Retreating monsoon

Also called: Post-monsoon season, Withdrawal of the monsoon · Topic: Climate of India and the Monsoon · NCERT: Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 4 "Climate"

Meaning

The retreating monsoon is the slow withdrawal of the southwest monsoon winds from India, from north-west to south-east. It begins in western Rajasthan in September and ends when the monsoon leaves the whole country around 15 October [2]. October–November, the time that follows, is called the post-monsoon season.

It matters because this is when the country splits in two. North India turns dry and gets the hot, sticky October heat. The east coast of the Peninsula gets its rainiest months, and post-monsoon cyclones strike the crowded east-coast deltas.

Explanation

How it happens (cause → process → result)

• The sun moves south:
• After the September equinox (about 23 September), the sun's overhead position moves towards the southern hemisphere.

• The monsoon trough (the low-pressure belt over the Ganga plain that pulls in the moist southwest winds) becomes weaker and also moves south.

• The winds weaken:

• Weaker low pressure pulls in weaker winds.

• By the end of September the monsoon has weakened. Northern India no longer gets moist winds from the sea.

• The result is a slow retreat:

• The monsoon arrives in sudden pushes ("bursts") in June.
• It leaves slowly, from north-west to south-east, over about 3 months.
• The low-pressure area moves south step by step and leaves the Peninsula by about mid-December (NCERT).

How IMD declares withdrawal

• IMD (India Meteorological Department) has used these rules since 2006. It checks them only after 1 September [2]:
• no rain over an area for 5 days in a row [2];
• an anticyclone (a high-pressure system in which winds blow outward) forms in the lower atmosphere, at 850 hPa (about 1.5 km up) and below [2];

• much less moisture, seen in satellite water-vapour images and tephigrams (charts of temperature and humidity at different heights) [2].

• Withdrawal is then declared region by region. IMD checks that the dry area is continuous, that moisture has dropped and that the weather has stayed dry for 5 days [2].

• Class 9 NCERT (Fig. 3.11) shows normal withdrawal dates as isochrones (lines joining places where an event happens on the same date).

The order of retreat

Stage	Time	Area the monsoon leaves / where the low lies
Withdrawal starts	1st week of September (NCERT); about 17 September (IMD revised normal, 2020)	Western Rajasthan
End of September	~30 September	Rajasthan, Gujarat, western Ganga plain, Central Highlands
Early October	1st week of October	The low moves over the northern Bay of Bengal
Mid-October	~15 October	Southwest monsoon leaves the whole country [2]
Early November	1st week of November	The low moves over Karnataka and Tamil Nadu
Mid-December	~15 December	The low leaves the Peninsula (NCERT)

Weather of the post-monsoon season

• October heat is the hot, sticky weather after the rains stop and before winter begins:
• The monsoon leaves → skies become clear → strong sunlight → day temperatures rise again.
• The land is still wet → water evaporates → the air stays humid.

• High heat plus high humidity → the weather feels oppressive (very uncomfortable).

• It ends quickly: temperatures fall fast in the second half of October, mostly in north India. Long nights and clear skies let the land lose heat quickly.

• The northeast monsoon takes over:
• As the ITCZ (Inter Tropical Convergence Zone, the low-pressure belt near the equator where the trade winds of the two hemispheres meet) moves south, dry northeasterly winds blow from land to sea. These are offshore winds.
• They start over the cold, dry land of Central Asia and north India, so they are mostly dry.

• While crossing the Bay of Bengal, they pick up moisture. When they hit the Coromandel coast, the moist air rises and cools, and it rains in October–December.

• Cyclones: cyclonic depressions (areas of low pressure with winds spiralling inward) form over the Andaman Sea and the south-east Bay of Bengal. They move west or north-west and cross the east coast. They give the Coromandel coast most of its rain.

• 2026 example: IMD expects above-normal maximum temperatures over most of India in October 2026. It also expects below-normal rain for the country as a whole, below 85% of the October LPA of 75.4 mm (base period 1971–2020) [5]. LPA (Long Period Average) is the average rainfall over a long base period, used as "normal".

