Ceilometer

A ceilometer is a device that uses a laser or other light source to determine the height of a cloud base.
An optical drum ceilometer uses triangulation to determine the height of a spot of light projected onto the base of the cloud. It consists essentially of a rotating projector, a detector, and a recorder. The projector emits an intense beam of light above into the sky at an angle that varies with the rotation. The detector, which is located at a fixed distance from the projector, uses a photoelectric cell pointing vertically. When it detects the projected light return from the cloud base, the instrument note the angle and the calculation gives the height of clouds.
A laser ceilometer consist on a vertically pointing laser and a receiver in the same location. It determines the height by measuring the time (δt) required for a pulse of light to be scattered back from the cloud base