Retreating Monsoon and North-East Winds: October Heat to Tamil Nadu Rains — Exam Notes

In one line: The retreating monsoon (September–November) is the southward withdrawal of the south-west monsoon, which reverses India’s winds to blow from land to sea as north-east trades — producing October heat in the north and the vital October–December rains of Tamil Nadu.

In one line: This page maps the entire withdrawal corridor — IMD dates, the ITCZ shift behind the wind reversal, October heat, Coromandel rainfall, cyclones, and the SW-vs-NE comparison table that examiners love.

What Is the Retreating Monsoon? (Quick Answer)

Read this definition first; everything else in these notes hangs on it. The retreating monsoon, also called the north-east monsoon or the winter monsoon, is the period from roughly mid-September to mid-November when the south-west monsoon withdraws from India. The low-pressure belt over the subcontinent collapses, winds reverse and blow from land to sea as the North-East Trade Winds, North India swelters under “October heat,” and Tamil Nadu — dry all summer — receives its main rainy season from moisture-laden north-east winds crossing the Bay of Bengal.

Timeline of Monsoon Withdrawal

The India Meteorological Department (IMD) has fixed withdrawal norms that examiners test directly:

• Withdrawal begins from extreme north-west India (west Rajasthan) around 17th September.
• It withdraws from most of the country by mid-October (the standard line: withdrawal from the entire country is completed by around 19th October, with the southern peninsula last).
• The line then moves south-eastward, and withdrawal from the southernmost peninsula completes by roughly mid-December.
• The withdrawal is gradual, not sudden — the monsoon “retreats” rather than “breaks,” which is why the season is called the retreating monsoon.

Memorise the three anchor dates: 17 September — start; mid-October — most of India; December — south. Reference: IMD’s monsoon withdrawal norms at mausam.imd.gov.in.

Why the Winds Reverse: ITCZ and Pressure Belt Shift

The reversal is pure mechanics, and examiners phrase the trap as “why do winds blow from land to sea in winter?” Answer it in this chain:

1. The sun’s apparent movement shifts southward after the September equinox, beyond the Tropic of Capricorn by December.
2. The Inter-Tropical Convergence Zone (ITCZ) — which sits over the Ganga plain in July — migrates south, following the sun.
3. The low pressure over north-west India is replaced by a developing high-pressure (anticyclonic) system over the land.
4. Pressure is now higher over land than over the surrounding seas, so winds blow from land to sea.
5. These outgoing winds, guided by the same Coriolis deflection, blow as the North-East (Winter) Monsoon — the Trades of the Northern Hemisphere.

October Heat: Causes and Characteristics

October heat is the most examined weather phenomenon of the season. The conditions:

• Clear skies after monsoon withdrawal let solar radiation pound the land uninterrupted — day temperatures rise sharply.
• But the ground and air still carry moisture deposited by the retreating monsoon, so humidity stays high.
• High temperature + high humidity = oppressive, sultry, exhausting weather — the combination that defines October heat. It is sometimes called the “Indian summer.”
• It is characteristic of North India, especially Delhi and the Gangetic plain, during October–November, before the western disturbances of winter set in.

Exam trap: candidates write “high temperature due to clear skies” alone. That is only half the answer. The humidity half — moisture left behind by the monsoon — is what makes it heat rather than just warmth.

Why Tamil Nadu Gets Rain in October–December

Here is the logic chain examiners want, in order:

1. The north-east winds blow from land to sea — dry and cold over most of India.
2. But while crossing the Bay of Bengal, especially its warm southern half, they pick up moisture.
3. These moisture-laden winds strike the Tamil Nadu coast (the Coromandel coast) and rise against the land, giving rain.
4. The Coromandel coast receives roughly half its annual rainfall in this October–December season — about 48% of Tamil Nadu’s annual total.

So the paradox that examiners love: the same winds that make the rest of India dry give Tamil Nadu its main rains.

Role of the Eastern Ghats and Rain Shadow

This answers the frequently asked reverse question — why is Tamil Nadu dry in June–September?

• During the south-west monsoon, the winds rise against the Western Ghats and dump torrential rain on the windward (western) slope — Kerala, coastal Karnataka, the Konkan.
• By the time the air descends on the eastern side, it is dry — the leeward rain-shadow zone. Tamil Nadu sits leeward of the Western Ghats.
• Hence Tamil Nadu’s rainfall calendar is inverted relative to the rest of India: its main rainy season is the north-east monsoon (October–December), not the south-west monsoon.

