Visibility Sensors Information
Visibility sensors detect atmospheric transparency and output a sensor equivalent visibility (SEV) range that represents the maximum distance that the human eye can see under given atmospheric conditions. They offer a standardized method for assessing visibility range when it is impaired by fog, cloud cover, snow, smoke, or other precipitation.
Common methods used to detect visibility range include infrared forward-scatter technology, transmissometers, and light detection and ranging (LIDAR).
Forward scatter sensors include an emitter and receiver. They emit an infrared light beam at an offset angle from the receiver. The amount of light scattered by particles in the air and received by the receiver determines the extinction coefficient, from which visibility range is calculated by applying either Allard's or Koschmieder's law.
Transmissometer sensors include an emitter and receiver. Like forward scatter sensors they use the extinction coefficient to calculate visibility range. The extinction coefficient is determined by measuring direct light transmissivity.
LIDAR is a light detection and ranging sensor used for long range visibility sensing. They transmit low-power laser pulses and measure back scatter from clouds and aerosols. They sense cloud height and vertical visibility range.
Applications for visibility sensors include meteorological and transportation applications including aviation, marine, and road condition assessments. Meteorologists use visibility sensors to remotely monitor the visibility range. They may be deployed in remote weather stations while the most common application for visibility sensors is for aviation when assessing visibility at runways.
ICAO 9837 – The Manual on Automatic Meteorological Observing Systems at Aerodromes describes methods to design or update automatic measurement systems for airports and understand the characteristics and limitations of such systems.
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