Monsoon comes from an Arabic word 'MAUSAM' which means season. Thus mansoon are seasonal winds which reverse their direction of flow with the change of season. They flow from sea to land during the summer and from land to sea during winter.
The theories regarding the monsoons are generally divided into following two broad categories :
1. Classical theory: Halley explained the monsoon as resulting from thermal contrast between between continents and oceans due to their different differential heating. In summer the sun shines vertically over the Tropic of Cancer resulting in high temperature and low pressure in Central Asia while the pressure is still sufficiently high for Arabian sea and Bay of Bengal. This induces air flow from sea to land and brings heavy rainfall to India and her neighbouring countries.
In winter the sun shines vertically over the Tropic of Capricorn the North Western part of India grows colder than Arabian sea and Bay of Bengal and the flow of the monsoon is reversed. it was lacking in the physical ingredient of the effect of rotation of the earth.
2. Modern Theory :
(a) Role of ITCZ : According of FEOHN monsoon is only the normal seasonal Migration of planetary winds following the sun. According to him the existence of Asian monsoon is not due to the contrast between land and sea but mainly due to the annual Migration of thermally produced planetary winds and pressure belts under Continental influence. the southeast trade winds of the southern hemisphere cross the equator and start flowing from Southwest to the north east direction under the effect of coriolis force.
