Factors that decide India's climate
Climate is the usual weather of a place over many years. For India, these things matter most:
- Latitude: the Tropic of Cancer passes through the middle. The south is in the hot tropics; the north is warm-temperate.
- The Himalayas: a very high wall in the north. It stops icy winds from Central Asia and makes the monsoon wind drop its rain in India.
- Land and water: land heats and cools fast; sea heats and cools slowly. This sets up pressure differences that drive the monsoon.
- Distance from the sea: coasts have a mild (equable) climate. Places far inland, like Delhi, are very hot in summer and cold in winter (extreme or continental climate).
- Altitude: air gets about 6.5 °C colder every 1 km up, so Shimla is cooler than Ambala.
- Relief: hills decide which side gets rain (windward) and which stays dry (leeward).
- Upper air: fast winds high in the sky (jet streams) and far-away seas also matter: a warm Pacific (El Niño) often means a weaker monsoon.
The nature of the monsoon
The word monsoon comes from the Arabic mausim, meaning season. A monsoon is a wind system that reverses its direction with the season.
How the south-west monsoon starts
- In May–June the sun is overhead near the Tropic of Cancer. The north-west plains become very hot and a low-pressure area forms there.
- The belt where the trade winds meet (the ITCZ, inter-tropical convergence zone) moves north over the Ganga plain.
- The south-east trade winds of the southern Indian Ocean cross the equator. The Earth's spin bends them to the right, so they reach India from the south-west.
- They have crossed thousands of kilometres of warm sea, so they are full of moisture.
- A high-level easterly jet stream over the peninsula helps the monsoon to burst.
Onset, breaks and retreat
The monsoon usually reaches Kerala around 1 June with a sudden heavy downpour (the burst). It covers the whole country by about mid-July. Rain does not fall every day: dry spells called breaks in the monsoon can last one or more weeks. From September the winds weaken and pull back (the retreat), leaving the north-west first.
Two branches
The Arabian Sea branch hits the Western Ghats and later reaches Gujarat and Rajasthan. The Bay of Bengal branch hits the north-east hills and then turns west along the Himalayas into the Ganga plain.
The four seasons of India
| Season | Months | What happens |
|---|---|---|
| Cold weather | Dec–Feb | Dry winds from land to sea. North India 10–15 °C, frost and fog. Western disturbances from the Mediterranean bring light winter rain to Punjab and snow to the Himalayas (good for wheat). The Tamil Nadu coast still gets rain. |
| Hot weather | Mar–May | Temperatures above 40 °C in the north-west. Hot dry wind called loo. Local storms: mango showers in Kerala and Karnataka, kalbaisakhi (nor'westers) in West Bengal and Assam. |
| South-west monsoon | Jun–Sep | Moist winds from the sea; about 75% of India's yearly rain. Heavy on the Western Ghats and north-east. |
| Retreating monsoon | Oct–Nov | Winds pull back; clear skies and humid heat (October heat). The Coromandel coast gets its main rain; cyclones form in the Bay of Bengal. |
Distribution of rainfall
India's average yearly rainfall is about 125 cm, but it is very uneven:
- Over 200 cm (very high): Western Ghats' windward side, Meghalaya and the north-east hills, Andaman and Nicobar. Mawsynram near Cherrapunji is among the wettest places on Earth.
- 100–200 cm: the eastern Ganga plain, eastern Madhya Pradesh, Odisha, the Konkan-Malabar belt.
- 50–100 cm: western Uttar Pradesh, Delhi, Haryana, inner Maharashtra, Karnataka, Telangana.
- Under 50 cm (low): western Rajasthan (Thar), Kachchh, Ladakh, and the rain-shadow part of the Deccan.
Rain falls less as we go from east to west in the north, and from the coast to the interior in the south. Rain shadow: the leeward side of the Western Ghats (e.g. Pune) gets much less rain than the windward side (e.g. Mumbai). Places with low rainfall also have the most variable rainfall, so they face droughts more often.
