Seasons of India - Southwest Monsoon or Rainy Season

• Coriolis force: The invisible force that appears to deflect the wind is the Coriolis force. 
• Burst of Monsoon: The duration of the monsoon is between 100- 120 days from early June to mid-September. Around the time of its arrival, the normal rainfall increases suddenly and continues constantly for several days. This is known as the ‘burst’ of the monsoon.
• Break of Monsoon: A Monsoon Break occurs when there are few or no rains for a few days during the rainy season.

Southwest Monsson or Rainy Season

• The South-West Monsoon season is the rainy season. This season lasts from June to September.
• During the summer months, the monsoon winds are directed towards the subcontinent due to the differential heating of land and sea. During the months of April and May, the large landmass to the north of the Indian Ocean becomes extremely hot. This causes in the formation of an intense low pressure system in the northwestern part of the Indian subcontinent.
• Since the pressure in the ocean to the south of the landmass is high, it attracts the South East (S.E.) Trade Winds that prevail in the southern hemisphere. After crossing the Equator, the S.E. trade winds are deflected to the right by the Coriolis force and reach the west coast as the South-West Monsoon. These winds bring heavy rain, as well as violent thunder and lightning. The sudden and violent onset of rain in the first week of June is known as the Monsoon Burst.
• However, the Break of Monsoon occurs when the SouthWest Monsoon fails to bring rainfall for two or more weeks and there is a dry period during the rainy season. Kerala is the first state to receive monsoon rains and the last to see them recede.
• As a result of the tapering topography of peninsular India the South-West Monsoon winds divides into two branches:
  1. The Arabian Sea Branch; and
  2. The Bay of Bengal Branch
• Rain reduces the temperature, providing some relief from the heat. However, there is relief only as long as it rains, because relative humidity is quite high during July and August. After a week of rain, the temperature in Mumbai drops by 10°C. Nagpur sees a maximum of 45°C in May which drops to 3 1°C in July.

1. The Arabian Sea Branch

The South-West Monsoon winds originating over the Arabian Sea further split into three branches: 

• Its one branch is obstructed by the Western Ghats. These winds rise up the slopes of the Western Ghats, become cool, and bring heavy rains to the windward side of the Sahyadris and Western Coastal Plains. After crossing the Ghats, these winds descend, become heated, and the humidity in the winds decreases. As a result, these winds produce little rainfall east of the Western Ghats, i.e. in the rainshadow area.
• The second branch of the Monsoon winds from the Arabian Sea hits the coast north of Mumbai. These winds blow further down the Narmada and Tapi river valleys, causing significant rainfall in Central India. This branch brings 15cm of rain to the Chota Nagpur Plateau. Following that, it enters the Ganga plains and joins the Bay of Bengal Branch.
• The third branch strikes the Saurashtra Peninsula and the Kutch. After that, it passes over western Rajasthan and along the Aravalis, causing little rain. The Arabian Sea Branch meets the Bay of Bengal Branch in Punjab and Haryana, and together these two branches cause rainfall in the western Himalayas.

2. The Bay of Bengal Branch

• The Bay of Bengal branch is directed towards the coast of Myanmar and part of south-east Bangladesh. However, due to the presence of the Arakan Hills along Myanmar's coast, a large portion of this monsoon is diverted towards the Indian subcontinent. As a result, the monsoons enter West Bengal and Bangladesh from the south and south-east rather than the south-west.
• This branch now splits into two, one running along the Ganga plains and the other along the Brahmaputra valley.
• These branches cause widespread rain in Ganga plains, Brahmaputra valley and Garo and Khasi Hills of Meghalaya. Cherrapunji and Mawsynram, located on the crest of the Khasi Hills' southern range, have the world's highest average annual rainfall.

The distribution of rainfall from the Southwest Monsoons is largely determined by relief or topography. For example, Mahabaleshwar on the windward side of the Western Ghats receives more than 250cm of rainfall, whereas Pune on the leeward side receives less than 70cm. Similarly, the presence of hilly ranges in the northeastern states causes heavy precipitation. Rainfall decreases as one travels from east to west. During this season, Kolkata receives about 120cm of rain, Patna 102cm, Allahabad 91cm, and Delhi 56cm.

During this season, the Tamil Nadu coast remains dry. This is due to the fact that the Tamil Nadu coast runs parallel to the Bay of Bengal branch of the south-west monsoon. It is also located in the rainshadow of the Arabian Sea branch of the south-west monsoon.