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Which quantity does a transmissometer measure?

  • A.The attenuation of a light beam over a fixed baseline
  • B.The concentration of water droplets within the fog, from which the visibility is subsequently calculated
  • C.The dew point
  • D.The size distribution of the aerosol particles suspended within the layer of air being sampled at the site

Answer: A

A transmissometer projects a beam over a baseline of a few tens of metres and measures how much reaches the receiver, from which the extinction coefficient and hence the visibility follow. Forward scatter sensors are now more common because they need no long baseline. Both infer visibility from optical extinction rather than measuring what an observer would see.

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