3.1. Thermal Concept

Halley, a noted astronomer, hypothesized that the primary cause of the annual cycle of the Indian monsoon circulation was the differential heating effects of the land and the sea. According to this concept monsoon are the extended land breeze and sea breeze on a large scale. 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.

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 Inter-tropical Convergence Zone (ITCZ) 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.

Halley’s concept is criticized on following lines:

♤ It fails to explain the intricacies of monsoon such as sudden burst of monsoon, breaks in monsoon, spatial and temporal distribution of monsoon.

♤ The low pressure areas are not stationary. The rainfall is not only convectional but a mix of orographic, cyclonic and convectional rainfall.