Ceiling (weather)
In weather reporting, the ceiling is the height of the lowest layer of cloud covering more than half the sky, measured above the ground. Along with visibility it determines whether an airport is usable: every instrument approach has a minimum ceiling and visibility, and if the reported weather is below them the crew cannot legally attempt to land. Small airports with only basic approaches may need a ceiling of 800–1,500 ft, while a major airport with ILS Category III can accept a few hundred feet or less. When a charter operator warns that low cloud may force a diversion to a larger airport, it is the ceiling they are watching.