Central Idea

• India Meteorological Department (IMD) has announced withdrawal of the monsoon.

What is Monsoon Withdrawal/Retreat?

• In India, retreating monsoon is the withdrawal of south-west monsoon winds from North India.
• The withdrawal is gradual and takes about three months.
• With the retreat of the monsoons, the clouds disappear and the sky becomes clear. The day temperature starts falling steeply.
• Monsoon rains weaken all over India except few southeastern states.
• It is helpful in Rabi crop cultivation.

Factors affecting the retreat

Two predominant factors cause the phenomenon:

(1) Land topography

• First, the low mountain range in each region runs from north to south, shielding it from west-bound winds that trigger summer monsoon.
• After summer, the range aids in the ‘orographic lift’ or rising of east-bound air mass from a lower to higher elevation, forming clouds and resulting in rain.

(2) Atmospheric convection

• The second factor is atmospheric convection or vertical movement of air.
• As the earth is heated by the sun, different surfaces absorb different amounts of energy and convection may occur where the surface heats up very rapidly.
• As the surface warms, it heats the overlying air, which gradually becomes less dense than the surrounding air and begins to rise.
• This condition is more favorable from September to February because of the role played by sea surface temperature or water temperature.

Immediate factors influencing withdrawal

• The withdrawal of the monsoon is based on meteorological conditions such as-

1. Anti-cyclonic circulation (dry air that is the opposite of a cyclone)
2. Absence of rain in the past five days and
3. Dry weather conditions over the region

When does it occur?

• The monsoon withdrawal is a long-drawn process and extends into mid-October, though the IMD considers September 30 to be the final day of the season over India.
• The rain after that is categorised as “post-monsoon” rainfall.

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