A ceilometer (from ceiling, the cloud base height) is a meteorological instrument that measures the height of the cloud base by emitting vertical laser pulses and timing the return of the backscattered signal. It is essentially a simplified lidar, optimised for a specific function: determining the cloud "ceiling".
Operation and precision
The ceilometer emits laser pulses (typically near-infrared, 905 or 1064 nm) at a frequency of several thousand per second. When the beam encounters a cloud base, part of the light is backscattered toward the receiver. The round-trip time, multiplied by the speed of light and divided by two, gives the distance to the cloud with an accuracy of ±5-10 m. Modern models can detect up to 3-4 simultaneous cloud layers up to 13 km altitude.
In aviation, ceilometer data is critical: it determines whether visual flight rules (VFR) or instrument flight rules (IFR) apply, and is essential for the METAR and TAF reports that pilots receive. A cloud ceiling below 60 m (200 ft) prevents most instrument approaches. All certified airports have at least one ceilometer alongside each runway.
Ceilometers are also used in research to monitor the planetary boundary layer, detect volcanic ash and aerosols, and in Saharan haze monitoring networks. In Spain, AEMET's network includes ceilometers at all major airports. See also: meteorological lidar, automatic weather station.