Monsoons in India
Monsoons are periodic winds which blow from sea to land for six months in summer and from land to sea for six months in winter. Monsoon winds prevail over India at different seasons.
South-West Monsoons: These are rain-bearing winds which prevail from about the end of May to the end of September. During summer, the sun’s rays fall vertically on the Tropic of Cancer making the Indian plains intensely hot. But the rays of the sun fall obliquely over the Indian Ocean during this period. The land is hotter than the sea, there is, therefore, low pressure over the land and high pressure over the sea. The winds blow from high to low pressure i.e., from the sea to the land, and are therefore wet winds. Because of the rotation of the earth, the monsoon winds blowing over India deflect to the right after crossing the Equator and become south-west winds. These are, therefore, called south-west monsoons.
India depends largely on these rain-bearing south-west winds. These winds give to India about 90% of the total rainfall. During their prevalence, the chief crops cultivated are rice, cotton, tobacco, tea, jawar and bajra.
North-East Monsoons (or Winter Monsoons): During the months of November to January i.e., in winter, the sun’s rays fall vertically on the Tropic of Capricorn. The air over the Indian Ocean during this period thus becomes hot and light and there is low pressure. The sun’s rays fall obliquely on the plains of India during these months with the result that the air over these plains is cold and heavy and there is high pressure. The winds, therefore, blow from plains to the Indian Ocean. While crossing the Equator, they deflect to the left and are known as north-east monsoons.
The North-East Monsoons bring only about 10% of the total rain to India as they are chilly and dry land winds. But the moisture that they pick from the Bay of Bengal, little as it is, is very useful. Wheat, barley, oats, oilseeds and sugarcane are cultivated during this season.