Monsoons are experienced in the tropical areas. June-September is the season for monsoon rains. The rainy season is relatively longer in the eastern regions. The quantity of rainfall keeps on decreasing as the monsoon winds move west. The north-east trade winds that blow during the winter months get replaced as the summer seasons sets in. There is continued low pressure over the north-west region. This attracts trade winds from the Southern Hemisphere. Coming form the Indian ocean, these winds cross over the Bay of Bengal and the Arabian Sea. They are then trapped by air circulation taking place over India. These winds are loaded with moisture and blow at a very fast speed. The rain caused by south-west monsoon in not uniform. The rain of Western Ghasts situated in the direction of the winds get more rainfall than the other side. Similarly, the hilly regions of the north-east get heavy rainfall. From east to west there is a constant decline in the amount of rainfall.
The cyclonic depression of formed at the head of the Bay of Bengal cause uneven and uncertain distribution of rainfall. These depressions move towards the low pressure monsoon trough which is not steadily placed. For a variety of reason, if moves northwards or southwards. When the axis of the trough move close to the Himalayas, there are heavy rains in the Himalayan regions. The plains get dry spells. With the approach of winter, the monsoon trough becomes weaker. There is reversal in the direction of surface winds with the approach of winter. The monsoons now withdraw from the Northern Plains. The shift in the low pressure conditions from north-western plains to the Bay of Bengal causes cyclones and rains in the Coromandel Coast.
