How the tallest mountains on Earth make rain, and keep India warm.
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These are the Himalayas, the highest mountains on Earth. They run for about 2,900 km. The tallest peak is Mount Everest, 8,849 m high. They are still growing a tiny bit every year, because India is still pushing into the rest of Asia.
In summer, the land in India gets much hotter than the sea (red means hot land). Hot air rises over the land, and wet air from the ocean rushes in to take its place. These big summer winds are the monsoon.
When air rises, it gets colder: up to about 10 degrees for every kilometre it climbs. Cold air can’t hold as much water vapour, the invisible water in the air. So the extra water turns into tiny droplets, and that’s a cloud! When the droplets bump together and get heavy, they fall as rain.
The wet wind runs into the Himalayas, and the smaller hills in front of them. It can’t go through, so it is pushed up. High up it gets cold, and the water in it falls as rain. The blue circles show how much rain falls in a year: the bigger the circle, the more rain. Cherrapunji, on the Khasi Hills, gets about 11 metres. That’s taller than a house!
By the time the air gets over the mountains, it has dropped almost all its water. So Tibet, behind the mountains, is dry. This is called a rain shadow. Lhasa gets less than half a metre of rain a year, and Leh gets about 10 cm.
The monsoon doesn’t arrive everywhere on the same day. It reaches Kerala, in the far south-west of India, around 1 June. Then it marches north, bringing rain to one place after another. Drag the calendar to watch it spread.
In winter, freezing air spreads out from Siberia, where it can drop below −40 °C. The Himalayas and Tibet block most of it, so India’s winters stay mild.
The summer monsoon is blowing in from the sea.
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The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).