The Indian Summer Monsoon also referred to as the Asian Summer Monsoon or Southwest Monsoon made its first visit to India during the Miocene epoch between 20-35 million years ago. The cause seems to have been the mighty 'uplift' of the Himalayas and the Tibetan Plateau - a process begun several million years earlier when the northward-moving Indian sub-continent collided with the Asian Plate. This collision is estimated to have occurred in the Paleocene epoch about 50 to 60 million years ago. Seabed sediments were analyzed for tiny opaline skeletons called diatoms and single-celled marine plants known as 'radiolaria' (described by Tom Pederson, a University of British Columbia marine scientist, as 'the signature of the monsoon') which are brought to the Ocean surface by the monsoon wind and allowed to sink again when it has passed. Core samples dating the winds indicate that the earliest blew during the Miocene epoch when the Himalayas had achieved a height commanding enough to beckon them in.
(Acknowledgement - Extract from 'Chasing the Monsoon' by Mr. Alexander Frater - Penguin Books - Page - 70. For a tourist interested in Cherrapunjee for its rainfall and in the Indian Summer Monsoon, the book is recommended for the passionate chasing of the monsoon.)
Over 70 percent of the annual rainfall over India is recorded during the southwest monsoon. The regions that receive the largest rainfall are along the west coast of India and the northeastern states of Assam, Meghalaya, Arunachal Pradesh and others besides the eastern India state of West Bengal. In these regions, orographic features play an important role, because the moisture laden monsoon winds strike against physical barriers by way of mountains.
In the subsequent pages, we are trying to trace the origin, passage, arrival and withdrawal of the monsoon in as simple language as possible. Those of you who are not interested in getting too technical, may restrict themselves to 'Monsoon Magic at Mesmeric Cherrapunjee'.