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• Middle Clouds: Altocumulus, Altostratus, and Nimbostratus

Cm Code	Priority (*)	Nontechnical Cloud Type Description
0	11	No Altocumulus, Altostratus or Nimbostratus
1	1	Altostratus, the greater part of which is semi-transparent; through this part the sun or moon may be weakly visible, as through ground glass.
2	2	Altostratus, the greater part of which is sufficiently dense to hide the sun or moon, or Nimbostratus.
3	10 (**)	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.
4	8	Patches (often in the form of almonds or fishes) of Altocumulus, the greater part of which is semi-transparent; the clouds occur at one or more levels and the elements are continually changing in appearance.
5	7	Semi-transparent Altocumulus in bands, or Altocumulus in one or more fairly continuous layers (semi-transparent or opaque), progressively invading the sky; these altocumulous clouds generally thicken as a whole.
6	6	Altocumulus resulting from the spreading out of Cumulus (or Cumulonimbus).
7	5, 9, 10 (***)	Altocumulus in two or more layers, usually opaque in places, and not progressively invading the sky; or Altocumulus together with Altostratus or Nimbostratus.
8	4	Altocumulus with sproutings in form of small towers or battlements, or Altocumulus having the appearance of cumuliform tufts.
9	3	Altocumulus of a chaotic sky, generally at several levels.
/	??	Altocumulus, Altostratus and Nimbostratus invisible owing to darkness, fog, blowing dust or sand or other similar phenomena, or more often because of the presence of a continuous layer of lower clouds.

(*) 1 is the highest priority and 11 is the lowest priority in designating Cm if more than one type is present.

(**) If no Cm 1, 2, 4, 5, 6, 7, 8 or 9 is present, priority is determined by whatever type has the greatest sky cover.

(***) This classification changes depending on what cloud type is most prevalent. As and Ns have a higher priority than Ac at two or more levels, and Ac at two or more levels has a higher priority than opaque Ac. (see flowchart for clarification).