Advancing Southwest Monsoon Season

It is a rainy season for most parts of India. It starts with the inflow of Southwest monsoons which strike the coast of Kerala normally in the first week of June and cover most of India by mid July. This weather continues till September.

The arrival of these 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 sudden onset of rain is called break of monsoons or the burst of monsoons.

The arrival of these winds may be delayed by a week or two depending upon the pressure conditions over northern plains and over the Indian ocean. 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.

(c) The characteristics of Southwest Monsoons

  1. These winds generally strike the Indian coast in the first week of June, but their arrival and departure may be before time or even it may be delayed.
  2. There may be dry spells in between rainy periods. Such long dry spells may even lead to failure of crops.
  3. At times, these winds skip over certain regions without any cause.
  4. The amount and timing of rainfall and intervening duration of wet and dry spells varies from year to year. This is known as the vagaries of the monsoons.
  5. The spatial distribution of rainfall is uneven - some regions may receive heavy rains while the others will have to be contented with meagre or scanty rains.
  6. Generally, these winds start retreating by the end of September. But, sometimes, their departure may be delayed till October or they may retreat even much earlier.

Summary

The main features of advancing Southwest monsoon season are:

  • 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.