Ceilometers are stand-alone instruments designed to measure cloud and aerosol height profiles using a laser based LIDAR (Light Detection And Ranging) technique. The light emitting component is a low power diode laser with the output power limited to an eye-safe level. They determine cloud base heights, penetration depths, mixing layer height and vertical visibilities. Within their operating range they reliably detect multiple cloud layers and cirrus clouds.
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 photo detector pointing vertically. When it detects the projected light return from the cloud base, the instrument notes the angle and the calculation gives the height of clouds.
This is a device that uses a beam of intense light to reflect off clouds to measure the height of the bottom of the cloud layer. For an individual to measure cloud height it is very difficult so the easiest way would to compare with a nearby mountain, if any. In many areas airports put out hourly reports on weather, including ceiling (cloud height).
They are most commonly used as part of an AWOS system at airports where cloud height and vertical visibilities are essential to the safe operation of the air traffic systems.
How it Works?
The device works day or night by shining an intense beam of light (often produced by an infrared or ultraviolet transmitter or a laser), modulated at an audio frequency, at overhead clouds. Reflections of this light from the base of the clouds are detected by a photocell in the receiver of the ceilometer. There are two basic types of ceilometers: the scanning receiver and the rotating transmitter.
The scanning-receiver ceilometer has its separate light transmitter fixed to direct its beam vertically. The receiver is stationed a known distance away. The parabolic collector of the receiver continuously scans up and down the vertical beam, searching for the point where the light intersects a cloud base. When a reflection is detected, the ceilometer measures the vertical angle to the spot; a simple trigonometric calculation then yields the height of the cloud ceiling. Many modern scanning-receiver ceilometers use a laser pulse to identify the height of a cloud’s base and top and various points in between to create a vertical profile of the cloud.
The rotating-transmitter ceilometer has its separate receiver fixed to direct reflections only from directly overhead while the transmitter sweeps the sky. When the modulated beam intersects a cloud base directly over the receiver, light is reflected downward and detected.
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