Monsoon Winds of India : The south-west and north-east monsoon, are the principal features in the meteorology of India. Temperature differences among the vast Indian Ocean, Bay of Bengal and Arabian Sea on one side and the India subcontinent with the Himalayan wall on the other side form the basis of the monsoons. The Tropic of Cancer passes through the centre of India. Therefore, the sun’s rays fall vertically on it on the 21st June.
During this time the temperature is very high near the Tropic of Cancer and it continues to be so for some time. As a result of this, in North and North-West India there is high temperature and correspondingly low pressure in summer. At this time there is high pressure in the Bay of Bengal, Arabian Sea, and Indian Ocean. So the wind from high pressure over the seas in the south rushes towards low pressure over India. The wind after crossing the equator deflects to the right and blows from the south-west. It is known as the South-West Monsoon.
The wind divides into two branches. One part is over the Arabian Sea and the other over the Bay of Bengal. The Arabian Sea branch causes heavy orographic rain on the western slopes of the Western Ghats.
As it crosses the Western Ghats and enters the Deccan, due to decrease in water vapor in the wind, rainfall decreases in the rain shadow Deccan plateau. It then passes on to the Eastern Ghats, central and northern India and there the rainfall increases to some extent. A small part flows straight to the Aravallis and so there is orographic rain on the southern side of the hills. But in Rajasthan it produces almost no rain as it gets no scope of condensation.
The Bay of Bengal branch enters Bangladesh, West Bengal, and Orissa. The wind goes to the north and north-east up to the foothills of the Himalayas and the plateau of Meghalaya. So there is very heavy orographic rain. Mousinram, a place near Cherrapunji, has recorded the heaviest the world. From there the wind turns to the left and passes over the plains of Northern India. The rainfall gradually decreases to the west. In Punjab, the rainfall is very poor and uncertain. The Arabian Sea branch to the monsoon also passes over these areas. Yet the rainfall in these areas is much less than that in Assam or in Bombay. Rainfall lasts from June to September. In winter, the conditions are reverse. Then there is comparatively high temperature and low pressure in the surrounding seas. So the wind from India goes to the south.
The winter monsoon in India generally blows from north-east. It is, therefore, called the North-East Monsoon. Of course, at places it blows from north or north-west. This wind is of land origin and therefore, contains little moisture. So in winter there is little rainfall in India. It blows over the Bay of Bengal, it picks up moisture. So in winter there is rainfall in Tamil Nadu coast.