Book: India: Physical Environment | Chapter 4
Climate
1. Learning Objectives
After reading these notes, you will be able to:
1
Explain the difference between weather and climate, and describe the unity and diversity of India’s monsoon climate.2
Identify the six factors that control India’s climate.3
Explain the mechanism, onset and breaks of the Indian Monsoon and the role of ITCZ, jet streams and El-Nino.4
Describe the four meteorological seasons of India and their characteristics.5
Understand the distribution of rainfall in India and the impact of monsoon on economic life and global warming.2. Introduction
Weather is the momentary (short-term) state of the atmosphere — it can change within a day or week. Climate is the average of weather conditions over a long period of time (50 years or more). India has a hot monsoonal climate which is the prevalent climate of South and Southeast Asia. Monsoon means the climate associated with seasonal reversal in the direction of winds.
🌍 Unity and Diversity in the Monsoon Climate
- The monsoon gives India a sense of climatic unity with the rest of Southeast Asia, but there is great regional diversity within India.
- Example of diversity: Climate of Kerala and Tamil Nadu is very different from Uttar Pradesh and Bihar — yet both have monsoon type climate.
- Temperature extremes:
→ Western Rajasthan: up to 55°C in summer; Drass (Ladakh): down to –45°C in winter
→ Thar Desert: 50°C day, drops to 15°–20°C at night
→ Tawang (Arunachal Pradesh): just 19°C on the same June day as Rajasthan’s 50°C - Rainfall extremes:
→ Mawsynram & Cherrapunji (Meghalaya): over 1,080 cm/year
→ Jaisalmer (Rajasthan): barely 9 cm/year - Despite all differences, India’s climate is monsoonal in rhythm and character.
3. Factors Determining the Climate of India
1. 🌐 Latitude
Tropic of Cancer passes through central India. Southern India (south of Tropic of Cancer) = tropical zone → high temperature all year, small temperature range. Northern India = sub-tropical → extreme climate, high daily and annual temperature range.
2. 🏔️ The Himalayan Mountains
Act as an effective climatic divide. Protect India from cold Arctic winds. Also trap monsoon winds, forcing them to shed moisture within the subcontinent.
3. 🌊 Distribution of Land and Water
Water heats/cools slowly; land heats/cools fast. Differential heating creates different air pressure zones → causes seasonal reversal of monsoon winds.
4. 🏖️ Distance from the Sea
Coastal areas → equable (moderate) climate. Interior areas (far from sea) → extreme climate. E.g., Mumbai = moderate; Delhi, Kanpur, Amritsar = extreme seasonal contrast.
5. 🏞️ Altitude
Temperature decreases with height. Same latitude — Agra: 16°C in January; Darjiling: 4°C in January (due to higher altitude).
6. ⛰️ Relief
Windward side receives heavy rainfall. Leeward side is dry (rain-shadow area). Ex: Western Ghats windward = heavy rain; east of Western Ghats = dry (rain-shadow).
4. The Nature of Indian Monsoon
Monsoon is a familiar but complex climatic phenomenon. Despite centuries of study, no single theory fully explains it. A real breakthrough came when it was studied at the global level instead of only regional level.
🗺️ Mind Map — Mechanism of Indian Monsoon
Indian Monsoon — Key Concepts🌡️ ITCZLow pressure zone at equator; shifts northward in summer
💨 Jet StreamsWesterly jet withdraws → Easterly jet sets in → Monsoon bursts
🔵 El-NinoWarm Pacific current; delays/disrupts Indian monsoon
🔴 Differential HeatingLand heats fast → Low pressure → Winds rush in from sea
🌊 Arabian Sea Branch3 sub-branches hit Western Ghats, Central India, NW India
🌊 Bay of Bengal BranchEnters NE India; splits towards Ganga plains and Brahmaputra valley
📌 ITCZ — Inter Tropical Convergence Zone
- ITCZ is a low pressure zone at the equator where trade winds converge and air tends to rise.
- In July, ITCZ shifts to 20°N–25°N (over Gangetic plain) → called the Monsoon Trough → creates thermal low over NW India.
- Due to this shift, SE trade winds of the Southern Hemisphere cross the equator, get deflected by Coriolis force, and blow as SW monsoon.
