CAVOK

CAVOK stands for Ceiling And Visibility OK. When it appears, it takes the place of the visibility, present weather and cloud groups entirely: instead of reporting each one, the observer states that none of them is worth reporting.

It may only be used when four conditions hold at once. Visibility is 10 km or more. There is no cloud below 5 000 ft, or below the highest minimum sector altitude if that is greater. There is no cumulonimbus and no towering cumulus at any height, however high. And there is no significant weather from the reporting table — no precipitation, thunderstorm, duststorm, fog, or anything else on that list.

The cumulonimbus rule catches people out. A sky can be almost empty and visibility unlimited, but a single CB anywhere above the aerodrome rules CAVOK out, because a CB threatens the approach regardless of how much clear sky surrounds it. CAVOK is a statement about hazards, not about how pleasant the day looks.