Where it occurs

• Western Rajasthan (north-west India): the monsoon leaves here first, around 17 September (IMD) [2]. It also arrives here last, so western Rajasthan has the shortest rainy season in India.
• Gujarat, the western Ganga plain and the Central Highlands: the monsoon leaves these areas by the end of September.
• Northern Bay of Bengal: the low pressure lies here in early October.
• Karnataka and Tamil Nadu: the low pressure lies here in early November. It leaves the Peninsula by mid-December.
• Coromandel coast (east coast of Tamil Nadu, about 8°N–13.5°N): rain from the northeast monsoon in October–December. Rain also falls on southern Andhra Pradesh, south-east Karnataka and south-east Kerala (NCERT).
• Normal onset of the northeast monsoon over coastal Tamil Nadu and south coastal Andhra Pradesh: 20 October [3][4].
• Tamil Nadu's normal October–December rain: ~443 mm [3].

• Tamil Nadu, Puducherry and Karaikal get ~48% of their annual rain in this season [5].

• Chennai: its climograph (Class 9 NCERT) shows October–November as its wettest months.

• Godavari and Krishna deltas (Andhra Pradesh) and Kaveri delta (Tamil Nadu): the main targets of post-monsoon cyclones. A few cyclones turn north to West Bengal, Bangladesh and Myanmar.
• Arabian Sea: cyclones here have been rarer than in the Bay of Bengal (NCERT). The first post-monsoon extremely severe cyclonic storms formed here in 2014 and 2015 [6].

Don't confuse with

• Onset (advance) of the southwest monsoon: the onset comes in sudden bursts in June. The retreat is slow and gradual and takes about 3 months.
• Northeast (winter) monsoon: the retreating monsoon is the withdrawal of the southwest winds. The northeast monsoon is the new, mostly dry, land-to-sea wind system that replaces them. It brings rain only after it crosses the Bay of Bengal.
• October heat vs summer (pre-monsoon) heat: October heat comes from clear skies plus high humidity from wet land. Pre-monsoon heat in April–June is mostly dry heat. Neither is caused by moist winds from the sea.
• Rain-shadow in summer vs winter rain in Tamil Nadu: in June–September, Tamil Nadu lies in the rain-shadow of the Western Ghats (the dry side of a mountain, away from the wind). So it gets more rain in October–December than in the southwest monsoon [3].

Prelims Hooks

• The southwest monsoon starts to withdraw from western Rajasthan. The normal date is ~17 September (IMD 2020 revised normal); NCERT says the first week of September.
• Order of withdrawal: western Rajasthan → Gujarat / western Ganga plain / Central Highlands → northern Bay of Bengal → Karnataka and Tamil Nadu → leaves the Peninsula.
• IMD's withdrawal rules (since 2006): 5 dry days in a row, an anticyclone at 850 hPa, and less moisture. They apply only after 1 September [2].
• The southwest monsoon leaves the whole country around 15 October [2].
• Trap: the "dry" northeast winds still bring rain to the Coromandel coast. Their moisture comes from the Bay of Bengal, not the Arabian Sea.
• Post-monsoon cyclones from the Andaman Sea hit the Godavari, Krishna and Kaveri deltas. Tamil Nadu, Puducherry and Karaikal get ~48% of their annual rain in October–December [5].

Mains Points

• Why the south-east coast gets winter rain (GS-I):
• The monsoon trough and the ITCZ move south → dry offshore northeast winds blow.
• These winds gain moisture over the Bay of Bengal, and Andaman Sea depressions add more rain on the Coromandel coast.

• Result: Tamil Nadu is in rain-shadow in summer, so its farms, tanks and reservoirs depend on October–December rain (~48% of its annual rain) [5].

• Cyclone risk on the east coast, and a new threat in the west (GS-III):

• Post-monsoon cyclones hit the crowded, low-lying Godavari, Krishna and Kaveri deltas. They bring storm surge, floods and crop loss.
• The Arabian Sea is warming. Its post-monsoon cyclone Genesis Potential Index (a measure of how favourable conditions are for cyclones to form) rose from 1998 to 2021 [7]. Model studies link its extremely severe storms to human-caused warming [6].

• Responses: impact-based forecasts and early warnings from IMD [5], and cyclone shelters and coastal planning on both coasts.

• Rabi season and water planning (GS-I/III):

• Weak post-monsoon rain dries the soil, lowers reservoir and groundwater recharge and raises irrigation demand for rabi crops. One example is the below-normal October 2026 rain expected over most of India [5].
• Responses: contingency crop plans and water conservation [5].

Related concepts

• October heat
• Northeast monsoon

Read more

• The retreating monsoon and the northeast monsoon