What Actually Causes the Monsoon?
It is not a season that simply arrives. The monsoon is the sea and the land swapping roles — and it comes down to how quickly each one heats up.
Every year, from around June, the wind over India changes direction and the rain arrives. It is so regular that farmers, railways and cities plan around it. But why does the wind turn around at all?
Land heats faster than water
Put a tray of sand and a tray of water in the sun. Come back in an hour: the sand is hot, the water is barely warmer. Water takes far more energy to heat up — and it also holds onto that heat far longer.
Now scale that up. Through spring, the Indian landmass bakes while the Indian Ocean stays comparatively cool. Hot air over the land rises, leaving lower pressure behind it. Air over the cooler sea flows in to fill the gap — and that sea air is loaded with moisture.
When that wet air is pushed up over the Western Ghats and later the Himalayas, it cools, and the water it carries falls out as rain. That is the southwest monsoon.
And in winter, it reverses
In winter the land cools quickly while the ocean stays relatively warm, so the pressure pattern flips and the wind blows from land to sea. That is the northeast monsoon, which brings rain to India's southeast coast.