Definition
A ground-based instrument that automatically measures the height of cloud bases above the surface by directing a vertical light beam (typically a laser) upward and timing the reflection from cloud particles.
Plain English
A device on the ground that shines a light straight up and measures how long it takes to bounce off the bottom of the clouds, using that timing to figure out how high the clouds are.
Context Anchor
Seen in automated airport weather systems and weather observations that report cloud height or ceiling.
Derivation
From 'ceiling' (the height of the lowest broken or overcast cloud layer) plus '-meter' (a measuring device). Knowing this makes the function obvious: it is literally a ceiling-measurer.
Why Pilots Care
Ceiling reports determine whether VFR flight is possible or IFR procedures are required, directly affecting go/no-go decisions and approach minimums.
Grounding Statement
A ceilometer samples the sky directly above its sensor, so it is measuring conditions at that spot, not every cloud around the airport.
Intuition Check
A ceilometer does not measure a building ceiling or the whole sky around an airport. It measures the height of clouds or obscuration above the weather sensor.
Example Sentence 1
The ASOS ceilometer reported a broken layer at 1,200 feet, putting the field below VFR minimums.
Example Sentence 2
Before departure the pilot checked the ceilometer data to confirm the reported ceiling met visual approach requirements.