Answer:
- The "thermal theory" proposed by Edmund Halley (1686 AD) is the most established classical theory of the origin of the monsoon. It is based on the differential heating of land and water:
Summer:
- The sun is perpendicular to the Tropic of Cancer. The Indian subcontinent (land) becomes much hotter than the Indian Ocean. This creates a low-pressure center in northwestern India, while high-pressure air prevails over the ocean. Winds blow from the ocean toward the land (the SW monsoon).
Winter:
- he sun moves southward. The land cools quickly (high-pressure air), while ocean remains relatively warm (low-pressure air). Winds blow from the land toward the ocean (the NE monsoon).
This theory considers the monsoon to be solely the result of surface winds. Despite the low pressure in April and May, there is no rainfall. There is no fixed date for the arrival of the monsoon. Also, the amount of rainfall varies every year even with the same pressure gradient.