ICAO Annex 3
4. Observing and reporting of meteorological elements
4.5.4 Reporting

4.5.4.3. Recommendation.- In local routine reports, local special reports, METAR and SPECI:

a) cloud amount should be reported using the abbreviations "FEW" (1 to 2 oktas), "SCT" (3 to 4 oktas), "BKN" (5 to 7 oktas) or "OVC" (8 oktas);
b) cumulonimbus clouds and towering cumulus clouds should be indicated as "CB" and "TCU", respectively;
c) the vertical visibility should be reported in steps of 30 m (100 ft) up to 600 m (2 000 ft);
d) if there are no clouds of operational significance and no restriction on vertical visibility and the abbreviation "CAVOK" is not appropriate, the abbreviation "NSC" should be used;
e) when several layers or masses of cloud of operational significance are observed, their amount and height of cloud base should be reported in increasing order of height of cloud base, and in accordance with the following criteria:

1) the lowest layer or mass, regardless of amount to be reported as FEW, SCT, BKN or OVC as appropriate;
2) the next layer or mass, covering more than 2/8 to be reported as SCT, BKN or OVC as appropriate;
3) the next higher layer or mass, covering more than 4/8 to be reported as BKN or OVC as appropriate; and
4) cumulonimbus and/or towering cumulus clouds, whenever observed and not reported in 1) to 3);

f) when the cloud base is diffuse or ragged or fluctuating rapidly, the minimum height of cloud base, or cloud fragments, should be reported; and

g) when an individual layer (mass) of cloud is composed of cumulonimbus and towering cumulus clouds with a common cloud base, the type of cloud should be reported as cumulonimbus only.

Note: Towering cumulus indicated cumulus congestus clouds of great vertical extent.