The term monsoon has been derived from the Arabic word ‘mausim’ or from the Malayalam word ‘monsin’ meaning season. Thus monsoons are seasonal winds that change their direction of flow with the change of season. The monsoon winds blow from sea to land during summers and from land to sea during the winters.
For Indians, monsoon means rainy season, while the seamen observe it as a system of Trade Winds in the Arabian Sea. To the meteorologists, it is characterised by a complete reversal in the direction of the prevailing winds between January and July over the Indian Subcontinent.
However, the phenomenon of monsoon continues to be a puzzle for the scientists and its origin is still shrouded in mystery. Several attempts have been made to explain the mechanism of the monsoons, but no satisfactory explanation is available till date.
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In AD 1686 Halley, an Englishman was of the view that the monsoons are caused by the differential heating of the land and sea. It causes contrast in the atmospheric pressure, which develops a wind that blows from the sea to the land. He conceived the concept of the summer and winter monsoons. This concept dominated the scene for about three centuries.
The monsoons do not develop equally everywhere and Halley’s thermal concept fails to explain the intricacies of the monsoon. Besides differential heating, the origin and development of the monsoon is also influenced by the shape of the continent, orography and the conditions of air circulation in the upper Troposphere.
Nowadays, Halley’s theory has lost much of its significance, but it has not been out rightly rejected. The modern theories based on air masses and jet stream is becoming more relevant.
The Air Mass Theory:
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The Southeast Trade Winds in the Southern Hemisphere and the Northeast Trade Winds in the Northern Hemisphere meet each other near the equator. The meeting place is called the Inter Tropical Convergence Zone (ITCZ). This is the region of ascending air, maximum clouds and heavy rainfall. The ITCZ shifts north and south of the equator with the change of season.
During the summer season, with the apparent migration of the sun northwards, the ITCZ shifts northwards. Thus, .the Southeast Trade Winds crosses the equator and blow in the southwest to northeast direction under the influence of the Coriolis force. These displaced Trade Winds are called the Southwest Monsoons.
The Jet Stream Theory:
It is the latest theory about the origin and development of the monsoons. Jet stream is a band of fast moving air at a height of about 12 km in the mid-latitudes. The wind speed in the westerly jet stream varies between 200 km and 400 km per hour.