Advancing Southwest Monsoon Season
June to September are the months of advancing South West monsoon season. By the end of May the monsoon trough further intensifies over north India due to high temperature in the region. The General direction of the wind during this season is from South-West to north-east.
These moisture laden winds first hit at Andaman and Nicobar Islands in the last week of May and Kerala coast in the first week of June with violent thunder and lightning. This weather continues till September.
The arrival of warm moisture laden winds brings a total change in weather conditions. Their arrival causes sudden rains which bring down the temperatures considerably. The decline in temperature is between 5°C to 10°C.
The peninsular shape of India divides these Southwest monsoons into two branches - Arabian Sea branch and Bay of Bengal branch.
(a) Arabian Sea branch of Southwest monsoons strikes the western coast of India and causes heavy rains on the western slopes of the Western Ghats. After crossing the Western Ghats, these winds cause less rainfall on the eastern slopes as they gain temperature while descending. This area is, therefore, known as rain shadow zone. This explains why interior parts of Maharashtra, Karnataka and Telangana get meagre rains from these winds.
Southwest monsoons striking along the coast of Saurastra and Kuchchh and pass over Rajasthan and beyond to meet the Bay of Bengal branch. These winds cause widespread rain in these states and western Himalayan region.
(b) The Bay of Bengal branch is divided into two sub branches after striking eastern Himalayas. One branch moves towards the east northeast direction and causes heavy rains in Brahmaputra valley and northeast hills of India.
The other branch moves towards northwest along the Ganga valley and the Himalayan ranges causing heavy and widespread rains over vast areas. In this region, the amount of rainfall decreases from east to west owing to the progressive decrease in humidity of these winds.
Main Features of Advancing Southwest Monsoon Season
- Low pressure conditions over northwestern parts of India and high pressure conditions over seas.
- The general wind direction particularly on the Arabian Sea and the Bay of Bengal is southwest to northeast. They cause wide spread rain interspersed with dry spells.
- The onset of monsoons is in the first week of June and withdrawal by the end of September.
- The weather is generally hot and humid during this season.