Climate

India's climate is dominated by the monsoon — perhaps the world's most important seasonal climate phenomenon, sustaining agriculture for over a billion people and shaping every aspect of Indian culture, economy, and planning. Understanding the monsoon mechanism, India's four seasons, regional climate variations, and the impact of ENSO on Indian agriculture is central to UPSC preparation — this chapter bridges physical geography with agriculture, disaster management, and economic planning.

The Indian monsoon accounts for ~75–80% of India's annual rainfall concentrated in June–September. Its failure causes droughts affecting hundreds of millions; its excess causes floods displacing millions annually.

PART 1 — Quick Reference Tables

Table 1: Controls of India's Climate

Table 2: India's Four Seasons

Table 3: Southwest Monsoon — Onset and Withdrawal

Table 4: Regional Climate Variations in India

Table 5: Onset of SW Monsoon — UPSC Key Dates

PART 2 — Detailed Notes

Controls of India's Climate: The Himalayan Role

The Himalayas play a crucial role in shaping India's climate in two ways:

1. Winter: Block the cold, dry winds from Central Asia and Siberia from reaching the Indian subcontinent. Without the Himalayas, India would have much colder winters (like the Tibetan Plateau or Central Asia at similar latitudes). The Himalayas explain why Lhasa (Tibet, 3,600 m, 30°N) gets –17°C in January while Patna (Bihar, 30 m, 25°N) gets just 10°C minimum — not the same extreme cold despite similar latitudes, because the plains are protected.

2. Monsoon: Force the moisture-laden southwest monsoon to rise and precipitate on the southern slopes, giving the sub-Himalayan region heavy rainfall. The Himalayas also trap the monsoon within the subcontinent, preventing it from escaping northward.

The Cold Weather Season (December–February)

During winter, a high-pressure zone develops over northwestern India due to cooling of the landmass. Winds blow from this high-pressure area outward — as northeast trade winds, which are dry (they blow from land to sea, gaining no moisture).

Characteristics:

• Low temperatures: Punjab gets 5–7°C; Delhi ~7°C minimum; northwest plains near 0°C with occasional frost
• Clear skies and low humidity in most of India
• Fog: Dense fog over the Indo-Gangetic Plain (temperature inversion + low wind speeds + moisture from irrigation/rivers)
• Western Disturbances: Extra-tropical Mediterranean cyclones travel eastward along the jet stream, bringing:
  • Light rainfall to Punjab, Haryana, Uttarakhand, UP, Himachal Pradesh
  • Snowfall to the higher Himalayas
  • Critical for rabi (winter) wheat crop
• Northeast monsoon (Tamil Nadu): Northeast winds blow from Bay of Bengal to Tamil Nadu coast, bringing rainfall to Tamil Nadu, southern Andhra Pradesh, and parts of Kerala. Chennai gets ~50% of its rainfall from this.

The Hot Weather Season (March–May)

As the Sun moves northward after the vernal equinox, temperatures rise rapidly over the Indian subcontinent. By May:

• Rajasthan, Pakistan: >45–50°C
• Thermal trough (heat low) intensifies over Thar Desert and northwestern India
• The ITCZ shifts northward toward India, destabilising the atmosphere

Loo: Hot, dry, dusty wind blowing from west to northwest over the plains of northern India during April–June. Can be fatal to livestock and vulnerable people. Temperatures during Loo can reach 45–48°C with very low relative humidity.

Pre-monsoon thunderstorms:

• Nor'Westers (Kalbaisakhi): Violent thunderstorms in West Bengal and Bangladesh, bringing brief relief in April–May; damage mango and banana crops but beneficial for jute
• Mango showers: Pre-monsoon rains in Kerala and Karnataka; help mango ripening
• Blossom showers (Kerala/Karnataka): Critical for coffee flowering

💡 Explainer: Southwest Monsoon Mechanism (Detailed)

The southwest monsoon is driven by the dramatic difference between the rapidly-heating Asian land mass and the slower-warming Indian Ocean.

Step-by-step mechanism:

1. May–June: The Sun is overhead the Tropic of Cancer. The northwest Indian subcontinent (Rajasthan, Pakistan) heats intensely → surface pressure falls → a powerful low-pressure system (thermal trough) develops.

2. ITCZ shift: The Inter-Tropical Convergence Zone (ITCZ) shifts northward from ~5–10°N (its normal equatorial position) to ~25°N over India — drawn by the low pressure. This is the critical shift.

