Most weather is fickle. A cloud appears or doesn’t; a storm turns up or wanders off. But every year, a whole season’s worth of rain arrives over India within a week or so of the same date. The monsoon reaches the Kerala coast around the first of June, sweeps north across the country over the following month, and brings around three-quarters of the year’s rain in four months. Something that regular needs an engine. Here is how it works.
It starts with a simple difference between land and sea: they don’t heat up at the same rate. Water has a remarkably high heat capacity. Kilogram for kilogram, it takes about four times as much energy to warm water by a degree as it does to warm dry soil or rock. And the sea mixes: sunlight warming the surface is stirred down by waves and currents into the water below, so the heat is shared through a deep layer. Land can’t do that. Sunlight warms only a thin skin at the surface, which heats up quickly. So in April and May, as the Sun climbs high over the northern half of the Earth, the Indian landmass bakes, and the ocean to its south warms only slowly.
Hot land heats the air above it. Warm air is less dense than cool air, so it rises, and as it rises it leaves behind a region of lower pressure near the ground. By late May, a vast area of low pressure has formed over north-west India and Pakistan. And there’s a giant helping hand to the north: the Tibetan Plateau, more than four kilometres high and thousands of kilometres wide, which heats the air at a height where there would normally be cold, thin atmosphere. It acts like a hot plate held up in the middle of the sky, pulling the whole circulation northward.
Air flows from high pressure to low, so the cooler, denser air over the ocean, where pressure is higher, begins to flow in towards the land to replace the rising air. That inflowing wind has spent days crossing thousands of kilometres of warm sea, and it’s loaded with water vapour. This is the monsoon wind.
When it reaches land, the moist air is forced upwards: over the Western Ghats, over the hills of the north-east, into the rising air over the heated plains. As air rises, it expands and cools, and cool air can’t hold as much water vapour, so the vapour condenses into clouds and falls as rain. That’s why the windward slopes of the Ghats get drenched while places just behind them, in their rain shadow, stay much drier, and why the hills of Meghalaya, catching air from the Bay of Bengal funnelled up against them, are among the wettest places on Earth.
Now comes the part that makes the monsoon so powerful. Every gram of water vapour that condenses into rain releases the energy it absorbed when it evaporated from the sea, a large amount of heat, into the surrounding air. That heat warms the air further, making it rise even more strongly, which draws in still more moist air from the ocean, which condenses and releases more heat. Once the monsoon is established, it partly runs on its own rain. The land started the engine; the condensing water keeps it roaring.
In autumn the whole thing runs backwards. The Sun moves south, the land cools faster than the sea, the low pressure fades, and by October the winds reverse, blowing from the cool land out towards the warmer ocean. That reversal is what the word monsoon really means: from the Arabic mawsim, a season, it describes not the rain but the wind that changes direction with the seasons.
Regular is not the same as identical, of course. The date can shift by a week or two, the rains can stall for weeks in a “break”, and some years are wetter or drier than others. Part of that variation comes from far away. In years when the eastern Pacific Ocean warms unusually, called El Niño, the circulation of air across the tropics shifts, and India’s monsoon is more often weaker than normal. Forecasting how strong each year’s monsoon will be, weeks ahead, remains one of the hardest and most important jobs in weather science, because for hundreds of millions of farmers, the answer decides the year.
But the basic engine never fails. As long as the Sun climbs north every spring, and land heats faster than the sea, the wind will turn, and the rain will come.