Topic 80: Climate of India – Monsoon, Rainfall Distribution, Climatic Regions

Q: Explain the mechanism of the Southwest Monsoon. How does it divide into two branches?

The **SW Monsoon** is triggered when the ITCZ shifts northward over India in June, attracting moisture-laden Arabian Sea winds to fill the thermal low over NW India/Thar Desert. It splits into: (1) **Arabian Sea Branch** — hits Western Ghats first; brings 2,500–4,000 mm to windward western coast but creates a rain-shadow over the Deccan; (2) **Bay of Bengal Branch** — enters NE India first; causes world's highest rainfall at Mawsynram (11,871 mm); then moves west along Himalayas to water the Ganga plains.

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Paper II

Unit 3

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1. India has a tropical monsoon climate with four distinct seasons: Winter (December–February), Pre-Monsoon/Hot-Dry Summer (March–May), SW Monsoon (June–September), and Retreating Monsoon/NE Monsoon (October–November).
2. The SW Monsoon originates from the Inter-Tropical Convergence Zone (ITCZ) shifting northward over India in June; splits into Arabian Sea Branch (hits Western Ghats first; brings 80% of rain to western India) and Bay of Bengal Branch (enters NE India/Bangladesh first; brings rain to Ganga plains).
3. Mawsynram (Meghalaya) receives the world's highest annual rainfall — 11,871 mm (Cherrapunji/Sohra: 11,777 mm); both in the Khasi Hills funnel-shaped valley. Jaisalmer (Rajasthan) receives the lowest rainfall in India — ~150 mm per year.
4. El Niño — warming of central/eastern equatorial Pacific (every 3–7 years) — weakens the Walker Circulation, suppresses SW monsoon, causing below-normal rainfall in India (drought risk). La Niña (cooling of Pacific) generally enhances SW monsoon, bringing excess rainfall.
5. Western Disturbances (WDs) are mid-latitude cyclonic weather systems originating over the Mediterranean/Atlantic, travelling east along the subtropical jet stream to bring winter rainfall to NW India (Punjab, Haryana, Himachal, Uttarakhand, J&K). They cause 5–10 rainy days per month in winter months (December–February).
6. The Tibetan Plateau acts as a thermal engine driving the monsoon — in summer, the plateau (avg 4,500 m) heats up rapidly, creating a powerful high-pressure heat source aloft; this heating enhances the upper-level anticyclone and intensifies the Arabian Sea branch of the monsoon.
7. India's average annual rainfall is about 1,187 mm — but distribution is highly uneven: the Northeast and west-facing Ghats receive >2,000 mm; the Thar Desert and rain-shadow Deccan receive <250 mm; and the Gangetic plains receive 600–1,200 mm.
8. Monsoon onset and withdrawal: SW Monsoon typically arrives at Kerala (Thiruvananthapuram) on June 1 ± 7 days; reaches Delhi by June 27–July 5; covers entire India by July 15; begins withdrawal from NW India in September; fully retreats by December 1 (replacing with NE monsoon).
9. Northeast Monsoon (NE Monsoon) — October–December; reverse flow of winter winds from land (high pressure) to Bay of Bengal; brings rainfall to Tamil Nadu, southern AP, and Sri Lanka; Tamil Nadu gets 60% of its annual rain from NE monsoon. Chennai receives most rain during October–December (NE monsoon), not SW monsoon.
10. Koppen's climate classification for India: Tropical Monsoon (Am — most of India); Tropical Savanna (Aw — Deccan Peninsula); Semi-arid Steppe (BSh — interior Deccan, Rajasthan); Arid/Hot Desert (BWh — Thar Desert); Humid Subtropical (Cwa — Ganga Plain); Montane (H — Himalayas, high altitudes).
11. Loo is a hot, dry, dusty wind that blows over the Indo-Gangetic Plain during April–June, bringing daytime temperatures to 45–50°C. It is strongest in Rajasthan, UP, and Bihar.
12. India has 29 agro-climatic zones (as per ICAR) and 15 agro-climatic zones (as per Planning Commission); Trewartha's classification identifies 6 climate types; Stamp's classification identifies 4; RPSC typically uses Koppen (6 types) or general regional classification.

