Clouds are classified into different groups:

High clouds, medium clouds and low clouds.

High clouds usually consist of solely ice crystals and have a base of between 18,000 and 45,000 feet (5,500 and 14,000 metres). These are sub-divided into 3 categories:

• Cirrus – white filaments
• Cirrocumulus – small rippled elements
• Cirrostratus – transparent sheet.

Medium clouds usually consist of either water droplets or a mixture of water and ice crystals. These have a base of between 6,500 – 23,000 feet (2,000 – 7,000 metres). There are 3 categories of medium clouds:

• Altocumulus – layered, with rippled elements and are generally white with a little shading
• Altostratus – a thin grey layer which allows the sun to appear through
• Nimbostratus – a low, thick, dark layer which rain or snow will fall from.

Low clouds mainly consist of water droplets (with the exception of cumulonimbus clouds which include ice crystals) and they have a base below 6,500 feet (2,000 metres).
There are 4 categories of low clouds:

• Stratocumulus – these are a series of white, layered rounded rolls;
• Stratus – these are grey and layered with a uniform base;
• Cumulus – these have a flat base with vertical rolls or towers often resembling
  cotton-wool;
• Cumulonimbus – these look like large cauliflower-shaped towers which bring thunderstorms, rain or snow.