A cloud group contains three different pieces of information
A group such as BKN018CB reports the amount of sky covered, the height of the layer base and, when included, a significant cloud type. Read it as: broken cloud, base 1,800 ft above aerodrome level, cumulonimbus.
In a METAR, cloud-base height is expressed in hundreds of feet above the aerodrome. It is not an altitude above mean sea level. Consider aerodrome elevation and the applicable charts before comparing it with terrain.
Coverage: how many oktas are covered
| Code | Coverage | Practical meaning |
|---|---|---|
FEW | 1–2 oktas | Few clouds |
SCT | 3–4 oktas | Scattered clouds |
BKN | 5–7 oktas | Broken cloud; may constitute the ceiling |
OVC | 8 oktas | Overcast; constitutes the ceiling |
Coverage does not describe layer thickness or route-wide extent. SCT020 does not automatically mean a 2,000 ft ceiling; BKN035 reports a broken layer based at 3,500 ft above the aerodrome.
Height: multiply the three digits by 100 ft
FEW008: few clouds at 800 ft.SCT025: scattered clouds at 2,500 ft.BKN100: broken cloud at 10,000 ft.///: height is unavailable or cannot be determined.
The height refers to the aerodrome observation, not the whole route. Compare the METAR with the TAF, charts, satellite imagery and the wider briefing.
What constitutes the ceiling
The ceiling is normally the base of the lowest BKN or OVC layer, or the vertical visibility when the sky is obscured. FEW and SCT do not constitute a ceiling, although they can remain operationally important.
In FEW012 BKN025 OVC060, the first ceiling is 2,500 ft, not 1,200 ft.
CB, TCU and vertical visibility
CB: cumulonimbus and associated convective hazards.TCU: towering cumulus with strong vertical development.VV003: vertical visibility of 300 ft when the sky is obscured.VV///: vertical visibility cannot be determined.
FEW020CB remains significant even though coverage is only FEW.
NSC, NCD and CAVOK are not synonyms
NSC means no significant cloud under the applicable criteria. NCD means that an automated system detected no cloud; it does not guarantee a completely clear sky beyond sensor limitations.
CAVOK combines visibility of at least 10 km, no significant weather, no cloud below the applicable threshold and no CB or TCU. It does not simply mean “clear sky”.
A reliable cross-check sequence
- Identify every layer.
- Convert the digits into hundreds of feet above the aerodrome.
- Find the first BKN, OVC or VV group.
- Check every layer for CB or TCU.
- Compare observation, forecast, route and flight time.