Monsoon

The wind that changes with the seasons

In summer the land heats up faster than the ocean. Warm air rises and moist wind rushes in from the sea — the monsoon. For months it brings rain to India and Southeast Asia: rivers fill, rice fields turn green. In winter it flips: dry wind blows from the land to the ocean. Move the season slider and watch the wind change direction!

A monsoon is a seasonal wind system born from the temperature difference between land and ocean. In summer low pressure sits over Asia and moist south-westerly air arrives from the Indian Ocean; the Himalayas block it and wring the vapour out as rain (Cherrapunji: 11,000 mm a year). In winter high pressure over Siberia sends dry north-easterly wind. The monsoon governs farming for 2 billion people in Asia; if late — drought, if too strong — floods. West Africa and Australia have monsoons of their own.

Fun facts

• The word "monsoon" comes from the Arabic "mausim" — season.
• India gets 75% of its yearly rain between June and September, during the monsoon.
• Ancient sailors rode the monsoon from Arabia to India and sailed back in winter.
• Monsoon clouds pile up against the Himalayas at 3 km — which is why Tibet is dry.