The word monsoon is used to connote a seasonal wind which flows with consistency and regularity for one part of the year and blows from the other direction for the other part of the year.
Monsoon rains are generated over different parts of the world due to the difference in response of land and sunlight and the resulting wind movement.
Over land, as a consequence of chemical composition and structure of the soil, only a few centimeters of the soil is heated by sunlight. But as the air has a low heat capacity, the atmosphere gets heated up rapidly.
On the other hand, the sea is warmed up slowly because of its larger heat capacity and also because sunlight penetrates up to 200 m into the water. Hence the air above the oceans remains cooler. The temperature difference between them lies in the range of 5-10 degrees Centigrade.
As a consequence of this, the warmer air over land begins to rise. To compensate this ascending air the denser air from the ocean has to move towards the land. (This is commonly known as sea breeze). This differential heating is the driving force for the monsoon winds.
During the northern summer (summer in the northern hemisphere) the sun is over the Tropic of Cancer (23.5 degrees). In this period the land in the northern hemisphere gets warmed up while the southern hemisphere is cooler as it receives less sunlight.
The land-sea contrast is much more in rectangular region shown in the figure than any other part of the world. Due to differential heating, the cooler moist air from the southern hemisphere starts moving towards the hot northern hemisphere. Once this airflow (called Somali current because it flows along the coast of Somali and Kenya region) crosses the equator the Coriollis force (a force arising due to the rotation of the Earth) turns the air to its right (in the Northern hemisphere). This flows over India and East Asia from the South West direction (hence the name south west monsoon). This moist air when aided by other factors results in abundant rain.
The Tibet Plateau on the Himalayas acts as a potential heat source for this kind of air movement. The lifetime of south-west monsoon is about 100 days which starts early June and ends by mid September. The arrival of monsoon is a gradual process starting with a transition period when the atmospheric characteristics change from a state of extreme dryness to high humidity and slight rainfall. Thunderstorm is a feature of pre-monsoon months of April and May over North-East India.
