H. Fohn suggested that monsoon system experienced in tropical Asia is a result of the seasonal changes in the planetary wind system resulting from the seasonal swing of temperature and pressure belts in this region in association with the changes in overhead position of sun. The planetary winds of tropics are trade winds. In the months of March and September, when sun is overhead in equatorial area low pressure belt is created near equator and north-east trade winds of northern hemisphere and south-east trade winds of southern hemisphere converge in this belt of low pressure. This zone is known as Inter tropical Convergence Zone or ITCZ. The ITCZ is associated with the zone of highest temperature and the lowest pressure. It is due to the low pressure here that the Trade Winds of the northern and the southern hemispheres converge here.
During summer solstice sun’s rays are vertical over the Tropic of Cancer. Therefore, all wind and pressure belts of the globe shift towards the north. At this point of time ITCZ shifts northwards and becomes NITCZ (Northern Inter Tropical Convergence Zone). It extends up to 30º N Latitude in South and South-East Asia. The excessive heating of Indian sub-continent further intensifies this process.
According to Flohn, at this point of time the equatorial westerlies of doldrums shift northward and then Eastwards under the influence of Coriolis force and get extended as south- west monsoon winds.
During winter season, due to southward shifting of ITCZ the pressure and wind belts the planetary system of north-east trade winds gets re-established over this region. These are called north-east winter monsoons. Therefore, are generally dry and devoid of rains. But on Tamil Nadu coast, they are on-shore and bring precipitation in winter months.