- In winter, ITCZ moves southward → winds reverse → NE monsoons.
📌 Onset of the Monsoon
- During April–May, sun over Tropic of Cancer → NW India heats intensely → strong low pressure forms.
- High pressure over Indian Ocean → attracts SE trade winds across the equator (between 40°E–60°E).
- These winds become SW monsoon after crossing the equator.
- Westerly jet stream withdraws from north Indian plain → Easterly jet stream sets in along 15°N → responsible for the burst of monsoon.
- SW Monsoon enters India over Kerala coast by 1st June → reaches Mumbai & Kolkata by 10th–13th June → covers entire subcontinent by mid-July.
📌 Break in the Monsoon
- If rain fails for one or more weeks during the SW monsoon period, it is called a Break in the Monsoon.
- In North India: breaks occur when rain-bearing storms are infrequent along the monsoon trough.
- On West Coast: dry spells occur when winds blow parallel to the coast.
⚡ El-Nino and the Indian Monsoon
El-Nino appears once every 3–7 years — it is a warm ocean current off the coast of Peru in the Eastern Pacific. It disturbs equatorial atmospheric circulation, leading to delayed or weak monsoons in India. The word El-Nino means ‘Child Christ’ (appears around Christmas). In 1990–91, a wild El-Nino event delayed monsoon onset by 5–12 days over most of India.5. The Southwest Monsoon Season
🌊 Monsoon Winds of the Arabian Sea (3 Branches)
- Branch 1 — Western Ghats: Winds climb Western Ghats (900–1,200 m) → cool down → windward side (Sahyadri + West Coast) gets 250–400 cm rainfall. After crossing, winds descend, heat up → rain-shadow area east of Western Ghats (Pune, Bengaluru) gets very little rain.
- Branch 2 — Narmada-Tapi Valley: Strikes coast north of Mumbai → moves along Narmada and Tapi valleys → heavy rainfall in Central India. Chotanagpur plateau gets 15 cm. Then merges with Bay of Bengal branch.
- Branch 3 — Saurashtra & Kachchh: Strikes Saurashtra Peninsula → moves over west Rajasthan along Aravalis → scanty rainfall. Joins Bay of Bengal branch in Punjab-Haryana → causes rains in western Himalayas.
🌊 Monsoon Winds of the Bay of Bengal
- Strikes coast of Myanmar and Bangladesh. Arakan Hills deflect it towards India.
- Enters India from south and southeast into West Bengal and Bangladesh (not southwest).
- Splits into two under influence of Himalayas:
→ One branch moves westward along Ganga plains up to Punjab.
→ Other moves up Brahmaputra valley (NE India) → sub-branch hits Garo and Khasi hills of Meghalaya. - Mawsynram (on crest of Khasi hills) = highest average annual rainfall in the world.
❓ Why does Tamil Nadu coast remain DRY during SW Monsoon?
(i) Tamil Nadu coast is parallel to the Bay of Bengal branch → winds don’t hit it directly.(ii) It lies in the rain-shadow area of the Arabian Sea branch (east of Western Ghats).
6. The Rhythm of Seasons
Meteorologists recognise four seasons in India based on climatic conditions. These are different from the traditional Indian six-season calendar (Vasanta, Grishma, Varsha, Sharada, Hemanta, Shishira).