Tropical Cyclones During Retreating Monsoon

• The warm Bay of Bengal — sea surface temperatures remain high in October–December — breeds cyclonic depressions and tropical cyclones.
• These systems typically track westward or north-westward, striking the eastern coast of India.
• Odisha, Andhra Pradesh, Tamil Nadu and West Bengal are the most vulnerable states (the Odisha Super Cyclone of 1999 struck in late October — a classic exam fact).
• Arabian Sea cyclones also form in this season but are fewer; the Bay remains the dominant breeding ground.
• Remember: more cyclones hit the east coast than the west coast, and retreating monsoon season + the pre-monsoon season are the two cyclone windows.

Difference Between SW Monsoon and NE Monsoon (Comparison Table)

Read the comparison table first, then the working notes, then attempt the drill at the end. This is the single most examined pair in Indian monsoon geography.

Economic and Agricultural Impact

• The north-east monsoon is the lifeline of Tamil Nadu’s samba paddy crop — the main rice crop of the state, sown with NE monsoon rains and harvested in January–February.
• It fills the reservoirs of the southern peninsula — Mettur, Vaigai, and the Chennai city water supply depend heavily on this season.
• A failed NE monsoon means drought, groundwater stress and drinking-water crises in Chennai and the Cauvery delta.
• On the flip side, cyclones in this season destroy standing crops, fishing fleets and coastal infrastructure across Odisha, Andhra and Tamil Nadu.

Previous Year Questions and Exam Relevance

• UPSC Prelims-style: “The majority of the rainfall on the Coromandel Coast is received from which monsoon?” — Answer: retreating / north-east monsoon.
• UPSC Prelims-style: Questions on why Tamil Nadu remains dry during the south-west monsoon (rain shadow of the Western Ghats).
• SSC CGL/CHSL staple: October heat = high temperature + high humidity after monsoon withdrawal from north India; withdrawal begins around 17 September from west Rajasthan.
• State PSCs (TNPSC, APPSC, OPSC): NE monsoon share of Tamil Nadu rainfall (~48%), samba crop, and east-coast cyclone vulnerability.
• Expect MCQ traps on Western Ghats vs Eastern Ghats as the rain barrier — it is always the Western Ghats that create Tamil Nadu’s rain shadow.

Quick Revision: Key Facts and Mnemonics

• Withdrawal starts: ~17 September, from west Rajasthan.
• Withdrawal from most of India: by mid-October; southern peninsula last.
• “17-10-12” — September 17 start, October complete for the north, December for the far south.
• October heat formula: Clear skies + leftover moisture = high temp × high humidity.
• Tamil Nadu NE monsoon share: ~48% (about half) of annual rainfall.
• Chennai rains in winter: NE winds cross Bay of Bengal → pick moisture → strike Coromandel coast. Mnemonic: “North-East winds Empty on land, Wet after the Bay.”
• Tamil Nadu dry in SW monsoon: leeward of the Western Ghats (rain shadow).
• Cyclone belt: east coast, October–December; Odisha 1999 Super Cyclone is the marker event.
• Samba crop = Tamil Nadu paddy fed by the NE monsoon.

Drill: Cover the table above and reconstruct the SW-vs-NE comparison from memory — direction, period, cause, share, regions. If you can rebuild five of seven rows, you are exam-ready on the retreating monsoon. Read these notes once tonight and once before your exam; the withdrawal dates and the 48% figure are the two marks you must not leak.

Frequently Asked Questions

Q: What is October heat and why is it called so?

October heat is the hot, humid, oppressive weather over North India in October–November, after the monsoon withdraws. Clear skies let solar heating build up, while moisture left by the monsoon keeps humidity high — the combination of high temperature and high humidity makes the weather feel like a second summer, hence the name.

Q: Why does Chennai receive rain in winter while most of India is dry?

The north-east monsoon winds blow from land to sea and are dry over most of India — but while crossing the Bay of Bengal they pick up moisture and strike the Tamil Nadu coast, bringing rain to Chennai and the Coromandel coast in October–December.

Q: When does the monsoon withdraw from India?

As per IMD norms, withdrawal begins from west Rajasthan around 17th September and is completed over most of the country by around mid-October, with the southern peninsula the last to see withdrawal (roughly by December).

Q: What percentage of Tamil Nadu’s rainfall comes from the NE monsoon?

Roughly half — about 48% — of Tamil Nadu’s annual rainfall comes from the north-east monsoon season (October–December), making it the state’s principal rainy season.

Q: Why are cyclones common on the east coast during the retreating monsoon?

The Bay of Bengal stays warm in October–December, and cyclonic depressions that form over it are steered west/north-westward by the prevailing winds towards the eastern coast, making Odisha, Andhra Pradesh and Tamil Nadu highly cyclone-prone in this season.

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