Definition
A ground-based weather instrument that measures cloud base height by sweeping a narrow light beam upward at a changing angle and detecting the angle at which the beam reflects off the underside of a cloud. The reported value is the height of the cloud base above the instrument, expressed in feet.
Plain English
A device on the ground that shines a moving light beam at the sky and works out how high the bottom of the cloud is by the angle of the reflection.
Context Anchor
Seen in FAA acronym and weather-observation references for airport equipment that helps report cloud ceiling.
Derivation
Ceilometer comes from 'ceiling' (the height of the cloud base) plus the Greek 'metron' meaning 'measure'. So a ceilometer is literally a 'ceiling measurer'. 'Rotating beam' describes how this older type works — the light beam rotates through a range of angles to find the cloud.
Why Pilots Care
Accurate ceiling data helps pilots determine if conditions are VFR or IFR and plan safe takeoffs and landings.
Intuition Check
Do not read “ceiling” here as the roof of a room. In aviation weather, ceiling means the height of the lowest cloud layer that significantly covers the sky.
Example Sentence 1
The reported 1,200-foot ceiling at the field came from the rotating beam ceilometer near the runway.
Example Sentence 2
Maintenance crews calibrate the RBC regularly to ensure reliable cloud height reports.