❄️ (i) The Cold Weather Season
Nov – Feb
- Sets in: mid-November in North India. Coldest months: December & January
- Mean temperature below 21°C in northern plains
- Night temperature can go below freezing in Punjab & Rajasthan
- Three reasons for extreme cold in north India: (i) Far from sea → continental climate (ii) Snowfall in nearby Himalayas → cold waves (iii) Cold winds from Caspian Sea & Turkmenistan → frost and fog
- Peninsular India: no well-defined cold season — sea moderates temperature
- Winds: High pressure over NW plains → feeble winds blow outward towards Indian Ocean
- Western Disturbances: Shallow cyclonic depressions from Mediterranean Sea bring winter rainfall in Punjab, Haryana, Delhi, western UP — beneficial for rabi crops
- NE monsoon brings rain to Tamil Nadu coast in Oct–Nov (picking moisture from Bay of Bengal)
🌡️ (ii) The Hot Weather Season
March – May
- Sun moves towards Tropic of Cancer → temperatures rise sharply in North India
- NW India: temperatures up to 48°C in May
- Deccan Plateau: ~38°C in March. Gujarat & MP: 38°C–43°C in April
- South India: milder — 26°C–32°C (moderating influence of ocean)
- ITCZ shifts northward, centred at 25°N in July → monsoon trough from Thar Desert to Chotanagpur
- Loo: Hot, dry, oppressive winds blowing in afternoon in northern plains from Punjab to Bihar
- Dust storms common in May in Punjab, Haryana, Eastern Rajasthan, UP
🌧️ (iii) The Southwest Monsoon Season
June – September
- SW monsoon arrives suddenly — called “burst” or “onset” of monsoon
- Brings down temperature by 5°C–8°C between mid-June and mid-July
- Two branches: Arabian Sea branch and Bay of Bengal branch
- Most parts of India receive rain in June–September
- Average annual rainfall in India: ~125 cm
🍂 (iv) The Retreating Monsoon Season
Oct – Nov
- By end of September, SW monsoon weakens → retreats from western Rajasthan (1st week September)
- Withdraws from Rajasthan, Gujarat, Western Ganga plain, Central Highlands by end of September
- By mid-December, completely removed from Peninsula
- Skies become clear → “October Heat” — hot and humid weather
- North India: dry weather. Eastern Peninsula: rainiest months (Oct–Nov)
- Cyclonic depressions from Andaman Sea → cross eastern coast of Southern Peninsula → cause heavy rain. Target: deltas of Godavari, Krishna, Kaveri
- Most of Coromandal coast rain comes from these cyclones
⛈️ Famous Local Storms of Hot Weather Season
🥭 Mango Shower
Pre-monsoon showers in Kerala & coastal Karnataka. Help in early ripening of mangoes.
🌸 Blossom Shower
Helps coffee flowers blossom in Kerala and nearby areas.
🌪️ Nor’westers (Kalbaisakhi)
Dreaded evening thunderstorms in Bengal & Assam. In Assam called “Bardoisila”. Useful for tea, jute, rice.
🌬️ Loo
Hot, dry, oppressive winds in northern plains (Punjab to Bihar). Most intense between Delhi and Patna.
7. Distribution of Rainfall
Average annual rainfall in India = ~125 cm. But there is great spatial variation — some areas get over 1,000 cm while others get less than 10 cm.
| Zone | Rainfall Amount | Regions |
|---|---|---|
| High Rainfall | Above 200 cm | West coast, Western Ghats, sub-Himalayan NE, Meghalaya hills (Khasi & Jaintia hills can exceed 1,000 cm) |
| Medium Rainfall | 100–200 cm | Southern Gujarat, east Tamil Nadu, Odisha, Jharkhand, Bihar, eastern MP, northern Ganga plain, Manipur |
| Low Rainfall | 50–100 cm | Western UP, Delhi, Haryana, Punjab, J&K, eastern Rajasthan, Gujarat, Deccan Plateau |
| Inadequate Rainfall | Below 50 cm | Parts of AP, Karnataka, Maharashtra, Ladakh, most of western Rajasthan |
📌 Snowfall
Snowfall is restricted to the Himalayan region only. Rest of India receives only rain.💰 Monsoons and the Economic Life in India
- About 64% of India’s population depends on agriculture — agriculture depends on SW monsoon.
- Except the Himalayas, all parts have temperatures above the threshold level to grow crops throughout the year.
- Regional variations in monsoon climate help in growing various types of crops.
- Variability of rainfall brings droughts or floods every year in some parts.
- Winter rainfall by temperate cyclones in north India is highly beneficial for rabi crops.
- Sudden monsoon burst causes soil erosion over large areas.
- Regional climatic variation is reflected in variety of food, clothes, and house types.
8. Global Warming
- Climate is changing globally due to both natural causes and human activities (large-scale industrialisation, burning fossil fuels).
- Greenhouse Gases: CO₂, methane (CH₄), chlorofluorocarbons (CFCs), nitrous oxide (N₂O) — trap long wave radiation → enhance the greenhouse effect.