CORE Key Points at a Glance

1
India has a tropical monsoon climate with four distinct seasons: Winter (December–February), Pre-Monsoon/Hot-Dry Summer (March–May), SW Monsoon (June–September), and Retreating Monsoon/NE Monsoon (October–November).
2
The SW Monsoon originates from the Inter-Tropical Convergence Zone (ITCZ) shifting northward over India in June; splits into Arabian Sea Branch (hits Western Ghats first; brings 80% of rain to western India) and Bay of Bengal Branch (enters NE India/Bangladesh first; brings rain to Ganga plains).
3
Mawsynram (Meghalaya) receives the world's highest annual rainfall — 11,871 mm (Cherrapunji/Sohra: 11,777 mm); both in the Khasi Hills funnel-shaped valley. Jaisalmer (Rajasthan) receives the lowest rainfall in India — ~150 mm per year.
4
El Niño — warming of central/eastern equatorial Pacific (every 3–7 years) — weakens the Walker Circulation, suppresses SW monsoon, causing below-normal rainfall in India (drought risk). La Niña (cooling of Pacific) generally enhances SW monsoon, bringing excess rainfall.
5
Western Disturbances (WDs) are mid-latitude cyclonic weather systems originating over the Mediterranean/Atlantic, travelling east along the subtropical jet stream to bring winter rainfall to NW India (Punjab, Haryana, Himachal, Uttarakhand, J&K). They cause 5–10 rainy days per month in winter months (December–February).
6
The Tibetan Plateau acts as a thermal engine driving the monsoon — in summer, the plateau (avg 4,500 m) heats up rapidly, creating a powerful high-pressure heat source aloft; this heating enhances the upper-level anticyclone and intensifies the Arabian Sea branch of the monsoon.
7
India's average annual rainfall is about 1,187 mm — but distribution is highly uneven: the Northeast and west-facing Ghats receive >2,000 mm; the Thar Desert and rain-shadow Deccan receive <250 mm; and the Gangetic plains receive 600–1,200 mm.
8
Monsoon onset and withdrawal: SW Monsoon typically arrives at Kerala (Thiruvananthapuram) on June 1 ± 7 days; reaches Delhi by June 27–July 5; covers entire India by July 15; begins withdrawal from NW India in September; fully retreats by December 1 (replacing with NE monsoon).
9
Northeast Monsoon (NE Monsoon) — October–December; reverse flow of winter winds from land (high pressure) to Bay of Bengal; brings rainfall to Tamil Nadu, southern AP, and Sri Lanka; Tamil Nadu gets 60% of its annual rain from NE monsoon. Chennai receives most rain during October–December (NE monsoon), not SW monsoon.
10
Koppen's climate classification for India: Tropical Monsoon (Am — most of India); Tropical Savanna (Aw — Deccan Peninsula); Semi-arid Steppe (BSh — interior Deccan, Rajasthan); Arid/Hot Desert (BWh — Thar Desert); Humid Subtropical (Cwa — Ganga Plain); Montane (H — Himalayas, high altitudes).
11
Loo is a hot, dry, dusty wind that blows over the Indo-Gangetic Plain during April–June, bringing daytime temperatures to 45–50°C. It is strongest in Rajasthan, UP, and Bihar.
12
India has 29 agro-climatic zones (as per ICAR) and 15 agro-climatic zones (as per Planning Commission); Trewartha's classification identifies 6 climate types; Stamp's classification identifies 4; RPSC typically uses Koppen (6 types) or general regional classification.

PREDICTED Predicted RAS Questions

Based on PYQ trends and 2026 syllabus analysis

1 5M Explain the mechanism of the Southwest Monsoon. How does it divide into two branches? 5 marks · 50 words

Model Answer

The SW Monsoon is triggered when the ITCZ shifts northward over India in June, attracting moisture-laden Arabian Sea winds to fill the thermal low over NW India/Thar Desert. It splits into: (1) Arabian Sea Branch — hits Western Ghats first; brings 2,500–4,000 mm to windward western coast but creates a rain-shadow over the Deccan; (2) Bay of Bengal Branch — enters NE India first; causes world's highest rainfall at Mawsynram (11,871 mm); then moves west along Himalayas to water the Ganga plains.

~50 words • 5 marks

CORE Key Points at a Glance

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PREDICTED Predicted RAS Questions