How the Indian monsoon works, and why forecasts matter

Land heats and cools faster than water. In summer the great landmass of South Asia, and the Tibetan plateau to the north, become very hot, the air above them rises and a belt of low pressure forms. The Indian Ocean to the south stays cooler, with higher pressure. Air flows from the high to the low, and the winds that cross the equator and bend towards India carry vast amounts of moisture picked up from the warm sea.

In winter the pattern reverses: the land is cold, the winds blow from the north-east and the air is mostly dry, except for the north-east monsoon that brings rain to the coast of Tamil Nadu in October to December.

The normal date for the monsoon to arrive over Kerala is 1 June, and it advances in two currents: an Arabian Sea branch up the west coast, and a Bay of Bengal branch that moves into the north-east and then west along the Gangetic plain. By about 8 July it normally covers the whole country. Withdrawal begins from the north-west, usually from about 17 September, and the rains retreat down the peninsula over the following weeks. The India Meteorological Department (IMD) revised these normal dates in 2020 to reflect recent decades.

On the way, the Western Ghats force the moist air upward, so the windward slopes of the coast get some of the heaviest rain in the country, while areas on the leeward side, which lie in a rain shadow, are much drier. Low-pressure systems that form over the Bay of Bengal and travel inland bring heavy spells to central India.

The monsoon does not rain continuously. It comes in bursts of days of heavy rain, called active spells, separated by lulls, called breaks, which can last a week or two. A long break in July can stress crops even in a year when the season total turns out normal.

El Niño, a warming of the central and eastern Pacific, tends to weaken the Indian monsoon, and the opposite phase, La Niña, tends to help it, but the link is not rigid. The Indian Ocean Dipole, a difference in sea temperature between the western and eastern Indian Ocean, can offset El Niño: in 1997 a strong El Niño was accompanied by a normal monsoon, thanks in part to a strongly positive dipole. Other factors include the snow cover in Eurasia in the preceding winter and the temperatures over land.

The IMD issues a long-range forecast in April and an update around the end of May. It expresses the expected rainfall as a percentage of the long-period average, which for the June to September season is about 87 centimetres. The categories have exact meanings.

• Normal: 96 to 104 per cent of the long-period average.
• Below normal: 90 to 95 per cent. Deficient: below 90 per cent.
• Above normal: 105 to 110 per cent. Excess: above 110 per cent.

A large part of India's workforce, close to half by most surveys, still depends on farming, and most farms depend on the monsoon for at least part of their water. A good monsoon supports rural incomes and holds down food prices, while a poor one can push up the prices of vegetables and pulses, and with them inflation. Reservoir levels also feed into drinking water and hydropower for the following year.

When reading a monsoon report, look beyond the national percentage. Ask how rainfall is spread across regions, whether there have been long dry spells, how sowing is progressing and how full the reservoirs are. A season can finish at a normal total after a flood in one week and a drought in another, and for a farmer that is not a normal year.

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