Why Monsoon in India Arrive in Summer?

Monsoon is derived from an Arabic word ‘mausin’ which refers to the seasonal reversal of winds. The Arab traders used to sail their ships to India with the help of these winds. Monsoon in India is an annual phenomena and acts as the rhythm to which most socio-cultural and economic phenomenon align.

Definition of Monsoon?

Monsoon is the seasonal reversal of winds in the tropical regions of the world. The wind direction changes from the North-Easterly winds during the winter season to South-Westerly winds in the summer season in northern hemisphere. The monsoon winds extend up to sub-tropical regions of India because of the extreme heating of the Indian subcontinent.

Mechanism of Monsoon

  • Before we study the mechanism of monsoon, we must understand few basic facts.
    Fig. 1: Global Wind Circulation. Source: The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001
    • Firstly, the winds blow from the high pressure regions to the low pressure regions.
    • Secondly, the tropical high pressure belt is located at 23.5° latitude and the equatorial low pressure belt is located at equator. Therefore, the usual direction of winds over India and Indian ocean is from the subtropical high pressure belt to the Equatorial low pressure belt (See Fig. 1). These winds are called trade winds. The zone of convergence of trade winds is called Inter-Tropical Convergence Zone (ITCZ). ITCZ is also known as the thermal equator. The winds do not blow perpendicular to the equator due to Coriolis Force.
    • Thirdly, the position of the Sun in relation to Earth’s latitudes changes due to the inclination of its axis at 23.5° to its orbital plane and revolution of Earth around the Sun. This means that the Sun moves from Tropic of Cancer on 21st June to Tropic of Capricorn on 22nd December and vice-versa.
  • From the above facts, we can conclude that the solar radiation received by Earth’s northern hemisphere increases as we approach 21st June. Thereafter, it decreases as we approach 22nd December. This shifting of the Sun between the tropics leads to seasons on Earth.
  • Coming back to the monsoon, one should understand that the rainfall will occur in India when the winds blow from South-westerly direction i.e. from Indian ocean to Indian sub-continent.

Why Does the Monsoon Arrive in Summer?

There are two primary forces which lead to the arrival of monsoon over India.

  • Firstly, during the summer months when the Sun moves north of Equator i.e. April to June, the Indian subcontinent becomes extremely warm. As a result, the ITCZ also moves towards the Indian subcontinent. The trade winds from Tropic of Capricorn cross the equator and deflect to right due to Coriolis Force and move in north-easterly direction towards ITCZ located over India. (Fig. 2)
  • Secondly, the warm weather and heat waves lead to formation of low pressure belt over the north and north-western India. Since, the water of Arabian Sea and Bay of Bengal is not heated as much as the land area, the water bodies have relatively higher atmospheric pressure. Hence, the winds start to blow from the water bodies towards the land area.

Two Branches of Monsoon

There are two branches of monsoon in India i.e. Arabian Sea Branch and Bay of Bengal Branch.

Fig.2: Monsoon In India
  • The Arabian Branch, blows from southwest to north east and showers rains on the western Ghats regions of India.
  • The Bay of Bengal Branch also blows from southwest to northeast and showers rains on the northeast India.
  • After collision with the Himalayas, the Bay of Bengal branch moves towards the west and northwest India. These winds are uplifted by the Aravalli Ranges and shower rain in northwestern India. The regions west of the Aravalli remain dry. The monsoon causes Orographic rainfall in India.

Click Here for Types of Rainfall.

Winter Monsoon or Returning Monsoon

During the winters in India, the wind direction is reversed. The winds move from the northeastern to southwestern direction. These winds carry moisture from the Bay of Bengal and showers rain on eastern coastal states in India such as Tamil Nadu, Andhra Pradesh and Orissa.

This cycle of reversal of winds continues while all other socio-economic and economic activities revolve around the monsoon.