Interests: Clouds: classification

Clouds are classified according to their shape and altitude. Altitudes are denoted by the following prefix:

  • "Cirro-": cloud base > 18,000 feet
  • "Alto-": 6,000 > cloud base < 20,000 feet
  • "Strato-": cloud base < 7,000 feet

The main cloud shapes are:

  • "Cumulus": fluffy, like cotton wool
  • "Stratus": layered, flat and/or sheet-like
  • "Cirrus": wispy or fibrous

A "-nimbus" suffix or a "nimbo-" prefix means a rain cloud.

The table shows cloud classifications along with descriptions, altitudes and any possible predictions that can be made from the cloud.

Altitude of cloud base/feet Cloud Type Description Weather Prediction
> 18,000 Cirrocumulus This is small, lumpy ice crystal clouds. They are formed when there is convection in the upper atmosphere. Convection in the upper atmosphere is an indicator of coming storms.
Cirrostratus Very high up in the atmosphere, this cloud is thin and white. It is composed of ice crystals. It forms a halo around the sun. Its presence indicates a warm front with lots of rain. When the halo disappears, it may be because the cirrostratus cloud has been replaced by altostratus cloud. This means the warm front - and hence the rain - is getting nearer.
Cirrus Composed of ice crystals. Is always white (except if the sun is low in the sky). It is often a sign of a change in the weather. Dense cirrus clouds could mean a storm is on its way.
6,000 - 20,000 Altostratus Formless sheet of light grey cloud. A hazy sun may shine through and light rain may fall. A warm front bringing continuous rain is close by.
Altocumulus Lumpy or blobby clouds caused by convection in the mid-atmosphere This type of cloud often forms a "mackerel sky". Convection in the mid-atmosphere signifies unsettled weather is on its way, with the possibility of storms.
< 7,000 Cumulus Fluffy, cotton-wool clouds. Showers may occur. Height:width ratio < 1:3 = fair-weather cumulus
Height:width ratio > 1:2 + convection in the atmosphere = possibility of cumulonimbus forming from cumulus.
Nimbostratus Cloud base is low, but cloud itself often stretches vertically upwards into the atmosphere. Nimbostratus clouds are dense, grey/blue and formless. When the sky is dark and overcast and it is raining, it is these clouds that are responsible. If it is not already raining, it is likely to start raining soon.
Stratocumulus Blobby or lumpy clouds, often forming a regular pattern of blobs. May appear to form a "mackerel sky". May precipitate drizzle. It will most likely be dull.
Stratus Layered clouds, of a uniform size. May be thick or thin. May be so low as to be touching the ground. In this case, the cloud is fog. Unreliable as a prediction aid.
Just above ground level to > 50,000 Cumulonimbus Large, dark, towering thunderclouds. Rain and hail precipitation. A thunderstorm is happening.

2003

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