India's climate is affected by two seasonal winds - the Southwest Monsoon and Northeast 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 a year in the country.
Southwest Monsoon (Summer monsoon):
During the summer, the continent of Asia heats up more than the surrounding ocean due to the differences in the way land and water heat. The warm surface creates a large area of low pressure over north-central Asia and a smaller one over India. The southwest summer monsoon is attracted to India by a low pressure area that's caused by the extreme heat of the Thar Desert and adjoining areas of the northern and central Indian subcontinent. Moisture-laden winds from the Indian Ocean rush in to the subcontinent to fill up the void, but because they can't pass through the Himalaya region, they're forced to rise. The gain in altitude of the clouds results in a drop in temperature, bringing rain. When the southwest monsoon reaches India, it splits into two parts –
1. Arabian Sea Branch
2. Bay of Bengal Branch
The Arabian Sea Branch of the Southwest Monsoon first hits the Western Ghats of the coastal state of Kerala, thus making the area the first state in India to receive rain from the Southwest Monsoon. This branch of the monsoon moves northwards along the Western Ghats with precipitation on coastal areas, west of the Western Ghats. The eastern areas of the Western Ghats do not receive much rain from this monsoon as the wind does not cross the Western Ghats.
The Bay of Bengal Branch of Southwest Monsoon flows over the Bay of Bengal heading towards North-East India and Bengal, picking up more moisture from the Bay of Bengal. The winds arrive at the Eastern Himalayas with large amounts of rain.After the arrival at the Eastern Himalayas, the winds turns towards the west, travelling over the Indo-Gangetic Plain at a rate of roughly 1–2 weeks per state pouring rain all along its way.
Northeast Monsoon (Winter monsoon):
During the winter, the flow of air reverses. The continent cools rapidly forming a large area of high pressure over north central Asia, known as the Siberian High, and a smaller area over India. Now the drier, colder air of the continent blows offshore creating the dry monsoon season. Many parts of southern India receive considerable rain from the northeast monsoon.Though the principal rainy season for interior Karnataka, Kerala and Lakshadweep is the Southwest Monsoon season, rainfall continues till December in this regions.TamilNadu, in particular, typically gets nearly half its annual rain during this monsoon. Parts of West Bengal, Orissa, Andhra Pradesh, Karnataka and North-East India also receive minor precipitation from the northeast monsoon.