Monsoons refer to a system of winds in the tropical regions under which the direction of winds is reversed completely between the summer and the winter seasons.
Under this system, the winds blow from land to sea in winter and from sea to land in summer.
Therefore, most of the rainfall in the regions influenced by the monsoons is received in the summer season while winter season is generally dry.
According to the traditional belief, the monsoon is caused by the differential heating of land and sea.
Due to a higher temperature over the land in summer, a low pressure area develops over the continents and the winds blow from neighbouring oceans towards the land.
These winds are of maritime origin and hence cause ample rainfall in summer.
On the other hand, the continents become colder than the neighboring oceans in winter.
As a result a high pressure area is developed over the continents.
Therefore, winds blow from land to sea in winter.
These winds, being of continental origin, are dry and do not cause rain.
This traditional theory of monsoon has been criticized by the German meteorologist Flohn.
He argues that the differential heating of land and sea is not enough to cause a seasonal reversal of winds at a global scale.
He has explained the origin of the monsoon on the basis of seasonal shift of the pressure and wind belts under the influence of the shift of the vertical rays of the sun.
According to this theory, as the vertical rays of the sun shift northwards over the Tropic of Cancer in summer season, the Inter-Tropical Convergence Zone (ITCZ) also shifts to north.
This results in the formation of a low pressure area over the northwestern parts of India.
This low pressure is further intensified by the high temperatures in this region.
This low pressure area sucks the air from the Indian Ocean towards the Indian landmass in the form of Southwest monsoons.
In winter season, the ITCZ shifts southwards and a mild high pressure is produced over northern parts of India.
This high pressure is further intensified by the equator ward shift of the sub tropical high pressure belt.
Due to high pressure over northern India, the winds start blowing from northeast as retreating monsoons.
According to recent observations, the origin of 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 causes 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.