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Ceilometers measure cloud height using triangulation or laser reflection. They are used in aviation and meteorology to monitor cloud ceilings and atmospheric composition. Optical ceilometers have two configurations: rotary transmitter and scanner-receiver. Laser ceilometers use LIDAR technology to study atmospheric structure, composition, and wind conditions.
A ceilometer is an instrument most often used to indicate the current height of a cloud base. An optical ceiling projects a modulated beam of light onto the bottom of a cloud. Its height can then be calculated using triangulation. A laser ceilometer determines height by measuring the time it takes for a pulse of laser light to reflect off a cloud base. Concentrations of aerosols, such as water vapor or pollutants in the atmosphere, can also be calculated from the backscatter effects of 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 of the sky is the cloud ceiling. This status is continuously monitored by ceilometers at major airports and the result is reported to the flight crews. Manufacturers of advanced ceilometers claim 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 for recording data. There are two basic configurations. In a rotary transmitter ceilometer, the searchlight sweeps the sky with a modulated beam of light. The detector is at a known distance from the projector and pointed vertically. When the light beam strikes a cloud base directly above the detector, the light is reflected downwards, detected, and the angle of projection at that moment is recorded.
In the scanner-receiver configuration, the projector is fixed and the detector moves. The projector vertically transmits a modulated beam of light. A parabolic detector, positioned at a predetermined distance, scans the beam up and down for light reflected from a cloud base. When detected, it provides the vertical angle for the intersection point of the light beam and the cloud. Both configurations provide the data needed to calculate the height of the cloud base using simple triangulation.
A laser ceilometer is a special application of a light sensing and distance system (LIDAR). As such, it was one of the earliest applications of laser technology. In meteorology, a laser ceilometer is used to study atmospheric structure and composition, aerosol concentrations, and cloud formation. Backscatter profiles of laser light can be analyzed to detect precipitation, the presence of particular gases and their concentration, as well as wind speed and turbulence. The ability to sense current and changing wind conditions has led to the use of this technology in the field of renewable energy.
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