The term monsoon was first used in Indian subcontinent to refer to the rainfall from the prevailing seasonal winds. Monsoon rainfall as defined in Hydrology occurs in any region that receives the majority of its rainfall during a particular season. This allows certain regions of North America, South America, Sub-Sahara Africa, Australia and East-Asia as monsoon regions.
The Tibetan Plateau uplifted some 50 million years ago with the collision of India and Asia. As a result the monsoon in India started some 8 million years ago.
The southwest summer monsoon
The southwest summer monsoon occurs from June through September. The Great Indian Desert (Thar Desert) and the northern and central India heats up during summer and cause a low pressure. To fill this void moisture laden winds from the Indian Ocean rush in. They travel all the way to the northern India and are blocked by the Himalayas. The Himalayas prevent the moisture laden winds from going any further and so they rise up. The air cools and precipitation occurs giving the monsoon rains. Some areas in India receive more than 10,000 mm of rains during monsoons.
Because of the shape of Indian Subcontinent, the winds branch out as the south western branch and the south eastern branch.
The south western branch is also known as the Arabian Branch and first hits the Western Ghats. Kerala is the first state to get the monsoon from the Arabian Branch around June 1. This branch brings a lot of rain to the west side of the Western Ghats but not much rain to the east of the Western Ghats.
The south eastern branch flows over the Bay of Bengal and travels over Bengal. They hit the Himalayas and produce rains over the north east India. Mawsynram, located about 16 kilometers from Cherapunjee is one of the wettest places in India. It gets about 12,000 mm of rain. After the winds strike the Himalayas they turn around and produce rains over the Indo-Gangetic plains.
Around September, the land starts to get cooler while the Indian Ocean retains most of its heat. There is low pressure over the ocean and winds blow from the land towards the Indian Ocean. This is known as the retreating monsoon or the North East monsoon. As this dry air flows over the Bay of Bengal, it picks up moisture and rains on the peninsular India. Chennai gets more rain from the North east monsoon than from the Southwest monsoon. Because the winds blow over the land to the sea, the retreating monsoon does not bring as much rain.
The agricultural practices of India are based around the arrival of the monsoons. The village farmers and the city dwellers look forward for the monsoons alike. Indian literature and culture revolves a lot around the monsoons. The monsoons are also a favorite theme in the Indian movies and the rains are glorified in numerous Bollywood songs