The cloud ceiling is the height reported in METARs when cloud cover is BKN (5–7 oktas) or OVC (8 oktas). It is one of aviation's most important parameters: a ceiling below 200 ft (60 m) with visibility <550 m prevents landings at most airports.
It is measured by ceilometers or visual estimation. In TAFs, predicted ceiling changes determine whether a flight can legally depart. IFR conditions (instrument flight) are declared with ceiling <1,000 ft or visibility <3 miles; VFR (visual) requires ceiling >3,000 ft and visibility >5 miles.