How Climate of India works?
Climate of India is par excellence as it experiences almost all types of climatic zones of the world. Indian climate contains hot desert type climate on one hand to the almost tundra type climate on the other. India is a tropical monsoon country, indicating the impact of its location in tropical belt and the monsoon winds. Although a sizable part of the country lies north of tropic of cancer and falls in the northern temperate zone but the shutting effects of the Himalayas and the existence of the Indian ocean have largely given India distinct tropical climatic characteristics. The Climate of India also have a profound bearing on the natural vegetation of India. Before going into the details of Indian climate, one must understand the weather of India first.
Climate of India – Weather conditions
An important aspect of the climate of India is the weather Conditions. The year is conveniently divided by Indian Metrological Department into following four seasons. They are : –
1) Cold weather season/ winter season,
2) Hot weather season/Summer season,
3) South-west monsoon season/Rainy season,
4) Retreating monsoon season
The cold weather season or winter season
It begins in early December and continues up of February.
- During winter season there is a general increase of temperature from north to south. Isotherms run in east-west direction almost parallel to the latitudes. The 210C 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 warmer than the eastern coast by about 1.70C.
- The peninsular region of the country however does not have a 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 bring considerable amount of precipitation over the area. This precipitation though in small amount is of great significance for the winter crop of wheat.
The Hot Dry Weather or summer season ( 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 vernal equinox, the temperature beings to rise continuously and rapidly.
- In May, the scene of highest temperature shifts to Rajasthan where temperature as high as 500C may be recorded.
- The maximum summer temperatures are comparatively lower in the southern parts of the country due to moderating effect of the sea.
- Because of the heating of the subcontinent the equatorial through moves northwards and lies at 250 N.
- Under such conditions hot dust laden strong winds blow over most part of North India, known as ‘Loo’.
- The strong dust storms resulting from the convective phenomena ( due to intense heating) are locally called ‘Aandhi’ in UP’, ‘Norwester’ in eastern India & ‘Kalbaishakhi’ in west Bengal. These storms bring some amount ;cherry blossoms’ in Karnataka ( suitable for coffee plantation). Elsewhere in south India this rainfall is called ‘Mango-Showers’.
- The southern parts of India do not experience any hot weather season as such.
- May is the hottest month in south India, while June is the hottest month in north west India.
The west Season / South west Monsoon Season ( June-September)
- During the summer months, differential heating of land and sea cause the monsoon winds to drift towards the subcontinent. During April and May, the sun shines vertically over the Tropic of Cancer and the large landmass in the north of the Indian ocean gets intensely heated which results in the formation of intense low pressure in the north-western part of the subcontinent.
- Due to high pressure at the Indian ocean the water here heats slowly as compared to the large landmass. Thus the low-pressure region attracts the south-east trade winds across the equator towards the landmass.
- The ITCZ therefore shifts northwards. At the same time withdrawal of the westerly jet stream from its position over the north Indian plain, south of the Himalayas. The south-west monsoon is thus the continuation of the south-east trades deflected towards the Indian subcontinent after crossing the equator.
- The easterly jet stream responsible for the burst of monsoon in India, sets in along 15°N latitude once the western jet stream has withdrawn itself from the region.
- A primary characteristic of monsoon is the seasonal reversal of winds. It is the monsoon that gives Indian climate a uniqueness.
- 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.
Arabian sea branch
- The Arabian sea branch contributes 65% of total humidity brought by the monsoon.
- The Indian subcontinent receives bulk of its rainfall ( about 78% ) during the south west monsoon period.
- The Arabian sea current of monsoon causes rainfall along the west coast, western Ghat, Maharashtra , Gujarat and parts of Madhya Pradesh and Rajasthan.
Bay of Bengal branch
- The Bay of Bengal branch of monsoon enters the Ganga plain after being deflected westwards by the Arakanyoma mountains.
- The Bay of Bengal branch is entrapped 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).
In general the distribution pattern of annual rainfall in climate of India shows two main trends :
1. It steadily declines towards the west and north-west from Bengal and Odhisha coasts and ,
2. From the west and the east coasts it exhibits a declining trend towards the interior parts of the peninsula.
- The weather and rain during this season are also affected by a number of cyclonic depressions which enter the country through Bay of Bengal and the Arabian Sea. About 20 to 25 such depression are developed during monsoon period. Of these some are very strong causing immense damage to life and property , e.g. super cyclone of Odisha in October, 1999.
- The normal date of onset of the S.W. monsoon is 20th May in Andaman ad Nicobar islands, 1st June on Kerala coast and by 25th July it covers whole of India.
- The east of coast of Indian remains almost dry during this season because it lies in the rain shadow are of the western Ghats and is parallel to the Bay of Bengal branch of monsoon.
- As the Arabian sea branch of monsoon cross the western Ghats, the places situated on the leeward side of the Ghats receive less rainfall.
- Kachchh, Saurashtra and western Rajasthan fail to get adequate rainfall. This is 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.
Retreating Monsoon Season
- By the end of September, the S.W. Monsoon becomes weak as the low pressure trough of the Ganga plain starts moving southwards 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 which is a unique feature of climate of India is much more gradual process than its onset . Starting from September , withdrawal 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 area 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 destructive tropical cyclones . Such cyclones are less frequent in the Arabian Sea.
Climate of India – Climatic Regions
The climatic division of climate of India is based upon Trewartha’s classification scheme, which is a modified form of Koppen’s system and it corresponds with the vegetative , agricultural and geographical regions of India. Main climatic regions of India include:
Tropical Rain Forest ( AW)
- It is found on the west coastal plain, the western Ghats and some parts of Assam .
- It is characterized by high temperature in winter not below 18.20C ; and in summer about 290C. The average rainfall exceeds 200 cm.
Tropical Savanna (AW)
- It is located in peninsular region except the semi-arid zone in the lee side of the Sahyadris. It is characterised by long dry weather throughout winter and early summer and high temperature ( above 18.20C), annual rainfall varies from 76 cm in the west to 150 cm. in the east.
The Tropical Semi-arid Steppe (BS)
- Prevails in the rain shadow belt running southward from central Maharashtra to Tamil Nadu. These regions fall in the lee side of the Sahyadris and Cardamom Hills.
- It is characterized by low rainfall which varies from 38 cm to 80 cm, high temperature between 200-300C.
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 120C ( January) to 350 C ( June)
Tropical Desert (BWh)
The area includes the western parts of Barmer, Jaisalmer and Bikaner district of Rajasthan. A large portion of Kutch peninsula along with Thar desert is also included . It is characterized by Scanty rainfall ( 30 cm average) with few parts receiving 12 cm annual rainfall . Temperature is above 350C.
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 Savanna. It is characterized by rainfall of 63.5 cm to 254 cm, most of it received during the south west monsoon season.
Mountain Climate (H)
The area lies above 6000 meter, sea-level. Examples are the Himalayas and Karakoram ranges. Temperature decreases with altitude . The Trans-Himalayas region particularly Laddakh has a dry and cold climate-what may be called cold desert drought is permanent.