Definition
A ground-based light that projects a narrow, vertical beam of light onto the base of a cloud layer. By measuring the angle from a known horizontal distance to the illuminated spot on the cloud, an observer can calculate the height of the cloud base above the ground.
Plain English
A light on the ground that shines straight up at the clouds so someone nearby can work out how high the clouds are by looking at the angle to the bright spot on the cloud.
Context Anchor
Seen in aircraft interior lighting descriptions, cockpit lighting checks, and night or low-light operations.
Derivation
In aviation weather, 'ceiling' refers to the height of the lowest broken or overcast cloud layer above the ground — the 'roof' of usable airspace below the clouds. A 'ceiling light' is simply the light used to measure that ceiling.
Why Pilots Care
Ensures safe visibility for crew and passengers during low-light operations.
Intuition Check
Do not confuse “ceiling” here with a cloud ceiling or the aircraft’s maximum altitude. In this term, it means the inside top surface of the aircraft cabin or cockpit.
Example Sentence 1
The weather observer used the ceiling light and a clinometer to determine that the overcast layer was at 1,200 feet.
Example Sentence 2
Cabin crew adjusted the ceiling lights before boarding to create a welcoming environment.