Chapter 4: Climate

This chapter explores India’s monsoon climate, characterized by seasonal wind reversals and regional variations. It covers climatic controls, factors affecting climate, seasonal patterns, rainfall distribution, and the monsoon’s role as a unifying bond, influencing agriculture, culture, and lifestyles.

1. Climate and Weather

Climate: The average weather conditions over a large area for over 30 years, including temperature, pressure, wind, humidity, and precipitation.

Weather: The short-term state of the atmosphere at a specific time and place, fluctuating daily.

Monsoon Climate: India has a monsoon climate, derived from the Arabic word ‘mausim’ (season), marked by seasonal wind reversals. Common in South and Southeast Asia.

2. Climatic Controls

Six Major Controls:

Latitude: Solar energy decreases from equator to poles, affecting temperature.
Altitude: Temperature decreases with height due to thinner air (e.g., hills are cooler).
Pressure and Winds: Influenced by latitude and altitude, affecting temperature and rainfall.
Distance from Sea (Continentality): Coastal areas have moderate climates; inland areas experience extremes.
Ocean Currents: Warm/cold currents with onshore winds affect coastal climates.
Relief: Mountains block winds or cause precipitation; leeward sides are dry.

3. Factors Affecting India’s Climate

Latitude: Tropic of Cancer divides India into tropical (south) and subtropical (north) zones, blending both climates.

Altitude: Himalayas (6,000 m average) block cold Central Asian winds, ensuring milder winters. Coastal areas (30 m max) are warmer.

Pressure and Winds:

Winter: High pressure north of Himalayas; northeast trade winds blow from land to sea, carrying little moisture, causing dry conditions.
Summer: Low pressure over interior Asia/northwest India; southwest monsoon winds from the Indian Ocean bring heavy rainfall.
Coriolis Force: Earth’s rotation deflects winds right in the Northern Hemisphere (Ferrel’s Law), shaping monsoon patterns.
Other Influences: Western cyclonic disturbances (winter rains, ‘mahawat’) and tropical cyclones (coastal rains).

4. The Seasons

Overview: India’s monsoon climate features four seasons: cold weather (winter), hot weather (summer), advancing monsoon (rainy), and retreating monsoon (transition).

Cold Weather Season (Winter, Mid-November to February):

Temperature: Decreases south to north (Chennai: 24–25°C; Northern Plains: 10–15°C). Warm days, cold nights; frost in north, snowfall in Himalayas.
Winds: Northeast trade winds (land to sea) keep most areas dry, except Tamil Nadu coast (sea to land).
Features: Clear skies, low humidity, feeble winds. Western cyclonic disturbances from Mediterranean bring ‘mahawat’ rains, vital for rabi crops.
Peninsular India: No distinct cold season due to sea’s moderating effect.

Hot Weather Season (Summer, March to May):

Temperature: Rises with sun’s northward shift (Deccan: 38°C in March; Gujarat/MP: 42°C in April; Northwest: 45°C in May). Peninsular India cooler due to oceans.
Pressure: Low-pressure trough from Thar Desert to Chotanagpur by May.
Features: ‘Loo’ (hot, dry winds in north), dust storms (temporary relief), localized thunderstorms (Kaal Baisakhi in West Bengal), pre-monsoon ‘mango showers’ in Kerala/Karnataka.

Advancing Monsoon (Rainy Season, June to September):

Mechanism: Southwest monsoon winds, triggered by intense low pressure, cross equator, bring moisture. Cover India in a month, except extreme northwest.
Rainfall: Western Ghats (over 250 cm), Northeast (Mawsynram, highest rainfall globally). Ganga valley rainfall decreases east to west. Rajasthan/Gujarat get scanty rain.
Features: Monsoon ‘breaks’ (wet/dry spells due to monsoon trough shifts). Trough over plains: good rain; near Himalayas: dry plains, heavy Himalayan rains causing floods.
Uncertainties: Irregular arrival/retreat, variable intensity, causing floods or droughts.

Retreating Monsoon (Transition, October to November):

Mechanism: Monsoon trough weakens, high pressure replaces it. Southwest winds withdraw by early October from Northern Plains.
Weather: Clear skies, high day temperatures, cool nights. ‘October heat’ due to high humidity.
Rainfall: Cyclonic depressions from Bay of Bengal bring heavy rain to eastern coasts (Godavari, Krishna, Kaveri deltas; Odisha, West Bengal). Coromandel Coast gets bulk rainfall.

