Formation of Heat Low
As the sun moves northwards across the equator in the northern hemisphere, the continents surrounding the Arabian Sea begin to receive large amounts of heat; not only in the form of radiation from the sun but also as heat emitted from the earth’s surface. Professor Budyko from Russia estimates the flux of heat from the earth’s surface into the atmosphere is the equivalent of 160 watts per square metre in the month of June as compared to 15 watts per square metre in December over the arid zones of northwest India, Pakistan, Saudi Arabia and the middle-eastern countries. As a consequence of this large input of power, a trough of low pressure forms over this region. It extends from Somalia northwards across Arabia into Pakistan and northwest India. Towards the end of May, the heat low is well established and a southwesterly wind spreads northwards over the Arabian Sea, the Bay of Bengal and the Indian sub-continent. The onset of southwesterly winds over the west coast of India is often sudden and is referred as the ‘burst’ of the monsoon.