Cloud codes and chart symbols

Description

In the 1939 edition of the International Cloud Atlas, the World Meteorological Organization assigned a unique identification code, and accompanying symbol, to cloud species, with the dropped middle letter denoting the altitude band to which the cloud belongs. Cloud height is based on the height of the cloud base, regardless of how high the cloud top may get. The cloud symbol is used when plotting station models on surface weather maps.

This 27 part classificatory system places much greater emphasis on the processes of cloud growth and decay.

Cloud codes and symbols

Low Clouds

Cloud base usually below 6,500 ft.

Low Clouds

Identification codeCloud symbolSpecificationExample cloud Genera/Species
CL1CL1.pngCumulus clouds with little vertical extent and seemingly flattened (humilis), are ragged cumulus other than that of bad weather (fractus)Cumulus humilis, Cumulus fractus
CL2CL2.pngCumulus clouds of moderate or strong vertical extent, generally with protuberances in the form of domes or towers, sometimes accompanied by other cumuliform clouds, all with their bases at the same level.Cumulus congestus, Cumulus mediocris
CL3CL3.pngCumulonimbus cloud, its summit lacking sharp outlines, being neither clearly fibrous, nor in the shape of an anvil.Cumulonimbus calvus
CL4CL4.pngStratocumulus formed from the spreading out of cumulus clouds, the remains of which may also be apparent in the sky.Stratocumulus cumulogenitus
CL5CL5.pngStratocumulus not resulting from the spreading out of cumulus clouds.Stratocumulus stratiformis, Stratocumulus castellanus, Stratocumulus lenticulari
CL6CL6.pngStratus in a more or less continuous sheet or layer, or in ragged sheets, or both, but no stratus fractus of bad weatherStratus nebulosus
CL7CL7.pngStratus fractus of bad weather or cumulus fractus of bad weather, or both (Pannus" pannus cloud).Stratus fractus
CL8CL8.pngCumulus and stratocumulus other than that produced by the spreading out of cumulus, with the bases of the two cloud types being at different levels. 
CL9CL9.pngCumulonimbus, the upper part of which is clearly fibrous (or cirriform), often in the shape of an anvil. This cloud may sometimes be accompanied by cumulonimbus calvus (without anvil).Cumulonimbus capillatus

 

Medium Clouds

Cloud base between 6,500 ft and 20,000 ft.

Medium Clouds

Identification codeCloud symbolSpecificationExample cloud Genera/Species
CM1CM1.pngAltostratus, the greater part of which is semi-transparent; through this part of the sun or moon may be weakly visible, as through frosted glass.Altostratus translucidus
CM2CM2.pngNimbostratus is a thicker for of altostratus opals, the greater part of which is dense enough to hide the sun or moon from view.Altostratus opals, Nimbostratus
CM3CM3.pngAltocumulus, the greater part of which is semi-transparent; the various elements of the cloud change only slowly and are all at a single level.Altocumulus translucidus, Altocumulus stratiformis
CM4CM4.pngPatches of altocumulus, often lenticular, the greater parts of which are semi-transparent; these clouds occur vat more than one level and are continually changing in appearance. 
CM5CM5.pngSemi-transparent altocumulus, either in bands or in one or more fairly continuous layers, which progressively invade the sky, growing thicker as they do so.Altocumulus stratiformis
CM6CM6.pngAltocumulus resulting from the spreading out of cumulus(or cumulonimbus) clouds.Altocumulus cumulogenitus.
CM7CM7.pngAltocumulus translucides, stratiformis or opaques in two or more layers, not progressively invading the sky; or a single layer of altocumulus opals or altocumulus stratiformis, not progressively invading the sky; or altocumulus appearing with altostratus and/or nimbostratus.Altocumulus duplicatus
CM8CM8.pngAltocumulus with sprouting either in the form of towers or castellations (castellanus) or small tufts (floccus).Altocumulus castellanus, altocumulus floccus
CM9CM9.pngAltocumulus of a chaotic sky, generally at several levels. 

 

High Clouds

Cloud base above 20,000 ft:

High Clouds

Identification codeCloud symbolSpecificationExample cloud Genera/Species
CH1CH1.pngCirrus clouds in the form of filaments, stands or hooks, not progressively invading the sky.Cirrus uncinus, cirrus fibratus
CH2CH2.pngDense cirrus, in patches or sheaves, which usually do not increase (spissatus); or cirrus with sprouting either in the form of small turrets or battlements (castellanus) or small cumuliform tufts (floccus).Cirrus spissatus, cirrus castellanus, cirrus floccus.
CH3CH3.pngDense cirrus, often in the form of an anvil, being the remains of the upper parts of a cumulonimbus cloud.Cirrus spissatus cumulonimbogenitus.
CH4CH4.pngCirrus in the form of hooks (uncinus) or filaments (fibratus) progressively invading the sky, generally thickening as they do so.Cirrus uncinus, cirrus fibratus.
CH5CH5.pngCirrus, often in bands, with cirrostratus, or cirrostratus on its own, progressively invading the sky, and growing denser as it does so, but the continuous veil of cloud does not reach 45 degrees above the horizon.Cirrostratus
CH6CH6.pngCirrus, often in bands, with cirrostratus, or cirrostratus on its own, progressively invading the sky, and growing denser as it does so, with the continuous veil of cloud does extending more than 45 degrees above the horizon although without the sky being totally covered.Cirrostratus
CH7CH7.pngA veil of cirrostratus which covers the entire sky.Cirrostratus nebulous, cirrostratus fibratus
CH8CH8.pngA veil of cirrostratus which neither covers the entire sky nor progressively invades it.Cirrostratus
CH9CH9.pngCirrocumulus appearing alone, or with cirrus and/or cirrostratus, as long as the cirrocumulus is predominant.Cirrocumulus stratiformis, cirrocumulus floccus, cirrocumulus lenticularis

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