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Which limitation do automatic visibility sensors have?

  • A.They require calibration against a human observer at least once during every hour of their operation
  • B.They sample a very small volume of air
  • C.They cannot operate at night, since they rely on the natural illumination available during the daytime hours
  • D.They measure only humidity

Answer: B

A forward scatter sensor samples a volume of a few cubic centimetres and assumes it is representative of the whole path, which fails in patchy fog. Human observers integrate over a much longer path and judge the visibility of actual objects. The discrepancy is a recognised issue at airports where fog is often highly variable across the site.

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