This also goes by the name of ‘Classical Theory’ presented by Hally in 1686 to explain the origin of the Asiatic monsoon. According to this concept Monsoons arc land and sea breezes on gigantic scale produced by the differential seasonal heating of continental and oceanic areas.
During northern winters (winter solstice) when sun’s rays fall vertically over the Tropic of Capricorn the huge landmass of Asia cools more rapidly than the surrounding oceans with the result that a strong high pressure centre is developed over the continent (near Lake Baikal and Peshawar). On the other hand low pressure centre is formed in the southern Indian Ocean. Hence there is an outflow of air from the high pressure land areas to the low pressure ocean areas resulting into north-east monsoon which is dry and devoid of moisture.
During summer season the temperature and pressure conditions are reversed. At the time of summer solstice the Sun’s noon rays fall vertically over the Tropic of Cancer and the huge landmass of Asia is heated forming low pressure area over the land. Moreover, the pole ward shift of the Intertropical Convergence Zone to a position over southern Asia reinforces the thermally induced low pressure centre. Because of the presence of the Himalayas the low pressure centre is bifurcated into two parts: (a) near Baikal Lake, and (b) near north-west India.
Conversely high pressure centre is developed in the southern Indian Ocean. The surface air flow is, therefore, from the highs over the oceans towards the lows over the heated land. Due to the intensity of the low pressure even wind from the southern part of the Indian Ocean from the neighbourhood of Australia is attracted towards the low. These south-east trade winds while crossing the equator are deflected towards right hand side (Ferret’s law) and become south-west monsoon over the northern Indian Ocean and the Indian subcontinent. Since these are onshore moist winds they produce rainfall wherever obstructed by topographical barriers.
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The thermal concept of monsoon as proposed by Edmund Hallcy was supported by a number of scholars like Anget, Hann, Koppcn and Bycrs. Miller.
One of the protagonists of this view observes, “Whereas monsoon climates appear to be very complex in detail, their fundamental principle, which of land and sea breezes on a large scale, remains simple and straightforward.” But recently a number of doubts have been expressed regarding the reliability of this concept.
1. Modern climatologists express doubt about the thermal origin of high (winter) and low (summer) pressure areas over the land. According to them the winter high is the outcome of the anticyclone conditions prevailing over this region due to the presence of southern jet. Similarly summer lows are the tropical cyclones found along the N.I.T.C. That is why the centre of these lows keeps on moving from east to west with the beginning of summer season (April onward) till it finally stabilises in north-western part of India in the last part of May or first week of June. As the origin of cyclonic and anticyclone centers is associated to the dynamic factors these cannot be termed as thermally induced.
2. Although high temperature and consequent low pressure take the north-western India in their grip since April but no rain starts till the middle of June except in the southern most part of the country. Similarly if monsoon winds have regular track there
should not be any break or interval during the rains. This phenomenon can be rightly explained through cyclonic rainfall which is erratic.
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3. Modern researches have shown that monsoon rains are wholly aerographical instead it is a combination of all the three types: aerographical, cyclonic as well as convectional. This is proved by high variability of the monsoon rains.
4. Instead of two broad seasons (winter and summer) the monsoon region has more (six in India) due to highly variable characteristic of the climate.
5. If monsoons are thermally induced then there should be anti-monsoon circulation in the upper air which is lacking in some of the cases.