High flyers

Clouds are classified according to type and to height.

Cirrus: Because of their height, cirrus clouds are composed of ice crystals and often look wispy or streaky. This is because of very strong winds at high levels. Cirrus clouds do not bring rain, but they can often be the first sign of an approaching warm front

Stratus: The low-level cloud that gives a dull, grey feel to the day. The name comes from the Latin for "layer", as it spreads out in a layer across the sky. It is often associated with drizzle, and, if it is low enough, becomes fog.

Cumulus: These are the result of convection (air rising due to heating by the Earth's surface). They are often white, fluffy clouds, hence their name - cumulus is from the Latin for "heaped". Cumulus clouds can grow to great heights, and then showers or thunderstorms are likely.

* High clouds (CH): base usually higher than 6,000 metres

Cirrus (Ci): no precipitation; dense patches may hide the sun. Cirrocumulus (Cc): no precipitation; sun or moon's position can usually be seen through it.

Cirrostratus (Cs): no precipitation. A halo often occurs. Outline of the sun normally visible. Can thicken to become altostratus.

Medium clouds (Cm): base usually between 2,000 and 6,000 metres; nimbostratus may be just above sea level.

Altocumulus (Ac): occasional precipitation. Can be thick enough to hide the sun or moon.

Altostratus (As): often continuous precipitation that reaches the ground, with sun or moon hidden. May thicken to become nimbostratus.

Nimbostratus (Ns): normally continuous precipitation (sometimes heavy), with sun or moon hidden.

Low clouds (LC): base usually below 2,000 metres

Stratocumulus (Sc): normally no precipitation but some possible over coast and hills. Can be thick enough to hide the moon or sun.

Stratus (St): precipitation can be considerable near coast or hills, but may come from higher clouds such as Ns. Can be thick enough to hide the sun or moon.

Cumulus (Cu): light precipitation is possible.

Cumulonimbus (Cb): always reported with showers, thunderstorms or hail. Squally winds are also common.

Photos: science photo library.

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