Monsoon and the economic life of India
- Farming: over half of India's farmland has no canal or tube-well water and depends on rain. The kharif crops (rice, cotton, soybean, pulses) are sown with the monsoon.
- Uneven and unreliable rain: a late onset, long breaks or early retreat can cause drought; too much rain in a short time causes floods. Both hurt crops.
- Food prices and jobs: a poor monsoon lowers harvests, raises food prices and cuts farm jobs and rural spending.
- Water and power: rivers, reservoirs and ground water are refilled by the monsoon; hydroelectric power depends on them.
- Culture: festivals like Onam, Baisakhi and Pongal follow the farming calendar set by the seasons.
Global warming and India
Greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapour) let sunlight in but trap some of the heat going out, like a blanket. This is natural and keeps Earth warm enough. But burning coal, oil and gas and cutting forests has added extra carbon dioxide, so the Earth has warmed by about 1.1–1.2 °C since 1850. This is global warming.
Effects for India: stronger heat waves, Himalayan glaciers shrinking (rivers like the Ganga depend on them), sea level rise that threatens coastal cities and the Sundarbans, and a monsoon with fewer rainy days but more extreme bursts, so both floods and droughts grow. Solutions: clean energy (solar, wind), saving power, public transport, planting trees and protecting forests.
Key formulas and definitions
- Monsoon = seasonal reversal of winds: summer sea → land (wet), winter land → sea (dry)
- Windward side = rain; leeward side = rain shadow
- Seasons: Cold (Dec–Feb) → Hot (Mar–May) → SW monsoon (Jun–Sep) → Retreating (Oct–Nov)
- Temperature falls about 6.5 °C per 1,000 m of height
- Average rainfall of India ≈ 125 cm; SW monsoon gives ≈ 75% of it
- El Niño (warm Pacific) → often weaker Indian monsoon
Worked examples
1. Mumbai and Pune are both in Maharashtra, but Mumbai gets about 240 cm of rain and Pune about 70 cm. Why?
Mumbai is on the windward side of the Western Ghats, where the wet south-west wind rises and rains. Pune lies behind the Ghats on the leeward side; the wind has already lost its moisture and is sinking, so Pune is in the rain shadow.
2. Chennai gets most of its rain in October–November, not in July. Explain.
In July the south-west wind blows parallel to the Coromandel coast and Tamil Nadu is in the rain shadow of the Western Ghats. In October–November the retreating monsoon winds blow from the north-east over the Bay of Bengal, pick up moisture and rain on the Tamil Nadu coast; cyclones add more rain.
3. The temperature at the foot of a hill is 32 °C. What will it be roughly at a hill station 2,000 m higher?
It falls about 6.5 °C per 1,000 m. Drop = 2 × 6.5 = 13 °C. Temperature ≈ 32 − 13 = 19 °C.
4. Why is Delhi's climate called 'extreme' while Mumbai's is 'equable'?
Mumbai is next to the sea, which heats and cools slowly and keeps the air mild; its summer–winter difference is small. Delhi is far inland, so land heating and cooling is not softened by the sea; summers go above 45 °C and winter nights near 5 °C.
5. If India's yearly rainfall is 125 cm and the south-west monsoon gives 75% of it, how much rain comes in June–September?
75% of 125 cm = 0.75 × 125 = 93.75 cm, about 94 cm.
6. In an El Niño year the monsoon was 15% below normal. Trace what may happen to a farmer's family in Vidarbha.
Less rain → kharif cotton and soybean fields dry up → lower harvest and income → loans may not be repaid → food and fodder cost more → family may migrate for work. This chain is why the monsoon is called the real finance minister of India.
Common mistakes
- Saying the monsoon is just 'the rainy season'. It is a wind system that reverses direction; rain is its result.
- Thinking the south-west monsoon rains on Tamil Nadu heavily. Tamil Nadu is in the rain shadow in summer and gets most rain from the retreating (north-east) monsoon.
- Mixing up windward and leeward. Windward faces the wind and gets rain; leeward is behind the hill and is dry.
- Believing global warming makes every place get more rain. It makes rain more uneven: heavier bursts, longer dry spells.