measure of cloud ceiling

Cloud ceiling: altitude of the base of the clouds, expressed in feet.

ceiling projector

Spotlight whose point of luminous impact on a cloud serves as a reference for an alidade or theodolite sighting.

theodolite

Instrument used to measure angles whose intervals indicate the height of a given point in relation to another.

alidade

Instrument whose sighting axis, by moving along a calibrated circle, measures a cloud’s angle in relation to the horizon, and thus its height.