It was Halley who propounded the thermal concept of the origin of Asiatic monsoon in a memoir presented to the Royal Society in 1686. This theory was, however, applied to all continental regions besides Asia.
According to this theory, the monsoons are considered to be gigantic convectional systems produced by differential seasonal heating of continental and oceanic areas. Monsoons are more than just seasonal winds and are also directly thermally induced. Thus, the origin of Asiatic monsoon may be ascribed to the thermal differences between land and sea.
During winter the huge landmass of Asia cools more rapidly than the surrounding oceans with the result that a strong high pressure centre develops over the continent.
On the other hand, the pressure over adjacent oceans is relatively lower. As a consequence the pressure-gradient directed from land to sea. Therefore there is an outflow of air from the continental landmass towards the adjacent oceans so that it brings cold, dry air towards the low latitudes.
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In summer the temperature and pressure conditions are reversed. Now, the huge landmass of Asia heats quickly and develops a strong low pressure centre.
Moreover, the pole-ward shift of the Intertropical Convergence Zone to a position over Southern Asia reinforces the thermally induced low pressure centre. The pressure over the adjacent oceans being high, a sea-to-land pressure gradient is established.
The surface air flow is, therefore, from the highs over the oceans towards the lows over the heated land. The air that is attracted into the centers of low pressure from over the oceans is warm and moist.
It may be mentioned here that the tropical maritime air mass, that is unstable and full of water vapour, cannot yield precipitation by itself. Some other factors such as atmospheric turbulence or landform barrier (mountains or plateaus) which make this unstable and moist air rise are required to bring about rainfall.
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The thermal concept of monsoon as proposed by Halley, and supported by many geographers and meteorologists such as Anget Hann, and Koppen compares it with land and sea breezes on a seasonal scale.
According to their belief, both of these phenomena derive solely from the contrast in temperature which is more aggravated in the case of monsoon simply because of the longer duration of the period.
This view was supported by Byers, an eminent meteorologist, who held that the ideal monsoon is largely due to thermal effect. Miller is also one of the protagonists of this view. He writes that “whereas monsoon climates appear to be very complex in detail, their fundamental principle that of land and sea breezes on a large scale remains simple and straightforward.”