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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.
device for measuring the height of cloud bases. One important use of the ceilometer is to determine cloud ceilings at airports. The device works day or night by shining an intense beam of light (often ultraviolet), modulated at an audio frequency, at overhead clouds. Reflections of this light from the base of the clouds are detected by a photocell in the receiver of the ceilometer.