The climate of India may be broadly described as tropical monsoon type. The term “monsoon” is derived from Arabic word ‘mausim’ which means seasonal reversal in the wind direction. The Indian Meteorological Department (IMD) designates four official seasons: 1. winter (From December to early April); 2. Summer or Pre-monsoon season (April to July in north-western India); 3. Monsoon or Rainy season (June-September); 4. Post-monsoon season (October-December). But traditionally, Indian note six seasons, each about two month long. These are spring (Sanskrit- Vasanta), late autumn (Hemanta) and winter (Shishira). These are based on the astronomical division of the 12 months into six parts. The ancient Hindu calendar also reflects these seasons in its arrangement of months.
India’s climate is affected by two seasonal winds — the north-east monsoon and the south-west monsoon. The north-east monsoon commonly known as winter monsoon blows from land to sea whereas south-west monsoon known as summer monsoon blows from sea to land after crossing the Indian Ocean, the Arabian Sea and the Bay of Bengal. The south-west monsoon brings most of the rainfall during the year in the country. It is now possible to make forecast about the monsoon rains successfully with developed models and trained manpower.
1. Tropical location, the presence of the Himalaya and Indian Ocean defines the special characteristics of Indian climate, but, it's broadly the tropical monsoon type. The region to the South of Tropic of Cancer experiences tropical climate, whereas the region of the North of the Tropic of Cancer experiences warm temperate climate.
2. The Northern Himalayan Range separates India from the rest of the India and hence, protects India from the bitterly cold and dry winds of Central Asia during winter. Moreover, it acts as a physical barrier for the rain bearing South-West monsoon.
3. India's large size also has significant impact on its climatic pattern, the areas near to the sea experience maritime climate, whereas the areas away from the sea experience continental climate.
4. Monsoon is the most dominating factor of Indian climate; it is the seasonal wind circulation, which completely reverses their directions of flow with the change in season.
5. The South-West summer monsoon from the Arabian Sea and the Bay of Bengal bring rainfall to the entire country. Besides, the North-East winter monsoon, travelling from land to sea causes rainfall along the Coromandal coast after getting moisture from the Bay of Bengal.
6. The Arabian branch of monsoon is more powerful than the Bay of Bengal branch. The onset of monsoon takes place first in Andaman and Nicobar Island and in Kerala coast in the Mainland of India.
7. Upper air circulation, the jet stream influences the monsoon as well as the climate in India. The easterly jet stream of the summer season helps with the onset of the South-West monsoon. The South branch of the jet stream in the winter season intensifies high pressure centres over the North-Western India.
8. Jet stream theory: It explains that the Westerly jet stream during the winter season bifurcated by the Himalayan ranges and its Southern branch located along the Southern slopes of the Himalayas intensifies the high pressure centres over the North-Western India in winter, but shift to the North with the onset of the monsoon. This periodic movement of the sub-tropical jet stream provides a useful indication of the onset and subsequent withdrawal of the monsoon.
9. The tropical cyclones generated in Arabian" Sea and Bay of Bengal during the South-West monsoon and the retreating monsoon seasons, influence the weather conditions of the Peninsular India. The Eastern coast is more prone to tropical cyclone as the number of depressions originated in the Bay of Bengal is more as compared to that of in the Arabian Sea. In the Western coast, Gujarat and Northern part of the Maharashtra are more affected as compared to the Southern part. But during the period of El-Nino, the Modoki Western coast gets more cyclones than eastern coast in India and during El-Nino period, Eastern coast gets unusually more cyclones.
10. The Western disturbances influence the winter weather conditions over most of the Northern plains and Western Himalayan regions. La Nina, which is the complete reversal of the El Nino, is the harbinger for the heavy monsoon showers in India.
11. El Nino is an occasional narrow warm ocean current appearing along the Peru coast in December replacing the Peru or Humboldt cold ocean current flowing over the region in normal years. The appearance of El-Nino anomaly reverses the thermal condition of the ocean surface (warm condition over the Eastern Pacific, Peru and cold in Western Pacific, Indonesia and also the lower atmospheric wind circulation that is conducive for weak monsoon and characterized with deficient rainfall and drought in India.
12. It has been noticed that whenever the surface pressure is high over the Pacific, the pressure over the Indian Ocean is low and vice-versa; this seesaw pattern of meteorological changes is named 'Southern Oscillation'. There is a close relationship between the appearance of the El-Nino and the negative Southern oscillation and this combined effect is called an ENSO event. According to incidence of rainfall, India is classified into 4 broad categories.
(I) Areas of very high rainfall: Areas receiving 200 cm and above rainfall includes North-East of India and the Western coast from Thiruvananthapuram to Mumbai.
(II) Areas of high rainfall: The areas receiving 100-200 cm includes Eastern slopes of Western Ghats, major parts of Northern Plain and East coasts.
(III) Areas of low rainfall: The areas receiving rainfall between 50-100 cm includes large part of Gujarat, Eastern Rajasthan etc.
(IV) Areas of very low rainfall: Areas receiving rainfall below 50 cm, includes arid and semi-arid areas of Western Rajasthan, Kutch and most of the region of Ladakh, Jammu and Kashmir.
The Monsoon climate is a characteristics property of south and south-east Asia and has influence over the economic activities of these regions. The Indian agriculture is considered a gamble against monsoon because agricultural activities over almost all parts of the India are very much dependent up on the monsoon rainfall. In fact, monsoon is the axis around which the Indian economy revolves.