The thermal theory was propounded by the British scholars. Dudley Stamp and Baker have played an important role in it. According to this theory, temperature is the main reason of the origin of monsoons. In summer, the sun rays fall perpendicularly in the Northern Hemisphere. It causes a great low pressure which results in the absence of the north-east trade winds. It is to be noted that the trade winds blow the whole year between 50 and 300 North and South latitudes.
The Southern trade winds which generally blow between 300 and 50 South latitudes, cross the Equator because of the Northern move of the thermal equatorial line. According to Ferrell’s Rule, these winds turn towards their right, i.e., North-East when they come in the Northern Hemisphere and blow on the whole Indian Peninsula. Since these winds come after a long sea journey, they full of water vapours. They bring rainfall in the Indian Peninsula by dividing themselves into two parts. Heavy rainfall occurs on the Western Slopes of the Western Ghats by the Arabian Sea Branch, and in the Andaman and Nicobar Islands and North-East India by the Bay of Bengal branch moves towards the low pressure of the North-West India. The amount of the rainfall keeps on becoming less because of shortage of water vapour when moves from the east to west.
South-East Trade Wind