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A ceilometer is an instrument most often used to indicate the current height of a cloud base. An optical ceilometer projects a modulated beam of light onto the bottom of a cloud. Its height can then be computed using triangulation. A laser ceilometer determines the height by measuring the time it takes for a pulse of laser light to be reflected back from a cloud base. Aerosol concentrations, such as water vapor or pollutants in the atmosphere, can also be calculated from the backscatter effects of the laser light.
This device is widely used in aviation and meteorology. The height of the lowest cloud layer below 20,000 feet (6,096 m) that covers more than half the sky is the cloud ceiling. This status is continuously monitored by ceilometers at major airports and the result is reported to flight crews. Makers of advanced ceilometers claim that their products can simultaneously measure multiple cloud layers up to 30,000 feet (9,144 m) when used in meteorological research.
An optical ceilometer consists of a projector, a detector and a means to record the data. There are two basic configurations. In a rotating-transmitter ceilometer, the projector sweeps the sky with a modulated beam of light. The detector is at a known distance from the projector and pointed vertically. When the beam of light hits a cloud base directly above the detector, the light is reflected downward, detected, and the angle of projection at that moment is recorded.
In the scanning-receiver configuration, the projector is fixed and the detector moves. The projector transmits a modulated beam of light vertically. A parabolic detector, stationed a predetermined distance away, scans the beam up and down searching for light reflected from a cloud base. When detected, this gives the vertical angle for the intersection point of the light beam and the cloud. Either configuration provides the data necessary to calculate the height of the cloud base by simple triangulation.
A laser ceilometer is a special application of a light detection and ranging (LIDAR) system. As such, it was one of the first applications of laser technology. In meteorology, a laser ceilometer is used to study atmospheric structure and composition, aerosol concentrations, and cloud formation. Backscatter profiles from laser light can be analyzed to detect precipitation, the presence of particular gasses and their concentration, as well as wind speed and turbulence. The ability to detect current and changing wind conditions has led to the use of this technology in the renewable energy field.
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