Climate and Vegetation | Additional Study Material for UPSC PDF Download

Climate

• The climate of India may be described as tropical monsoon type.
• There are four seasons in Indian climate: 
  • the cold dry season (December-February); 
  • hot weather season (March-May);
  • The south-west monsoon season (June-September).
  • the retreating south-west monsoon season (October-November).
  • The mean temperature in winter is about 24° to 25°C in southern India and about 10° to 15° C in the northern plains.
  • The light and dry winds from this high pressure centre blow outwards generally in the north-easterly direction. These are north-east monsoons. 
• Rainfall from north-east monsoons is confined to the coastal region of Tamil Nadu where these winds become moist by absorbing humidity from Bay of Bengal. 
• Occasionally cyclones from the Mediterranean Sea travel eastward and bring light rainfall in the north-western part of Ganga Plain.
• The period of hot weather season is characterized by a progressive rise in temperature which from 26° C in mid-March goes as high as 45° C in mid-May particularly in the Northern Plain. 
• The southern part of India does not experience such a high temperature (mean temperature around 30°C). 
• The hot air over the northern plain particularly over the north-western part forms a low pressure centre.
• The amount of rainfall in general declines with the increasing distance from the sea. Thus, the amount of rainfall in Calcutta is 199 cm and in Delhi is 56 cm. 
• The wettest parts of India, however, lie on the interior plain on the windward slopes of mountains.
• Thus, the western slope of Western Ghats and southern slope of north-eastern hills receive more than 250 cm of rainfall.
• During the retreating season of monsoons which commences from October, the south-west monsoons begins to decrease rapidly, particularly in Northern India and by December the winter sets in.
• The retreating monsoons are generally dry except on the coast of Tamil Nadu where they cause considerable rainfall in November and December.
• The projecting Indian Peninsula divides the south-west monsoons into two branches: 1. the Arabian Sea branch and 2.  the Bay of Bengal branch. The Arabian Sea branch opens on the East Coast of India about the first week of June and causes rainfall over the western parts of Kerala, Karnataka, Maharashtra, Gujarat and Madhya Pradesh. The Bay of Bengal branch brings rainfall to West Bengal, and the adjoining states, Sub-Himalayan region, and the whole of Northern Plain. 
• With the sole exception of the east coast of Tamil Nadu, every part of India receives its bulk of the annual rainfall from south-west monsoons.
• The amount of rainfall especially from Bay of Bengal branch decreases with the increasing distance from the sea.
• Thus, Calcutta receives a rainfall of 119 cm., Patna 105, Allahabad 76, and Delhi 56.
• However, the wettest regions are those where mountains intercept the south-west monsoons, e.g., the southern slopes of the Himalayas and the western slopes of the Western Ghats.
• There are only two regions which receive rainfall in winter: 
  1.  the north-western part of the Great Plains, and 
  2. the Coromandel Coast. 
• The rainfall in Tamil Nadu or the Coromandel Coast is caused by the north-east monsoons.
• The most important characteristic of Indian Monsoons is the complete reversal in wind direction with the change of seasons. 
• Winds become on-shore (i.e., from the sea) during summer and offshore (ie., from the land) during winter.
• The period of Retreating Monsoons extends between September and November. 
• The south-west monsoons retreat from Punjab by mid-September, the Ganga Delta by late October, and the southern part of India by early November. 
• The weather during the season of retreating monsoons is characterised by high day temperature, pleasant nights, and clear sky.
• North-East Monsoons are the winds blowing out from the land-mass of north-west India into the Indian Ocean from December to February. 
• The direction of these winds is north-westerly down the Ganga valley, northerly in the delta, and north-easterly over the Bay of Bengal. 
• Tamil Nadu coast is such an area in India where north-east monsoons reach from the Bay of Bengal.
• The intermittent rains in April-May, before the monsoon opens in full force in June are called Pre-Monsoon or Mango showers.
• In the dry north-western part of India, the hot winds known as loo are most frequent. Sometimes the violent dust storms arise. 
• Winds of the same kind sometimes arise in West Bengal and the adjoining north-eastern area, but, instead of dust, they bring showers of rain. These thunderstorms are called Kalbaisakhi.

Vegetation

• The major types of forests are: (1) Tropical evergreen forests, (2) Monsoon deciduous forests, (3) Thorny forests, (4) Mountain forests, and (5) Tidal or Mangrove forests.
• Tropical evergreen forests are found on the Western Ghats and the sub-Himalayan regions in the east where the rainfall is 200-300 cm.
• Monsoon deciduous forests are found in the larger part of the Deccan Plateau where the rainfall ranges 100 – 200 cm.
• Thorny forest grow in the desert regions of Rajasthan and South Punjab where rainfall is below 80 cm.
• Mountain forests occur in the Himalayas above 1000 metres and in South India above 1500 metres elevation.
• Tidal forests are found in the estuaries of the Ganga and the Mahanadi.