Monsoon the wind system that dominates the climate of South Asia and the area around the indian ocean with seasonal reversals of direction caused by the differential heating and cooling of landmass and oceans between summer and winter. The wind blows from the northeast (towards the sea) in winter (the dry-monsoon) and from the southwest (towards the land) in summer (the wet-monsoon). The word 'monsoon' is derived from the Arabic word 'mawsim' which means seasons. The monsoon cycle is believed to have started about 12 million years ago with the uplift of the himalayas.

During the summer season, a centre of low pressure develops over the western part of India because of the intense heating of the landmass, while high pressure develops over the relatively cooler Indian Ocean. This pressure difference causes the winds to flow from the high-pressure area to the low-pressure area, which is from ocean to land in this case. This flow pattern is known as the summer monsoon circulation. As a result, the summer monsoon winds bring in enormous amount of moisture, causing heavy rainfall in the subcontinent, especially in Bangladesh and the neighbouring states of India.

The summer monsoon flow of the Indian Ocean has two branches - the Arabian Sea branch and the bay of bengal branch. The Arabian Sea branch dominates the weather scenes in central and peninsular India, while the Bay of Bengal branch dominates the scenes in Bangladesh, northeastern India, Gangetic Plain and the southern slopes of the Himalayas. It enters Bangladesh in late May or early June, and continues to flow toward the centre of low pressure that lies over the west-central part of India.

During the winter season, the landmass of India cools at a faster rate than the adjacent watermass (ie, the Indian Ocean). Consequently, a centre of high pressure develops over the northwestern part of India, while low pressure develops over the relatively warmer Indian Ocean. As a result of this pressure distribution, winds flow from the interior of India toward the Indian Ocean, which is known as the winter monsoon circulation. A stream of this circulation flows down the Gangetic Plain toward Bangladesh, and then to the Bay of Bengal. Since the wind in this season is characterised by the land-to-ocean flow, nearly dry conditions prevail during this season with a meagre rainfall distribution. [Rafique Ahmed]

See also barsha; climate; season.