High clouds, C_H
base usually 20, 000 ft or above over British Isles)

Type

Precipitation (ppn) etc.

Range of cloud base

C code

Cirrus (Ci)

No ppn. Halo may occur. Dense patches may veil or hide the sun

Usually 20,000–40,000 ft

0

Cirrocumulus (Cc)

No ppn. Position of sun/moon can usually be seen

If at a non-aviation station the height cannot reasonably be estimated, the British practice is to use a nominal height of 25,000 ft, and 35,000 ft for any higher cloud

1

Cirrostratus (Cs)

No ppn. Halo often occurs. Outline of sun normally visible

Cs may thicken to become As

2

Medium clouds, C_M
(base usually between 6,500 and 20,000 ft over British Isles, although Ns may lower to near earth's surface)

Type

Precipitation (ppn) etc.

Range of cloud base

C code

Altocumulus (Ac)

Ac Castellanus occasionally produces ppn. Can be thick enough to hide sun/moon

Usually 6,500–20,000 ft. If at a non-aviation station the height cannot reasonably be estimated, the British practice is to use a nominal height of 10,000 ft, and 15,000 ft for any Ac or As above

3

Altostratus (As)

Often continuous ppn reaching the ground with sun/moon hidden. Thinner As shows sun/moon as ground glass appearance

Altostratus may thicken with progressive lowering of the base to become Ns

4

Nimbostratus (Ns)

Normally continuous ppn (sometimes moderate/heavy) with sun/moon hidden

Usually between the surface and 10,000 ft

5

Low clouds, C_L
(base usually below 6,500 ft over British Isles)

Type

Precipitation (ppn) etc.

Range of cloud base

C code

Stratocumulus (Sc)

Normally no ppn, but slight ppn possible over coasts/hills. Can be thick enough to hide sun/moon

Usually between 1,000 ft* and 4,500 ft but may often be observed to 6,500 ft

6

Stratus (St)

Near coasts/hills, ppn can be considerable, but it may be falling from higher cloud such as Ns. Can be thick enough to hide sun/moon. However when thin, sun/moon can be clearly visible

Usually between the surface and 2,000 ft but may sometimes be observed to 4,000 ft

7

Cumulus (Cu)

Light showers are possible

Usually between 1,000 ft* and 5,000 ft, but may sometimes be observed to 6,500 ft. After initial formation, a rise in temperature often leads to a rise in cloud base

8

Cumulonimbus (Cb)

Always reported when showers/thunderstorms/hail occurs. Squally winds are also common

Usually between 2,000 ft* and 5,000 ft, but may sometimes lower to near surface, or be as high as 6,500 ft

9

*At stations substantially over 500 ft above sea level the base will often be less.