Cyclones

Cyclones are huge storms that form over warm ocean water. As the sea is heated, the warm, moist air above it rises.

As it rises, its water vapour condenses into raindrops. When water evaporates it takes in heat; when the vapour condenses again, that heat is given back to the air.

That heat warms the rising air even more, so it climbs higher still and the pressure underneath drops even lower. Air from all around rushes in, and starts rising too. The Earth's rotation sets all this moving air spinning.

The cycle repeats and builds a very low-pressure centre with fierce winds whirling round it. That spinning system of clouds, wind and rain is a cyclone.

At the centre is the region of lowest pressure, the eye. Strangely, the wind at the eye is calm, but all around it are howling winds and heavy rain. Over the ocean a cyclone's winds are far stronger than an ordinary thunderstorm's. Once it reaches land, its supply of warm, moist air is cut off and it slowly weakens.

Even as it weakens it can leave damage that takes months or years to repair:

• Cyclone Amphan (2020) had winds of up to 270 km/h;
• the wind pushes sea water onto the shore as a wall of water 3 to 12 m high, flooding the coast and places far inland;
• heavy rain makes rivers overflow and can trigger landslides;
• sea water spoils drinking water, and its salt makes farmland less fertile;
• fallen trees and debris block roads, so help can't get through;
• power cuts can last for days.

Staying safe: follow weather reports and the warnings from the India Meteorological Department (IMD). Weather satellites now track cyclones and predict their path, which saves lives. If you live where cyclones are common, keep an emergency kit of essentials ready, and during a cyclone move quickly to the nearest official cyclone shelter.