Cloud classification

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.

Related pages: Cloud classification | C_L guide | C_M guide | C_H guide | Cloud index