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Weather and climate


Cirrocumulus cloudsCirrocumulus clouds When water vapour cools, it turns back into liquid water. This is called condensation. Water droplets form around tiny particles in the air, such as sea salt and dust. Millions of these droplets gather together to form clouds. In the highest clouds, the water freezes into ice crystals. Clouds form at different heights and have different shapes. There are a variety of names to describe them.

Cloud types and altitudesCloud types and altitudes

Types of cloud

The names of clouds describe their shapes and altitudes using a combination of meanings. They are all made up of three basic shapes: cumulus (“heaped”), stratus (“layered”) and cirrus (“feathery”). High-altitude clouds have the prefix (first part of a word) cirro-, middle-altitude clouds alto- and low altitude clouds strato-.

It is possible to forecast the weather from observing the different types of clouds. Highest at 8000 metres (about 25,000 feet) and above are cirrus, made of tiny ice crystals. Thin and wispy, they signal fine, dry, settled conditions.

Cirrocumulus are small, regular-shaped clouds that look almost like fish scales and make a so-called “mackerel sky”.

Altostratus and altocumulus form at medium heights and often mean that rain is on the way.

Cumulus are the “cotton wool” clouds of a summer’s day.

Stratus are low clouds that cover the whole sky like a flat, pale grey sheet.

Nimbostratus clouds are even lower and usually bring heavy rain or snow.

The biggest and most impressive cloud is the cumulonimbus. It towers 5000 metres (16,000 feet) or more, with a fluffy white top and flat, grey “anvil” base. It usually brings fierce storms with thunder and lightning.

Cumulonimbus clouds can tower up to 23,000 m (75,000 ft). Their anvil-like shapes are created by high-altitude winds shearing off the clouds' tops.

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