- an automatic device for measuring and recording the height of clouds by projecting a modulated beam of light onto a cloud base, receiving the reflection of light through a photoelectric apparatus, and computing the height by triangulation.
- a device for determining the cloud ceiling, esp by means of a reflected light beam
- An instrument for calculating the altitude of the lowest cloud layer in the sky. A ceilometer consists of a projector that shines an intense beam of light (usually laser light) at the cloud layer and a photoelectric cell that detects the light as it is reflected back. The distance between the projector and the photoelectric cell and the angle at which the light is detected are used to calculate the altitude of the cloud layer by triangulation; alternatively, the time required for the light to be reflected back to the detector may be used to measure the distance, as in radar.