From AMS Glossary
The general term for instruments that measure the height of cloud bases.
Cloud-height indicators may be classified according to their principle of operation. One class of instrument is based on height determination by means of the principle of triangulation. A beam of light projected from the ground onto the base of the cloud is observed visually or electrically from a remote point. The height of the cloud is determined trigonometrically from a measurement of the angle defined by the light projector, the observation point, and the light spot on the cloud plus the distance between the light projector and the observation point. Examples of instruments based on this principle are the ceilometer and the ceiling light. A second class of instrument is based on pulse techniques. The time required for a pulse of energy to travel from a radiator located on the ground to the cloud base and back to the ground is measured electrically. The height of the cloud is computed from this transit time and a knowledge of the propagation velocity of the pulse. Instruments based on this principle include the pulsed-light cloud-height indicator and vertically directed cloud-detection radar.