east-west through the middle of the country roughly parallel to the tropic of cancer connecting Tapi estuary and Mahanadi delta.
  • The western coast is wanner than the eastern coast by about 1.7°C.
  • The peninsular region of the country however does not have well defined cold weather season.
  • A high pressure system develops over north and north-western part of the country from where cold and dry winds blow outwards.
  • During the cold weather season a number of cyclonic depressions travel eastwards from the Mediterranean sea to North India. These depressions called western disturbances being considerable amount of precipitation over the area.

II. The Hot Dry Weather (March to May)

  • The north Indian region experiences a well defined hot weather season between cool and mainly dry winter and the west monsoon season.
  • With the northward March of the sun towards the Tropic of Cancer after venial equinox, the temperature begins to rise continuously and rapidly.
  • In May, the scene of highest temperature shifts to Rajasthan where tem-perature as high as 50°C may be recorded.
  • The maximum summer temperatures are comparatively lower in the south-ern parts of the country due to moderating effect of the sea.
  • Because of the heating of the subcontinent the equatorial trough moves northward and lies at 25°N.
  • Under such conditions hot dust laden strong winds blow over most parts of North India, known as 'Loo'.
  • The strong dust stroms resulting from the convective phenomenon (due to intense heating) arc locally called `Andhi' in UP', `Norwester' in eastern India & 'Kalbaishakhi' in West Bengal.
  • These storms bring some amount of precipitation which is called 'cherry blossoms' in Karnataka (suitable for coffee plantation). Elsewhere in South India this rainfall is called 'Mango-Showers'.

III. The Wet Season/South West Monsoon Season (June—September)

  • During this period an extensive low pressure area develops over north west India and Pakistan which is called monsoon trough.
  • This trough attracts south west monsoonal winds.
  • Due to tapering of the southern peninsula the south west monsoon winds are bifurcated and enter the country in two branches—the Arabian Sea branch and the Bay of Bengal branch. The sudden outbreak of rainfall in this period is called monsoon burst.
  • The Arabian Sea branch contributes 65 per cent of total humidity brought by the monsoon.
  • The Indian subcontinent receives bulk of its rainfall (about 78 per cent) during the south west monsoon period.
  • The Arabian sea branch of monsoon causes rainfall along the west coast, western ghat, Maharashtra, Gujarat and parts of Madhya Pradesh and Ra-jasthan.
  • The Bay of Bengal branch of Monsoon enters the Ganga plain after being deflected westwards by the Arakan Yoma mountains.
  • The Bay of Bengal branch is in trapped in the deep tunnel shaped valley of Garo, Khasi and Jaintia hills which is surrounded by high hills on three sides. Due to this, heaviest rainfall occurs at Mawsynram (1141 cm) and Cherapunji (1087 cm).
  • The weather and rain during this season arc also affected by a number of cyclonic depressions which enter the country through Bay of Bengal and the Arabian Sea.
  • The normal date of onset of the S.W. monsoon is 20th may in Andaman and Nicobar islands, 1st June on Kerala coast and by 15th July it covers whole of India.
  • The east coast of India remains almost dry during this season because it lies in the rainshadow area of the Western Ghats and is parallel to the Bay of Bengal branch of monsoon.
  • As the Arabian Sea branch of monsoon crosses the Western Ghats, the places situated on the leeward side of the Ghats receive less rainfall.
  • Kachchh, Saurashtra and western Rajasthan fall to get adequate rainfall due to absence of mountain barrier in Kachchh, parallel position of the Aravali ranges to monsoon winds and the shutting effect of the hot & dry air from Baluchistan.
  • The monsoon trough of low pressure does not remain stationary rather it moves north and south over North India, affecting greatly the distribution of rainfall in the country.

IV. Retreating Monsoon Season

  • By the end of Seprember, the S.W. monsoon becomes weak as the low pressure trough of the Ganga Plain starts moving southward in response to the southward march of the Sun. As a consequence monsoon also starts retreating by completely reversing its direction of flow.
  • The withdrawal of monsoon start from September and is completed in mid- December from the south-eastern coast of Tamil Nadu.
  • The direction of retreating monsoon becomes North-Easterly (hence called N.E. monsoon after full development). It causes rains in the coastal areas of Tamil Nadu. Elsewhere the season is marked by dry weather.
  • Weather during this period is also influenced by a number of very violent and distractive tropical cyclones. Such cyclones are less frequent in the Arabian Sea.

Climate Regions of India

The climatic division of India is based upon Trewartha's scheme, which is a modi-fied form of Koppen's system and it corresponds with the vegetative, agricultural and geographical regions of India. Math climatic regions of India include.

  • Tropical Rain Forest (AW): It is found on the west coastal plain, the west-ern ghats and some parts of Assam. It is characterised by high temperature in winter not below 18.2°C; and in summer about 29°C. The average rain-fall exceeds 200 cm.
  • Tropical Savana (AW): It is located in peninsular region except the semi-arid zone in the leeside of Assam. It is characterised by high temperature in winter not below 18.2°C; and in summer about 29°C. The average rainfall exceeds 200 cm.
  • Tropical Semi-arid Steppe (BS): Prevails in the rain-shadow belt running southward from central Maharashtra to Tamil Nadu in the leeside of the Sahyadris and Cardamom Hills. It is characterised by low rainfall which varies from 38 cm to 80 cm, high temperature between 20° and 30°C.
  • Tropical and Sub-Tropical Steppe (BSh): Occurs over Punjab extending to Kutch region. The Thar desert is in the west and the more humid climate of the Ganga plain and the peninsula to its east and south respectively. Characterised by the annual rainfall of 30.5 cm to 63.5 cm, temperature from 12°C (January) to 35°C (June).
  • Tropical Desert (BWh): The area includes the western parts of Barmer, Jaisalmer and Bikaner districts of Rajasthan. A large portion of Kutch peninsula along with Thar Desert is also included. It is characterised by scanty rainfall (30 cm average) with few parts receiving 12 cm annual rainfall. Temperature is above 35°C.
  • Humid Sub-tropical with Dry Winter (CWa): The area includes south of the Himalayas, East of the tropical and sub-tropical steppe and north of tropical Savana. It is characterised by rainfall of 63.5 cm to 254 cm, most of it is received during the south west monsoon season.
  • Mountain Climate (H): The area lies above 6000 metre of sea-level. Examples are the Himalayas and Karakoram ranges. Temperature decreases with altitude. The Trans-Himalayan region particularly Ladakh has a dry and cold climate—what may be called cold desert. Drought is permanent.