- India has a tropical monsoon climate: seasonal reversal of winds brings most of the year's rain between June and September.
- The south-west monsoon provides about three-quarters of annual rainfall; its long-period average (1971–2020) is about 87 cm.
- The Himalayas block cold Central Asian winds and trap monsoon winds, giving India a warmer climate than other places at the same latitude.
Controls of India's Climate
- Latitude — the Tropic of Cancer splits India into a tropical south and a subtropical north.
- The Himalayas — a barrier to cold winds and a trap for moist winds.
- Distance from the sea — coasts have an equable climate; the interior is continental (e.g., Delhi's hot summers and cold winters).
- Altitude and relief — hill stations are cooler; leeward sides of the Western Ghats lie in a rain shadow.
- Pressure and winds — shift of the ITCZ, jet streams and western disturbances.
The Four Seasons (IMD)
| Season | Months | Main features |
|---|---|---|
| Winter (cold weather) | December–February | Clear skies; western disturbances bring rain and snow to the north-west, which helps rabi crops |
| Summer (hot weather) | March–May | Low pressure over the north-west; the loo blows over the northern plains; thunderstorms in the east |
| South-west monsoon | June–September | Most of the year's rainfall; "breaks" in between wet spells |
| Post-monsoon (retreating) | October–November | "October heat"; cyclones in the Bay of Bengal; north-east monsoon rain on the Tamil Nadu coast |
Kerala onset (normal)June 1
Covers all India (normal)July 8
Withdrawal begins~Sept 17
LPA (1971–2020)~87 cm
Wettest placeMawsynram
El Niño effectWeaker monsoon
How the Monsoon Works
- Differential heating: in summer the land heats faster than the ocean, creating low pressure over north-west India and high pressure over the Indian Ocean.
- ITCZ shift: the Inter-Tropical Convergence Zone moves north to about 20–25°N over the Gangetic plain, forming the monsoon trough. South-east trades cross the Equator, turn right (Coriolis effect) and blow in as south-westerlies.
- Jet streams: the subtropical westerly jet shifts north of the Himalayas in early June, and a tropical easterly jet develops over peninsular India — both linked to the monsoon's sudden "burst".
- Tibetan Plateau: intense summer heating of the plateau strengthens upper-air circulation.
- Ocean–atmosphere links: El Niño (warm eastern Pacific) usually weakens the monsoon; La Niña usually strengthens it; a positive Indian Ocean Dipole (warmer western Indian Ocean) supports good rain. The Madden–Julian Oscillation influences active and break spells.
Onset, Advance and Withdrawal
- The monsoon reaches Kerala around June 1 and, under IMD's revised normal dates (2020), covers the whole country around July 8.
- It arrives in two branches: the Arabian Sea branch (strikes the Western Ghats, very heavy rain on the windward side) and the Bay of Bengal branch (deflected by the Arakan hills towards the north-east and then west along the Ganga plain).
- Withdrawal starts from north-west India around September 17.
- IMD calls seasonal rain "normal" when it is 96–104% of the LPA.
Rainfall Distribution
Highest rainfall falls on the Western Ghats and the north-east: Mawsynram and Sohra (Cherrapunji) in Meghalaya are among the wettest places on Earth because the Khasi hills funnel moist air upward. The lowest rainfall is in western Rajasthan and Ladakh. The Coromandel coast gets most of its rain from the north-east monsoon (October–December).
Local Winds and Storms
| Name | Where | What |
|---|---|---|
| Loo | Northern plains | Hot, dry, dusty afternoon winds in May–June |
| Mango showers | Kerala, coastal Karnataka | Pre-monsoon showers that help mangoes ripen |
| Blossom showers | Kerala, Karnataka (coffee areas) | Make coffee plants flower |
| Kalbaisakhi (Nor'westers) | West Bengal, Assam, Odisha | Violent evening thunderstorms; good for tea, jute and rice; called "Bardoli Chheerha" in Assam |
- Cause-and-effect statements: "El Niño strengthens the monsoon" (false) or "a positive IOD supports monsoon rain" (true).
- Local wind ↔ region: loo (north plains), mango showers (Kerala), kalbaisakhi (Bengal/Assam).
- Why Tamil Nadu is dry in July: it lies in the rain shadow of the Western Ghats for the south-west monsoon.
Q1. Which phenomenon usually weakens the Indian south-west monsoon?
✅ Answer: (c) El Niño
💡 Explanation: El Niño — warming of the eastern Pacific — is usually linked to below-normal monsoon rain.
Q2. Most of the annual rainfall on the Coromandel coast of Tamil Nadu comes from:
✅ Answer: (a) The north-east monsoon
💡 Explanation: Tamil Nadu lies in the rain shadow for the south-west monsoon and gets most rain in October–December.
Q3. Pre-monsoon showers in Kerala that help mangoes ripen are called:
✅ Answer: (d) Mango showers
💡 Explanation: Mango showers occur in April–May along the Kerala and Karnataka coasts.
Q4. Under IMD's revised normal dates, the monsoon covers the whole country around:
✅ Answer: (b) July 8
💡 Explanation: IMD revised the normal date for covering all of India to July 8 in 2020.
Q5. Winter rainfall in north-west India is mainly caused by:
✅ Answer: (a) Western disturbances
💡 Explanation: Western disturbances originating over the Mediterranean bring winter rain that helps wheat.
🔁 Revise in 60 Seconds
- Seasons: winter (Dec–Feb), summer (Mar–May), SW monsoon (Jun–Sep), retreating (Oct–Nov).
- Onset Kerala June 1 → all India July 8 → withdrawal from Sept 17.
- El Niño weakens, La Niña and positive IOD support the monsoon.
- LPA ~87 cm; "normal" = 96–104% of LPA.
📌 Prelims Traps to Avoid
- ✘ Western disturbances come from the Mediterranean, not the Bay of Bengal.
- ✘ The leeward Deccan interior is in a rain shadow, even though the Western Ghats are very wet.
- ✘ "October heat" is due to high temperature and humidity after the monsoon retreats.
How does the Indian monsoon work?
Summer heating creates low pressure over north-west India, pulling the ITCZ north. Moist south-east trades cross the Equator, turn into south-westerlies and bring rain from June to September, aided by shifts in the jet streams.
How does El Niño affect the Indian monsoon?
El Niño, an abnormal warming of the eastern Pacific, usually weakens the south-west monsoon and is associated with below-normal rainfall and droughts in India.