What instrument measures cloud ceiling for aviation weather reporting?

• A. Hygrometer
• B. Barometer
• C. Seismometer
• D. Ceilometer

Answer: (D)

Cloud ceiling is the height of the lowest cloud base that affects aviation operations, so you need an instrument that can detect where the cloud base begins. A ceilometer is designed for this purpose. It sends a beam of light upward—typically a laser or infrared light—and monitors the backscatter as it encounters cloud droplets. When the backscattered signal sharply increases at a certain height, that point marks the cloud base. The device converts that height into the ceiling height used in aviation weather reports like METAR.

The other instruments measure things unrelated to cloud base: a hygrometer gauges humidity, a barometer measures atmospheric pressure, and a seismometer detects ground motion. Hence, the ceilometer is the one that directly provides the cloud ceiling information.