5. Distribution of Rainfall

High Rainfall: Western coast, Northeast (>400 cm annually, e.g., Mawsynram).

Low Rainfall: Western Rajasthan, Gujarat, Haryana, Punjab, Deccan interior, Leh (<60 cm).

Moderate Rainfall: Rest of India. Snowfall in Himalayas.

Variability: High in low-rainfall areas (Rajasthan, Gujarat), causing droughts; high-rainfall areas prone to floods.

Reasons for Low Rainfall: Rajasthan/Gujarat: Arid due to distance from sea, rain shadow. Deccan interior: Leeward side of Western Ghats. Leh: High altitude, cold desert.

6. Regional Variations

Temperature:

Summer: Rajasthan desert (50°C); Pahalgam, J&K (20°C).
Winter: Drass, J&K (-45°C); Thiruvananthapuram (22°C).
Diurnal Range: Thar Desert (50°C day, 15°C night); minimal in Andaman, Kerala.

Precipitation:

Meghalaya (>400 cm); Ladakh, Rajasthan (<10 cm).
Most areas: June–September rain; Tamil Nadu: October–November (retreating monsoon).

Lifestyle Impacts:

Rajasthan: Thick walls, flat roofs for heat insulation.
Tarai/Goa/Mangalore: Sloping roofs for heavy rain.
Assam: Houses on stilts for flood protection.

7. Analysis of Table I (Climatic Data)

Rainiest Stations: Shillong (225.3 cm), Mumbai (183.4 cm).

Driest Stations: Jodhpur (36.6 cm), Leh (9.2 cm).

Equable Climate: Thiruvananthapuram (stable temperatures, 26–28°C), Chennai (24–33°C).

Extreme Climate: Leh (-2 to 10°C, high altitude), Jodhpur (14–33°C, desert).

Retreating Monsoon Influence: Chennai, Thiruvananthapuram (high October–November rain).

Hottest Stations:

February: Thiruvananthapuram (27.3°C), Chennai (25.7°C).
April: Nagpur (32.7°C), Chennai (30.4°C).
May: Nagpur (35.5°C), Delhi/Jodhpur (33.3°C).
June: Jodhpur (33.9°C), Delhi (33.3°C).

Key Observations:

Thiruvananthapuram/Shillong rainier in June than July: Early monsoon onset in south/northeast.
Mumbai rainier in July than Thiruvananthapuram: Western Ghats trap monsoon; Thiruvananthapuram gets early rain.
Chennai less rainy in southwest monsoon: Lies in rain shadow of Western Ghats; gets retreating monsoon rain.
Shillong rainier than Kolkata: Higher altitude, orographic rain in Khasi Hills.
Kolkata rainier in July: Peak monsoon; Shillong peaks in June due to early onset.
Delhi rainier than Jodhpur: Monsoon reaches Delhi; Jodhpur in arid zone.

8. Monsoon as a Unifying Bond

Role: Monsoons provide a rhythmic seasonal cycle, binding India through water for agriculture, shaping river valleys, and influencing culture (festivals, food).

Himalayas: Protect from cold Central Asian winds, ensuring higher temperatures than similar latitudes.

Peninsular Plateau: Moderate temperatures due to sea proximity.

Uncertainties: Variable rainfall, floods, and droughts are typical, yet unify India’s agricultural and cultural life.

9. Summary Table

Season	Months	Features	Regions Affected
Cold Weather	Mid-Nov to Feb	Cool days, cold nights, mahawat rains, snowfall in Himalayas	Northern India, Himalayas
Hot Weather	Mar to May	High temperatures, loo, dust storms, mango showers	Northwest, Deccan
Advancing Monsoon	Jun to Sep	Heavy rainfall, breaks, floods, droughts	Western Ghats, Northeast
Retreating Monsoon	Oct to Nov	October heat, cyclonic rains, clear skies	Eastern Coast

10. Summary and Key Takeaways

Monsoon Climate: Defined by seasonal wind reversals, with regional variations in temperature and rainfall.

Controls: Latitude, altitude, winds, sea distance, ocean currents, and relief shape India’s climate.

Seasons: Four distinct seasons drive agriculture and lifestyles.

Unifying Bond: Monsoons unify India’s diverse regions through water, agriculture, and culture, despite variability.