Science Original explainer
Three out of every four drops of rain India gets arrive in four months. Here is how the monsoon works.
Every year, between June and September, India waits for the same guest: the southwest monsoon. About 75 per cent of the country's annual rainfall arrives in these four months. It fills reservoirs, waters the kharif crop, recharges groundwater, and sets the mood of the economy for the year. A good monsoon means full granaries and steady food prices. A weak one means drought, costlier vegetables and worried farmers. No other weather event matters as much to India.
Yet the monsoon is not rain that simply happens to fall. It is a giant seasonal reversal of wind, driven by the difference between how fast land and sea heat up. Understanding that reversal explains why the rains arrive when they do, why they sometimes fail, and why forecasters watch the Pacific Ocean as closely as the Indian sky.
The word monsoon comes from the Arabic word mausim, meaning season. In winter, winds blow from the cold land of Central Asia out towards the warmer Indian Ocean, bringing dry weather. In summer, the land heats up much faster than the sea. Hot air rises over the Indian subcontinent, creating a low-pressure area, and moist air rushes in from the ocean to fill it. That moist air is the monsoon.
As the damp air moves inland, it is forced upwards by hills and mountains, cools, and drops its moisture as rain. This is why the Western Ghats get some of the heaviest rainfall in the country: the Arabian Sea branch of the monsoon hits those mountains first. The northeast of India, where the Bay of Bengal branch meets the Himalayan foothills, is also among the wettest places on Earth.
The India Meteorological Department, founded in 1875, marks the official onset of the monsoon over Kerala, usually around 1 June. From there it advances in two branches. The Arabian Sea branch moves up the west coast, bringing rain to Kerala, Karnataka, Goa and Maharashtra. The Bay of Bengal branch sweeps across eastern India and the northeast, watering Assam and Meghalaya.
By mid-July the monsoon usually covers the whole country. It then stays for about four months, with spells of heavy rain broken by dry gaps. From mid-September it begins to withdraw, starting from the northwest, and by mid-October it has mostly left. A second, smaller rainy season, the northeast monsoon, then brings rain to Tamil Nadu and parts of Andhra Pradesh between October and December, contributing roughly 10 to 15 per cent of annual rainfall.
Agriculture is the most direct reason. Nearly 45 per cent of India's net sown area still has no irrigation and depends entirely on monsoon rain. Farming employs close to 46 per cent of the population and contributes about 18 per cent of the country's gross value added. The kharif crops sown with the monsoon, rice, maize, cotton, pulses, soybean and oilseeds, feed the country and shape rural incomes.
The effects spread far beyond the farm. Food and beverages account for roughly 46 per cent of India's consumer price index, so a bad monsoon pushes up vegetable and grain prices for everyone. Reservoirs filled by the monsoon supply drinking water and hydroelectric power. A weak monsoon can slow the whole economy; a strong one lifts it.
The monsoon is reliable in its arrival but not in its strength. The biggest spoiler is El Nino, a warming of the Pacific Ocean near South America that shifts global wind patterns. In El Nino years, India often gets less rain. In 2026, for example, the weather department first forecast 92 per cent of the long-period average, then revised it down to 90 per cent as El Nino conditions were expected to develop. The long-period average itself is 868.6 millimetres of rain for the June to September season, based on 1971 to 2020.
Too much rain is also a problem. Heavy spells cause floods in cities and in river plains, damage standing crops, and spread water-borne disease. Climate change is making both extremes more likely: longer dry gaps inside the season, and short bursts of very heavy rain. The total rainfall in a season can look normal while farmers still suffer, because what matters is when and where the rain falls.
The weather department issues its first long-range forecast in April and updates it in late May, predicting the season's rainfall as a percentage of the long-period average. It uses ocean temperatures, wind patterns, snow cover in Eurasia, and computer models. The forecasts have improved a lot, but predicting the exact timing of rain over a particular district weeks ahead remains hard, which is why farmers also watch short-range forecasts and local rain gauges.
The monsoon will keep deciding India's year for the foreseeable future. Dams, canals and groundwater have reduced the dependence, but they have not removed it. For a country where almost half the farmland still waits for the sky, four months of wind and rain remain the most important economic event of the year.
This explainer was written by The Daily Short's desk from the sources listed above. It is general information, not professional advice. Spot an error? Tell us via the Contact page and we will correct it. Follow our official Instagram @the.daily.short.