Monsoon implies the system of tropical winds is reversed completely between the summer and the winter season. Under this system the winds blow from land to sea in winter and from sea to land in summer.

The different theories of causes of monsoons are as under:

1. Traditional theory: Under this theory monsoons are caused by the differential of heating of land and sea.

2. Flohn theory: The vertical rays of the sun shifts northwards over the Tropic of Cancer in the summer season. The ITCZ also shifts to the north. This results in the northwestern parts of India. This low pressure sucks the air from the Indian ocean towards the Indian land mass in the form of southwest monsoons.

3. Modern theory: According to the origin, the Indian monsoon is influenced by a number of other factors, besides the differential heating of land and sea and the seasonal shifts of pressure and wind belts. One of the most important factors among these is the system of subtropical westerly and tropical easterly Jet streams. The subtropical westerly jet streams blowing over India in winter cause a high pressure over Northern India. It thus intensifies the northeast monsoons. This jet stream shifts northwards beyond India in summer season and tropical easterly jets develop over India in this season. The behaviour of these Jet streams is partly responsible for the variations in the time of onset of southwest monsoons over India.