Climate of India

Ask anyone in India what controls the year, and the answer is the same: the rains. India has a monsoon type of climate, ruled by winds that reverse their direction with the seasons. They bring a long dry spell followed by a burst of rain on which the whole country depends. Understanding why the monsoon happens, what controls it, and how the seasons follow one another is one of the richest and most heavily tested parts of Indian geography.

It helps to begin with two terms that are often confused. Weather is the state of the atmosphere over an area at a particular time (today's temperature, rainfall and wind). Climate is the sum total of weather conditions averaged over a long period, usually more than thirty years. Weather changes from day to day, whereas climate is the settled pattern.

The word monsoon comes from a term meaning "season", and it refers to the seasonal reversal in the direction of winds through the year.

Several factors work together to shape the climate, and they make excellent exam points.

• Latitude: the Tropic of Cancer passes through the middle of India. So the south is tropical and hot while the north is warm temperate. A line on the map makes this division precise. The 15°C January isotherm, which runs roughly along the Tropic of Cancer and the Vindhya belt, is conventionally taken as the boundary between the tropical south and the sub-tropical north. South of this line, even January stays above 15°C.
• Altitude: India has high mountains in the north. The Himalayas keep out the bitterly cold winds from Central Asia. This makes India warmer than other places at the same latitude. Temperature falls as one climbs higher.
• Pressure and winds: the seasonal change in air pressure over the land and sea drives the monsoon winds.
• Distance from the sea: coastal areas have a mild, even climate. Places deep in the interior have a continental climate of hot summers and cold winters.
• Relief (mountains): highlands force moist winds to rise and shed their rain. The windward side of the Western Ghats is very wet, while the leeward side lies in a dry "rain shadow".

The monsoon is caused by the differential heating of land and sea and the seasonal shift of pressure and winds.

In summer the land heats up faster than the sea. A region of low pressure forms over north-western India, while the sea stays relatively cool with higher pressure. Winds always blow from high pressure to low pressure. So moisture-laden winds rush in from the Indian Ocean towards the land. These are the south-west monsoon winds, and they bring the main rains from June to September.

In winter the opposite happens. The land cools faster than the sea, and high pressure builds over the land. Dry winds blow outward from the land towards the sea. These are the north-east monsoon winds. They are dry, but after crossing the Bay of Bengal they bring some rain to the Tamil Nadu coast in winter.

The monsoon is famous for its uncertainty. Its arrival, called the burst of the monsoon, can be early or late. The total rain also varies from year to year. This is why Indian agriculture is often described as a gamble on the monsoon.

India has four main seasons.

The cold weather season (winter), from about December to February, has clear skies, low temperatures and dry north-east winds.

The hot weather season (summer), from March to May, brings rising temperatures and low pressure over the north. Towards its end come violent local storms, such as the Kaal Baisakhi in the east and the loo, a hot dry wind, in the north.

The advancing monsoon (the rainy season), from June to September, is when the south-west monsoon sweeps over the country and brings most of the year's rain.

The retreating monsoon, in October and November, is when the monsoon winds withdraw. The weather becomes hot and humid, a condition known as October heat, before the cool dry winter returns.

Rainfall in India is very uneven, both from place to place and from time to time. The heaviest rain falls on the windward slopes of the Western Ghats and in the north-eastern states. Mawsynram and Cherrapunji in Meghalaya are among the wettest places on Earth. In contrast, western Rajasthan, the interior of the Deccan and parts of Ladakh receive very little rain.

There is also a clear gradient along the Himalayas themselves. Himalayan rainfall decreases from east to west. The Bay of Bengal branch of the monsoon drenches the eastern Himalayas first, while the western Himalayas, far from this moist current, stay much drier. This has a striking effect on mountain vegetation. Heavier rainfall raises the snowline and lets plant life climb higher. So alpine vegetation reaches about 4000 m in the eastern Himalayas but only about 3000 m in the western Himalayas. The cause is the extra monsoon rain in the east, not nearness to the Equator or to the sea.

This unevenness has deep consequences. Areas of low or unreliable rain face frequent droughts, while areas of heavy rain may suffer floods in the same season. Most of the rain is concentrated in just a few monsoon months. So storing water for the rest of the year is a constant challenge.

The simple idea of land and sea heating differently explains a lot. But the full mechanism of the monsoon involves the whole atmosphere and even far-off oceans.

A central player is the Inter-Tropical Convergence Zone (ITCZ), a belt of low pressure near the Equator where the trade winds meet. In the northern summer it shifts northward over the heated Indian landmass. The moist south-east trade winds of the southern hemisphere then cross the Equator, become the south-west monsoon, and are drawn into this low. The intense heating of the Tibetan plateau strengthens this low-pressure pull. High above, the position of the subtropical jet stream and the appearance of the tropical easterly jet control the timing of the monsoon's burst. It is the shifting of all these together, not heating alone, that makes the monsoon.

The Indian monsoon is also tied to the distant Pacific Ocean through a phenomenon called the El Nino Southern Oscillation (ENSO). El Nino is an unusual warming of the surface waters of the central and eastern Pacific. In El Nino years this warming disturbs the global wind pattern and tends to weaken the Indian monsoon. Many of India's worst drought years have coincided with El Nino. Its opposite phase, La Nina (a cooling of the Pacific), is generally linked with a stronger monsoon. Watching the Pacific has therefore become part of forecasting India's rains.

The south-west monsoon does not arrive everywhere at once. It usually bursts onto the coast of Kerala around the 1st of June. It then advances rapidly, splitting into the Arabian Sea branch and the Bay of Bengal branch, and covers the whole country by mid-July. After September, as the land cools, the monsoon begins to withdraw. It retreats gradually from the north-west first and leaves the southern peninsula last. This retreat brings the north-east monsoon rains to the Tamil Nadu coast.

Using Koeppen's scheme, which links climate to temperature, rainfall and vegetation, India can be divided into several climatic regions. They range from the tropical wet climate of the west coast and the north-east (heavy rain all season), through the tropical wet-and-dry climate of most of the interior, the semi-arid and hot desert climates of the north-west, to the cold mountain climate of the Himalayas. This shows how, under one broad monsoon system, India still contains a wide range of local climates.

Each climatic region favours its own crops, and examiners love to describe a climate and ask which crop fits it.

• Pepper: a crop of the hot, humid, tropical wet west coast, above all Kerala. It grows on hill slopes up to about 1100 m, needs roughly 200 cm of rain a year, and thrives where temperatures stay between about 15°C and 30°C through the year.
• Mustard: a cool-season crop of the dry north-west, grown in winter.
• Cotton: suited to the drier, tropical wet-and-dry interior of the Deccan.
• Tobacco: a warm-region crop that tolerates moderate rainfall, unlike rain-soaked pepper.

So a description of hot and humid conditions, heavy rain near 200 cm, and cultivation on hill slopes points straight to pepper on the west coast.