Ceilometer

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Laser Ceilometer

A ceilometer is a device using 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 ( $δ t$ ) required for a pulse of light to be scattered back from the cloud base

$distance = \frac {c \delta t}{2}$ where c is the light speed in the air.

Because of light diffusion by air molecules, ceilometers are not effective beyond 4,000 m (13,123 ft) in height. Additionally, for cloud base determination purpose, due to the ceilometers ability to pick up any particle in the air (dust, precipitation, smoke, etc...), it will give occasional false readings. As an example, depending on the threshold used, falling Diamond dust (Ice Crystals) may cause the ceilometer to report a cloud height of zero, even though the sky is clear.

Using these last properties, ceilometers will have other uses. Since the instrument will note any returns, it is possible to locate any faint layer where it occurs, additionally to the cloud's base, by looking at the whole pattern of returned energy. Furthermore, the rate at which diffusion happens can be noted by the diminishing part returned to the ceilometer in clear air, giving the coefficient of extinction of the light signal. Using these data could give the vertical visibility and the possible concentration of air pollutants. This has been developed in research and could be applied for operational purpose.