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• High Clouds: Cirrus, Cirrocumulus and Cirrostratus

Ch Code	Priority (*)	Nontechnical Cloud Type Description
0	8	No Cirrus, Cirrocumulus or Cirrostratus.
1	7 (***)	Cirrus in the form of filaments, strands or hooks, not progressively invading the sky.
2	7 (***)	Dense Cirrus, in patches or entangled sheaves, which usually do not increase and sometimes seem to be the remains of the upper part of a Cumulonimbus; or Cirrus with sproutings in the form of small turrets or battlements, or Cirrus having the appearance of cumuliform tufts.
3	6	Dense Cirrus, often in the form of an anvil, being the remains of the upper parts of Cumulonimbus.
4	5	Cirrus in the form of hooks or of filaments, or both, progressively invading the sky; they generallly become denser as a whole.
5	4 (**)	Cirrus (often in bands converging towards one point or two opposite points of the horizon) and Cirrostratus, or Cirrostratus alone; in either case, they are progressively invading the sky, and generally growing deser as a whole, bu the continuous veil does not reach 45 degrees above the horizon.
6	4 (**)	Cirrus (often in bands converging towards one point or two opposite points of the horizon) and Cirrostratus, or Cirrostratus alone; in either case, they are progressively invading the sky, and generally growing denser as a whole; the continuous veil extends more than 45 degrees above the horizon, withou the sky being totally covered.
7	2	Veil of Cirrostratus covering the celestial dome.
8	3	Cirrostratus not progressively invading the sky and not completely covering the celestial dome.
9	1	Cirrocumulus alone; or Cirrocumulus accompanied by Cirrus or Cirrostratus, or both, but Cirrocumulus is predominant.
/	??	Cirrus, Cirrocumulus, and Cirrostratus 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 8 is the lowest priority in designating Ch if more than one type is present.

(**) If no Ch 7, 8 or 9 is present, priority is determined by whatever type has the greatest sky cover.

(***) If no Ch 3, 4, 5, 6, 7, 8 or 9 is present, priority is determined by whatever type has the greatest sky cover.