Celometer
A celometer is an apparatus being used to measure the height of the ceiling Nuage ux thanks to the return of a beam of light.
The first celometers used a rotary drum provided with luminous reflectors with narrow beam and a receiver (Electric eye) at a certain distance. When the drum turns, the beam is projected towards the base of clouds and is turned over towards the ground by diffusion. The receiver which can see only with the vertical, one can note the angle of elevation at the time when a signal is received there cloud and, by triangulation, the height of this last.
More recently, the celometers Laser were developed. They emit a luminous impulse and the height of the ceiling is calculated by the measurement of time between the emission and the return of the signal ():
where is the Speed of light.
The celometer is not used only for measurement of the cloudy ceilings. Indeed, the light passing through the atmosphere is reflected by any particle being there (dust, ashes, droplets, etc). Part of the beam of light can thus be turned over towards the sensor before reaching the base of the cloud. This weaker signal also will be received and positioned. It can give an estimate of the vertical visibility, presence of Précipitation S and quantity of pollutants in the air by the calculation of the coefficient of extinction of the signal.
Finally, the cloud having a more or less large density, the beam will be turned over on a certain thickness before being completely blocked by the cloud. The thickness of this layer of return of ceiling gives an idea of the type and density of the cloud. In general, the data of celometers do not exceed 4 km since the beam of light disperses gradually by diffusion on the molecules of air.
See too
External bond
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the laser celometer of the University McGill.