3. Mascarene High: A persistent high-pressure cell in the southern Indian Ocean (~20°S, near Mascarene Islands/Réunion) intensifies in June. SE trade winds from this high blow northward, cross the equator, and are deflected eastward (Coriolis) to become southwesterly winds — the Southwest Monsoon.

4. Jet stream migration: The subtropical westerly jet stream (which lies south of the Himalayas in winter) migrates north of Tibet (~40–45°N) in summer. This removes the upper-level high-pressure ridge that previously blocked southwesterly moisture penetration into India.

5. Upper-level Easterly Jet Stream: Develops over India at ~15°N at 9 km altitude in June–July. Creates upper-level divergence over India, which reinforces the surface low pressure and maintains the monsoon.

6. Two branches enter India:

   • Arabian Sea branch: Hits Western Ghats → heavy rain on windward slopes → crosses Ghats (less moisture) → continues NE into central India
   • Bay of Bengal branch: Moves NE, hits Meghalaya hills → world's highest rainfall → turns west along Ganga plains

7. Breaks in monsoon: The monsoon doesn't rain continuously — there are "break monsoon" periods where rainfall over India temporarily halts for 1–2 weeks while the ITCZ retreats to the Himalayas. During breaks, NE India (hills) gets heavy rain but the plains are dry.

The Retreating Monsoon (Post-Monsoon, October–November)

After September, the low-pressure trough over northwest India weakens as the land cools. The monsoon begins retreating southward:

• The Bay of Bengal becomes the main source of moisture
• Cyclone season peaks in October–November — Bay of Bengal cyclones hit the coasts of Andhra Pradesh, Odisha, Tamil Nadu (Cyclone Fani 2019, Cyclone Amphan 2020)
• Tamil Nadu and parts of southern Andhra Pradesh receive rainfall from the Northeast monsoon (October–December) — these states get ~30–50% of their annual rainfall from this

The retreating monsoon is significant for UPSC because it explains the seasonality of cyclone hazards in India.

🎯 UPSC Connect: ENSO, IOD, and India's Monsoon

ENSO (El Niño Southern Oscillation):

• El Niño (warming of central-eastern Pacific) → weakens Walker Circulation → reduces moisture in Indian Ocean → weaker SW monsoon → drought risk in India
• La Niña → opposite → stronger monsoon → flood risk
• ~50% of El Niño years see deficient Indian monsoon; correlation is probabilistic, not deterministic

IOD (Indian Ocean Dipole):

• Positive IOD (warm western IO, cool eastern IO) → more moisture toward India → tends to strengthen monsoon; can offset El Niño
• Negative IOD → weaker monsoon; compounds El Niño

India's Climate Risk:

• 55% of agricultural area is rainfed (no irrigation)
• A 10% deficiency in monsoon rainfall can reduce agricultural GDP by 1.5–2%
• The Food Corporation of India (FCI) buffer stock policy and import/export policies are explicitly designed to manage monsoon variability

PART 3 — Frameworks & Analysis

India's Rainfall Seasonality: Key Comparison

Comparison: Monsoon and Western Disturbances

Exam Strategy

Prelims Traps:

• Tamil Nadu gets most rainfall from the Northeast monsoon (October–December), NOT the SW monsoon — the Western Ghats create a rainshadow for Tamil Nadu during SW monsoon.
• Loo winds are hot, dry, dusty winds over northern and northwestern India in summer — NOT a monsoon phenomenon.
• Kalbaisakhi (Nor'westers) occur in West Bengal during April–May — pre-monsoon convective storms.
• The SW monsoon first arrives at Kerala (~June 1), NOT Mumbai or Delhi.
• El Niño typically weakens the SW monsoon (but not always). The IOD can partially offset El Niño.
• Monsoon withdrawal begins from the northwest (Rajasthan) first, completing in December.

Mains Frameworks:

• Monsoon mechanism: 6-step sequence (thermal trough → ITCZ shift → Mascarene High → jet stream migration → easterly jet → two branches).
• Climate and agriculture: monsoon variability → kharif crop risk → food security → government response (MSP, buffer stocks, insurance).
• Climate change and India: monsoon changing character (more intense events, longer dry spells) → NAPCC + missions.

Practice Questions

1. UPSC Prelims 2021: Which state of India receives rainfall primarily from the Northeast monsoon? (Tamil Nadu)
2. UPSC Prelims 2019: The phenomenon known as "Loo" refers to: (Hot, dry, dusty winds over north and northwest India in summer)
3. UPSC Mains GS1 2017: Discuss the factors that explain the uneven spatial distribution of rainfall in India.
4. UPSC Mains GS1 2020: Explain the mechanism of the Indian monsoon and discuss how climate change is altering its behaviour.