Definition
The height above the ground or water of the base of the lowest layer of cloud below 6,000 metres (20,000 feet) covering more than half the sky.
Plain English
The height of the bottom of the lowest cloud layer that covers more than half the sky, as long as that layer is below 20,000 feet. If clouds cover only a small portion of the sky, there is no ceiling.
Context Anchor
Seen in aviation weather reports, forecasts, and flight planning when deciding whether cloud height is high enough for the flight.
Derivation
From the everyday word 'ceiling,' the upper inside surface of a room. In weather use, the cloud base acts like a roof above the aircraft, limiting how high you can fly visually.
Why Pilots Care
Determines whether VFR flight is allowed or IFR procedures and minimums must be used.
Analogy
Think of walking under a low roof outside. The important number is not how thick the roof is, but how much space you have between the ground and the roof above you.
Intuition Check
Do not read ceiling as the highest cloud or the top of the clouds. In this context, it means the height of the bottom of the lowest cloud layer that covers more than half the sky.
Example Sentence 1
The METAR reported a ceiling of 1,500 feet, so the pilot reviewed the instrument approach before departure.
Example Sentence 2
With the reported ceiling at 2500 feet we were still legal for the VFR cross-country.