There are several theories which explain the origin of monsoon. Air mass theory is one modern theory which explains the mechanism of monsoon. In the summer the sun shines vertically over the Tropic of Cancer and the Inter-Tropical Convergence Zone (ITCZ) shifts northwards. The southwest trade winds across the southern hemisphere cross the equator and start flowing from Southwest to the northeast direction under the influence of Coriolis force. These displaced trade winds are called southwest monsoon when they blow over India subcontinent. In the month of July the ITCZ shifts to 20degree to 25drgree North latitude and is located in Indo-Gangetic plain and the southwest monsoon blow from the Arabian Sea and Bay of Bengal.