- Mean annual surface temperature of earth has been increasing for past 150 years.
- By the year 2100, global temperature is projected to rise by about 2°C.
⚠️ Consequences of Global Warming
→ Melting of glaciers and sea ice → rise in sea level→ Sea level predicted to rise 48 cm by end of 21st century → increased annual flooding
→ Spread of insect-borne diseases like malaria
→ Shift in climatic boundaries — some regions wetter, others drier
→ Changes in agricultural patterns and ecosystem
Summary — Quick Revision
1
Weather = short-term atmospheric state. Climate = average over 50+ years. India has a hot monsoonal climate.
2
India has unity (monsoonal) but also great diversity — temperature ranges from –45°C (Drass) to 55°C (Rajasthan).
3
6 factors control India’s climate: Latitude · Himalayan Mountains · Land & Water distribution · Distance from Sea · Altitude · Relief.
4
ITCZ shifts to 20°–25°N in July → monsoon trough → SW monsoon arrives. Easterly jet stream = responsible for burst of monsoon.
5
SW Monsoon enters Kerala by 1 June → Mumbai & Kolkata by 10–13 June → entire India by mid-July.
6
Arabian Sea branch has 3 sub-branches: Western Ghats, Narmada-Tapi valley, Saurashtra-Kachchh. Bay of Bengal branch: enters from south/SE, splits into Ganga plains + Brahmaputra valley.
7
Tamil Nadu is dry in SW monsoon: (i) coast is parallel to Bay of Bengal branch (ii) lies in rain-shadow of Arabian Sea branch.
8
4 seasons: Cold Weather (Nov–Feb) · Hot Weather (Mar–May) · SW Monsoon (Jun–Sep) · Retreating Monsoon (Oct–Nov).
9
Average annual rainfall = ~125 cm. Mawsynram (Meghalaya) = highest rainfall in the world. Jaisalmer = barely 9 cm/year.
10
Global warming → rise of 2°C by 2100. Sea level rise of 48 cm by 21st century end. Causes floods, disease spread, ecosystem change.
Important Terms to Remember
- Weather: Momentary or short-term state of the atmosphere at a particular place and time.
- Climate: Average weather conditions of a place over a long period (50 years or more).
- Monsoon: Climate associated with seasonal reversal in the direction of winds; from Arabic word ‘Mausim’ (season).
- ITCZ (Inter Tropical Convergence Zone): Low pressure zone at the equator where trade winds converge and air rises. Shifts northward in summer → creates monsoon trough over India.
- Jet Streams: Fast-flowing air currents in the upper atmosphere. Westerly jet stream controls western disturbances; its withdrawal allows the easterly jet stream to trigger the monsoon burst.
- El-Nino: Warm ocean current appearing off Peru coast every 3–7 years. Disrupts Indian monsoon — ‘Child Christ’ as it appears near Christmas. Opposite = La-Nina.
- Burst of Monsoon: Sudden onset of rain with violent thunder and lightning when moisture-laden winds hit the coast.
- Break in the Monsoon: Dry spell of one or more weeks during the SW monsoon period.
- Loo: Hot, dry, oppressive wind blowing in northern plains (Punjab to Bihar) during hot weather season.
- Nor’westers (Kalbaisakhi): Dreaded evening thunderstorms in Bengal and Assam. ‘Kalbaisakhi’ means ‘calamity of Baisakh month’. Known as ‘Bardoisila’ in Assam.
- Mango Showers: Pre-monsoon showers in Kerala and coastal Karnataka that help ripen mangoes.
- Western Disturbances: Shallow cyclonic depressions from Mediterranean Sea that bring winter rainfall to NW India (Punjab, Haryana, Delhi) — very beneficial for rabi crops.
- October Heat: Hot and oppressive weather in October-November due to high temperature + humidity when monsoon retreats.
- Rain-shadow Area: Leeward (sheltered) side of a mountain that receives very little rainfall. Example: East of Western Ghats.
- Greenhouse Gases: CO₂, CH₄, CFCs, N₂O — trap long-wave radiation → warm the earth → cause global warming.