1. Southwest monsoon season is also known as the rainy season in India. 

2. The onset of Southwest monsoon season takes place over the southern tip of the country’ by the first week of June, advances along the Konkon coast in early June and covers the whole country by 15th July. 

3. Jet stream and ELNino are the two factors that determine the occurrence of Southwest monsoon. 

4. Due to the high temperature over north India creates a low-pressure trough which draws the moisture-laden winds from the Indian Ocean towards the Indian landmass. 

5. The sudden approach of monsoon wind over south India with ‘ lightning and the thunder indicates the onset of south-west monsoon. This is also known as “break or burst of monsoon”.

6. The monsoon winds strike against the southern tip of the Indian landmass and get divided into two branches.

• (a) Arabian sea branch (of Southwest monsoon)
• (b) The Bay of Bengal branch (of Southwest monsoon)

(a) Arabian sea branch of South west monsoon:

• The Arabian sea branch of south west monsoon gives heavy rainfall to the west coast of India as it is located in the wind wards side of the Western Ghats.
• The other part which advances towards north strikes against the Himalayan mountains results in heavy rainfall in north.

As Aravalli mountains lie parallel to the wind direction. Rajasthan and the western parts do not get much rainfall.

(b) Bay of Bengal branch of south west monsoon:

• The Wind from Bay of Bengal moves towards north east India and Myanmar.
• This wind is trapped by Garo, Khasi and Jaintia hills and gives the heaviest rainfall to Meghalaya (at Mawsynram).
• This wind gets deflected towards west.
• When the wind moves from east to west rainfall decreases as it looses its moisture.
• Tamil Nadu receives only a meager rainfall as the state is located on the leeward side.

About 75% of India’s rainfall is from the south west monsoon.