- The Climate of India has marked influences on the growth, distribution and development of vegetation types in India.
- Atmospheric and meteorological influences, principally moisture, temperature, wind, atmospheric pressure and evaporation of a region collectively form the climate.
- India lies north of equator between latitudes 6° and 38° N and longitude 68° and 97° 25’ E. The Himalaya mountain is present in its north. India is surrounded on its south, east, and west by Indian Ocean, Bay of Bengal and Arabian Sea respectively.
The country as a whole can be divided into three main parts:
(i) The mountain wall,
(ii) The plains of northern India, and
(iii) The plateau of Peninsular India.
- Climate of India is not homogeneous throughout but it is highly variable.
- The country stands in the tropical and subtropical belts and the climate is greatly modified by oceans and typography of several mountain ranges, as Himalayan ranges mountain ranges of Meghalaya and Arunachal in North east and Western Ghats with straight and Eastern Ghats in South.
- The climate in South and East is more typically tropical rainy, near temperate at high altitudes in the north (Kashmir), and semiarid and arid (dry hot) in the north-western part.
- The temperature is usually high throughout the year in most parts of the country except at high altitude in extreme north where snowfall occurs during winter and temperature goes below 0°C for short period.
- The amount of rainfall varies greatly in different periods of the year and in different parts of the country; some parts receiving above 400 cm annual rainfall and some receiving as little as 20 cm annual rainfall or even no rains at all in some years.
- The distribution of annual rainfall in India is shown in Fig. 11.1. Sir John Elliot was the first man to prepare a detailed climate map of India and later Kendew (1952) CWB. Normand (1937), Stamp (1946) and Waheed Khan (1959) also described the seasons and the climates of India.
The Indian Meteorological Department (IMP) has adopted the following 4 seasons:
I. Cold season (from mid December to February).
2. Hot season (from March to mid of June).
3. Rainy season (from mid June to mid September).
4. Season of retreating monsoons (from mid of September to mid December).
1. Cold season:
- The season starts in December and persists up to February.
- In this season the atmospheric temperature goes very down, chilly winds blow from west or north-west to east or south-east direction, air is dry and fresh and the sky is clear.
- The cyclonic rains are prevalent at the top of Peninsula, foothills of Himalayas and the margins of the plains in the Punjab and extending up to U.P. and southward into Rajasthan, Maharashtra and M.R.
- The winter rain, although light, greatly influences the growth of vegetation because runoff is very little, evaporation rate is very low and a large amount of it is available to the root zone.
2. Hot season:
- In the beginning of March, temperature starts rising and humidity decreases.
- The onset of summer season is indicated by leaf-fall and almost simultaneous appearance of new leaves and blooming in many plants as Mango, Amaltas (Cassia fistula), Gulmohar (Delonix regia) and so on.
- The temperature continues to rise till middle of June when mercury level may touch 45° to 49°C in upper Gangetic plains.
- The high temperature causes decrease in atmospheric pressure in most parts of the country.
- This results in hot winds which blow from south and south-east to north and north-west. These winds carry clouds with them.
3. Rainy season:
- This season starts with the bursting of the advancing monsoon winds during the middle of June or beginning of July.
- The monsoon winds arising in the Indian Ocean and passing over the Bay of Bengal enter the Gangetic plains and produce maximum rains in the north-eastern parts of the country and heavy rains in the plains of Bengal, Bihar, U.P., Orissa, Punjab, part of M.P. Kashmir, etc.
- The amount of rainfall decreases towards west.
- The annual rainfall in these regions ranges between 100 and 200 cm. Parts of Punjab, Rajasthan, Gujarat and Maharashtra receive only scanty rainfalls and a part of Rajasthan west of Aravalli ranges shows arid desert conditions with almost no rains in some years.
- The monsoon winds arising over Arabian Sea produce heavy rains in South India and western coasts and lesser rains in the interior.
- The rain is not continuous throughout the season but it continues up to middle or end of September interrupted by bright or cloudy rainless days.
- The mountain ranges play significant role in the distribution of rainfall. Aravalli ranges are largely responsible for the arid climate in western Rajasthan.
4. Season of retreating monsoons:
- Towards the end of September the monsoon winds start retreating and produce only occasional showers.
- The sky becomes clear. The temperature decreases gradually and in the mid December cold sets in.
- Due to sudden variations in temperature local cyclones develop in the region of Bay of Bengal and the east coast of Tamil Nadu and cause isolated rains in certain areas.
Climatic Regions of India:
On the basis of rainfall climatic conditions in different physiography determine the geographic and not temperature called climate regions. India can be roughly divided into the following four climatic zones; wet zone, Intermediate zone, Dry zone and arid zone.
1. Wet zone:
- The average annual rainfall in this zone is more than 200 cm.
- Wet zone comprises western Ghats including western coastal part of Mumbai, Karnataka, Kerala, Assam, eastern half of Bengal, Tarai region of U.R, Orissa, Chhattisgarh and North Bihar.
- Natural vegetation includes normally evergreen and semi-evergreen tropical forests in the west coasts and Assam but dry deciduous forests (mostly Shorea robusta) in other parts.
- Paddy is the main agricultural crop.
2. Intermediate zone:
- The mean annual rainfall in this zone is above 100 cm but less than 200 cm.
- The zone comprises south-west Bengal, Bihar, Orissa, Part of M.P., east U.R, north-eastern Andhra Pradesh, eastern slopes of Western Ghats and east Tamil Nadu.
- Natural vegetation includes dry deciduous forests dry deciduous thorn forests and scrubs. Rice is the major agricultural crop.
3. Dry zone:
- This zone receives annual rainfall between 50 and 100 cm and includes western U.R, N-E. Punjab, Delhi, S-E. Mumbai, Eastern part of Rajasthan, and western half of Andhra Pradesh.
- Natural vegetation includes dry deciduous forests, shrubs and thorn scrubs. Wheat and millets are the major agricultural crops.
4. Arid zone:
- This zone receives rainfall less than 50 cm in a year and includes south-western parts of Rajasthan and Punjab, N-E. Mumbai and N-W. Gujarat.
- Natural vegetation includes thorny scrubs and short-lived herbs.