📌 CDS Focus 2022–2026:Extremely high weightage. The exam tests the mechanism of Indian monsoon (ITCZ, Jet streams, onset, retreat), 4 seasons of India, Western Disturbances (Rabi crop connection), local winds (Loo, Nor’westers, Mango showers), factors influencing climate, distribution of rainfall, and Koppen’s classification. Get the monsoon mechanism steps exactly right.
1. Factors Influencing India’s Climate
🌐 Geographical Factors
Latitude: India spans 8°–37°N; tropical & sub-tropical. Tropic of Cancer (23.5°N) divides India.
Altitude: Himalayas block cold Central Asian winds; Western Ghats trigger orographic rainfall
Himalayas: Act as a barrier — keep cold Arctic air out; keep monsoon rain in. Without Himalayas, India would be a desert
Indian Ocean position: Three sides surrounded by sea; moderates temperature
Distance from sea: Interiors (Rajasthan, MP) have continental climate (extreme temperatures)
🌬 Atmospheric Factors
Jet Streams: Sub-tropical westerly jet stream in winter over India keeps it warm; Tibetan anticyclone (summer) pushes jet north, allowing SW monsoon
ITCZ (Inter-Tropical Convergence Zone): Shifts north in summer → triggers monsoon onset
El Niño: Weakens Indian monsoon → drought years
La Niña: Strengthens monsoon → above-normal rainfall
Western Disturbances: Extra-tropical cyclones from Mediterranean; cause winter rains in NW India (Punjab, Haryana) → critical for Rabi crops (wheat)
MONSOON MECHANISM
2. Mechanism of the Indian Monsoon
Fig. 2.1 — SW Monsoon Mechanism: Step-by-Step Process
3. Four Seasons of India
Season
Period
Key Features & CDS Facts
Winter (Cold Weather)
December – February
Western Disturbances bring rain to NW India (Punjab, Haryana, J&K) → critical for Rabi crops (wheat, mustard). Northern plains: fog, frost. South India remains warm. North-East Monsoon gives rain to Tamil Nadu in this period.
Summer (Hot Weather)
March – May
Intense heat; Loo (hot, dry winds) blow in NW India (Rajasthan, Punjab, UP). Dust storms (andhi) in Rajasthan. Nor’westers / Kalbaisakhi in West Bengal & Assam (thunderstorms). Mango showers in Kerala & Karnataka (pre-monsoon showers benefiting mango crop).
SW Monsoon (Rainy)
June – September
Onset: Kerala ~1 June. Covers all India by mid-July. Accounts for 75% of India’s annual rainfall. Arabian Sea branch & Bay of Bengal branch. Western Ghats windward (west face) gets heavy rain; leeward (east face) = rain shadow.
Retreating Monsoon / NE Monsoon
October – November
SW monsoon retreats from NW India. Winds reverse (become NE). Tamil Nadu, SE Andhra get most rainfall during this phase (Bay of Bengal cyclones). October is called “October Heat” (hot & humid as ocean cools). Post-monsoon cyclones form in Bay of Bengal.
Fig. 3.1 — Rainfall Distribution in India: Heavy, Moderate & Scanty Zones
Topic BLocal Winds of India — CDS DirectFrequently Tested
Loo
Hot, dry, dusty wind blowing over NW India (Punjab, Haryana, Rajasthan, western UP) during summer (May–June). Temperatures can reach 45–48°C. Causes dehydration and heat stroke. Blows from W to E during afternoon hours.
Nor’wester / Kalbaisakhi
Violent pre-monsoon thunderstorm in West Bengal, Assam, and Bangladesh (May–June). Blows from NW. Locally called “Kalbaisakhi” in Bengali. Brings relief from heat but causes damage to crops. Beneficial for jute and early rice in Bengal.
Mango Showers
Pre-monsoon showers in Kerala and Karnataka (April–May). Help in the ripening of mangoes. Also help in the growth of coffee (called “coffee blossoms”).
Western Disturbances
Extra-tropical cyclones originating in the Mediterranean Sea, moving eastward across Iran, Afghanistan, Pakistan, and into NW India. Bring winter rains (Dec–Feb) to Punjab, Haryana, HP, J&K. Critical for Rabi crops (wheat, barley, mustard). Also bring snowfall to Himalayas.
