Visibility

Visibility refers to the greatest distance at which a deep color object, subtending an angle between 0.5° to 5°, can be seen and recognized by an observer with normal vision. Suspended particulates in the air, dust, fog, haze, precipitation and spray will affect visibility.

Visibility measurement

1)Human observation

Reference objects for visibility measurement:

(a)  Visibility map: a map with a series of concentric circles is used to determine distance of the reference objects from the observer.

Visibility map
(b)   Panoramic photograph: landmarks in a photo providing quick and convenient reference.
The landmarks shown on a weather photo taken at Central Pier
in the Hong Kong Observatory’s “Regional Weather “ webpage
2)Automatic measurement

Visibility meter

Operating principle of a visibility meter:
The more particulates in the air, the higher the intensity of the received scattered light, and the lower the visibility.

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