The fraction of the sky (celestial dome) covered by cloud is called sky cover, cloud cover, or cloud amount. It is measured in eights (oktas) according to the World Meteorological Organization. Table 6-7 gives the definitions for different cloud amounts, the associated symbol for weather maps, and the abbreviation for aviation weather reports (METAR).
|Table 6-7. Sky cover. Oktas= eighths of sky covered.|
|Sky Cover (oktas)||Symbol||Name||Abbr.||Sky Cover (tenths)|
|2||2 to 3|
|6||7 to 8|
* “Few” is used for (0 oktas) < coverage ≤ (2 oktas).
** See text body for a list of abbreviations of many obscurations.
Sometimes the sky is obscured, meaning that there might be clouds but the observer on the ground cannot see them. Obscurations (and their abbreviations) include: mist [BR; horizontal visibilities ≥ 1 km (i.e., ≥ 5/8 of a statute mile)], fog [FG; visibilities < 1 km (i.e., < 5/8 statute mile)], smoke (FU), volcanic ash (VA), sand (SA), haze (HZ), spray (PY), widespread dust (DU).
For aviation, the altitude of cloud base for the lowest cloud with coverage ≥ 5 oktas (i.e., lowest broken or overcast clouds) is considered the ceiling. For obscurations, the vertical visibility (VV) distance is reported as a ceiling instead.