⚠️ Climate CDS Traps: (1) Mawsynram (Meghalaya) = wettest place on Earth (not Cherrapunji). Both are in Meghalaya East Khasi Hills. (2) Tamil Nadu gets most rain during the retreating NE monsoon (Oct–Dec), NOT SW monsoon. (3) Western Disturbances → Rabi crops (winter; wheat). SW Monsoon → Kharif crops (summer; rice, cotton). (4) Loo blows in NW India (not South India). (5) Rain shadow of Western Ghats = eastern leeward side = parts of Maharashtra, Karnataka are dry. (6) ITCZ moves north in summer (not south) — this is what triggers the SW monsoon.
📐 Formula Sheet & Key Facts — GC08
Monsoon Onset Dates
Kerala: 1 June (normal) Mumbai: ~10 June Delhi: ~29 June Rajasthan: ~15 July
Monsoon Retreat Dates
NW India: September Mainland India: ~15 October Tamil Nadu coast: November–December 75% of India’s rain from SW Monsoon
Rainfall Zones
Heavy (>200 cm): W. Ghats, NE India Moderate (100–200): E. coast, E. India Scanty (<50 cm): Rajasthan, Ladakh Wettest: Mawsynram, Meghalaya
Key Winds
Loo: Hot, dry; NW India (summer) Nor’wester/Kalbaisakhi: W. Bengal Mango Showers: Kerala, Karnataka W. Disturbances: NW India (winter)
Mawsynram (Meghalaya) has replaced Cherrapunji as the wettest place on Earth based on average annual rainfall records (~11,871 mm/year). Both are in the East Khasi Hills of Meghalaya. Cherrapunji held the record for highest rainfall in a single month. The Bay of Bengal branch of the monsoon is funnelled by the valley geography into this region. Agumbe (Karnataka) is the wettest place in South India.
Q2. The state which receives maximum rainfall from the North-East (retreating) monsoon is: CDS PYQ
(a) West Bengal(b) Kerala(c) Tamil Nadu(d) Odisha
✔ Answer: (c) Tamil Nadu
Tamil Nadu is the only state in India that receives most of its annual rainfall (about 60%) from the retreating NE monsoon (October–December), NOT from the SW monsoon. As the SW monsoon retreats, winds blow from NE across the Bay of Bengal, picking up moisture and depositing it on the Tamil Nadu coast. This is why Chennai is relatively drier in June–September (SW monsoon) compared to October–December.
Q3. Western Disturbances are associated primarily with: CDS PYQ
(a) SW monsoon rains in South India(b) Pre-monsoon thunderstorms in West Bengal(c) Winter rainfall in NW India, vital for wheat(d) Heavy orographic rainfall on Western Ghats
✔ Answer: (c) Winter rainfall in NW India, vital for wheat
Western Disturbances are extra-tropical cyclones that originate in the Mediterranean/West Asia region and travel eastward, bringing winter rain (December–February) to northwest India — Punjab, Haryana, Himachal Pradesh, J&K, and northern Rajasthan. This winter rainfall is essential for the Rabi crop cycle, especially wheat cultivation. They also bring snowfall to the Himalayas. The pre-monsoon storms in Bengal are Nor’westers (Kalbaisakhi).
Q4. The “Loo” is a local wind associated with: ⚡ Tricky
(a) Hot dry summer winds in NW India(b) Cool sea breeze along western coast(c) Pre-monsoon thunderstorms in Assam(d) Cold winter winds in the Himalayas
✔ Answer: (a) Hot dry summer winds in NW India
The “Loo” is a hot, dry, dusty wind that blows during May–June afternoons across NW India (Rajasthan, Punjab, Haryana, western UP). Temperatures associated with Loo can reach 45–48°C. It causes heat strokes and desiccation. It is equivalent to the Sirocco (N. Africa), Harmattan (W. Africa), and Foehn (Alps) as a hot desert-origin wind.
Q5. The onset of the South-West monsoon over Kerala normally occurs around: CDS PYQ
(a) 1st May(b) 1st June(c) 15th June(d) 1st July
✔ Answer: (b) 1st June
The South-West monsoon normally makes its onset over Kerala around 1 June (with a variation of ±7 days). It then advances progressively northward: reaches Mumbai ~10 June, Delhi ~29 June, and covers the entire country by mid-July. The retreat begins from northwest India in early September and the monsoon fully withdraws from the Indian mainland by ~15 October.
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