According to the thermal concept, during summer the intense heating of land results in the formation of a permanent thermally induced low in the north-western part of the subcontinent. This thermal low extends up to 700 mb level.
Besides, during the high sun period the Intertropical Convergence Zone is displaced to the northern plains of India, sometimes reaching the Himalayas. On the contrary, there are high-pressure cells formed over the extensive areas of Indian Ocean and Arabian Sea. There being winter in Australia, a high pressure cell develops over there also.
During the hot, dry season (April-May) when temperatures rise rapidly and pressures over land decrease, the warm and moist air from over the adjacent seas starts blowing towards the above- mentioned low pressure centre.
However, in the beginning the maritime air masses are drawn only from a short distance. But by the end of May or the first week of June, when the low pressure centre has fully developed, the pressure-gradient is steepened so that even the trade winds from the southern hemisphere are drawn towards the thermal low positioned in the northwestern region of the sub-continent.
The southerly trades on crossing the equator are deflected to their right ‘in accordance with Ferrel’s Law. Now, the originally south-east trade winds become south-westerly blowing towards north-east.
It may be pointed out that with the apparent movement of the mid-day sun towards the north the doldrums also are displaced north of the equator. At the same time the intertropical convergence also shifts to about latitude 30°N.
Southwesterly on-shore winds blowing towards the centre of low pressure over northern India traverse thousands of miles over the warm tropical ocean. They are, therefore, full of moisture and have a great potential for heavy precipitation.
The south-west monsoon, as it is called in this region, is split into two branches by the shape of Peninsular India. They are known as: (a) the Arabian Sea branch, and (b) the Bay of Bengal branch.
Since these moisture-laden summer winds in this region blow from south-west, the windward coasts get the maximum amount of monsoon rainfall. These on-shore winds by themselves are not capable of yielding precipitation which, as a matter of fact, results from a number of rain-making processes.