Thermal origin of monsoons was explained by Sir Edmund Halley in 1686. This is known as the ‘classical theory’ of origin of Asiatic monsoon. According to this concept, monsoons are land and sea breezes on gigantic scale produced by the differential seasonal heating and cooling of continental and oceanic areas.
The sun is vertical over the Tropic of Cancer in summer season of northern hemisphere and the Indian landmass at this time gets heated to a greater extent than the neighbouring sea. This leads to formation of low pressure conditions over the Indian subcontinent in comparison to over Indian Ocean. Therefore, thermally induced pressure gradient is produced from ocean towards Indian sub-continent leading to the onset of south-westerly winds blowing from Indian Ocean towards India. These winds, called southwest monsoon, blowing from sea towards land carry a large amount of moisture and cause copious rainfall over the landmass.
Land also cools faster. Hence, the Indian Ocean is warmer than the Indian subcontinent in winter. This causes the pressure gradient to be reversed towards sea. This altered pressure gradient leads to the onset of winds blowing from northeast to southwest, i.e., winds blowing from Indian subcontinent towards Indian Ocean. This wind system is called the northeast monsoon. Since the winds are at this time blowing from land towards sea, they carry little moisture. The winter season over the Indian landmass thus remains largely dry.
Fig. : Monsoon – the Traditional View
The thermal theory of monsoon treats this system as a system of land and sea breezes operating at sub-continental level. The direction of winds reverses seasonally rather than daily, unlike the true local land and sea breeze.
CRITICISM OF THERMAL THEORY OR CLASSICAL THEORY OF MONSOON
1) This theory visualizes monsoon as regional surface winds only.
2) It fails to explain the uncertain and irregular character of dynamic monsoon.
3) Modern climatologists express doubt about the thermal origin of low (summer) and high (winter) pressure areas over the land (the Indian sub-continent). According to them, the position of low- and high-pressure areas changes suddenly. These sudden changes are not exclusively related to thermal conditions rather to dynamic factors. According to them the winter high is the outcome of the anticyclonic conditions prevailing over Indian sub-continent due to the presence of southerly westerly jet streams. The summer season low pressure areas are also associated with the cyclonic lows.
4) Monsoon rainfall is not wholly orographic rather it is an amalgamation of all the three types: orographic, cyclonic and convectional.