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 code	Cloud symbol	Specification	Example cloud Genera/Species
CL1		Cumulus clouds with little vertical extent and seemingly flattened (humilis), are ragged cumulus other than that of bad weather (fractus)	Cumulus humilis, Cumulus fractus
CL2		Cumulus 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
CL3		Cumulonimbus cloud, its summit lacking sharp outlines, being neither clearly fibrous, nor in the shape of an anvil.	Cumulonimbus calvus
CL4		Stratocumulus formed from the spreading out of cumulus clouds, the remains of which may also be apparent in the sky.	Stratocumulus cumulogenitus
CL5		Stratocumulus not resulting from the spreading out of cumulus clouds.	Stratocumulus stratiformis, Stratocumulus castellanus, Stratocumulus lenticulari
CL6		Stratus in a more or less continuous sheet or layer, or in ragged sheets, or both, but no stratus fractus of bad weather	Stratus nebulosus
CL7		Stratus fractus of bad weather or cumulus fractus of bad weather, or both (Pannus" pannus cloud).	Stratus fractus
CL8		Cumulus and stratocumulus other than that produced by the spreading out of cumulus, with the bases of the two cloud types being at different levels.
CL9		Cumulonimbus, 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 code	Cloud symbol	Specification	Example cloud Genera/Species
CM1		Altostratus, 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
CM2		Nimbostratus 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
CM3		Altocumulus, 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
CM4		Patches 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.
CM5		Semi-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
CM6		Altocumulus resulting from the spreading out of cumulus(or cumulonimbus) clouds.	Altocumulus cumulogenitus.
CM7		Altocumulus 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
CM8		Altocumulus with sprouting either in the form of towers or castellations (castellanus) or small tufts (floccus).	Altocumulus castellanus, altocumulus floccus
CM9		Altocumulus of a chaotic sky, generally at several levels.

High Clouds

Cloud base above 20,000 ft:

High Clouds

Identification code	Cloud symbol	Specification	Example cloud Genera/Species
CH1		Cirrus clouds in the form of filaments, stands or hooks, not progressively invading the sky.	Cirrus uncinus, cirrus fibratus
CH2		Dense 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.
CH3		Dense cirrus, often in the form of an anvil, being the remains of the upper parts of a cumulonimbus cloud.	Cirrus spissatus cumulonimbogenitus.
CH4		Cirrus in the form of hooks (uncinus) or filaments (fibratus) progressively invading the sky, generally thickening as they do so.	Cirrus uncinus, cirrus fibratus.
CH5		Cirrus, 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
CH6		Cirrus, 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
CH7		A veil of cirrostratus which covers the entire sky.	Cirrostratus nebulous, cirrostratus fibratus
CH8		A veil of cirrostratus which neither covers the entire sky nor progressively invades it.	Cirrostratus
CH9		Cirrocumulus 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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• Cloud Classification: Genera and Species