Prevailing visibility in
aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot a ...
is a measurement of the greatest distance visible throughout at least half of the
horizon, not necessarily continuously. To take the prevailing visibility,
controllers reference a number of visual reference points: usually
building
A building, or edifice, is an enclosed structure with a roof and walls standing more or less permanently in one place, such as a house or factory (although there's also portable buildings). Buildings come in a variety of sizes, shapes, and fu ...
s,
hill
A hill is a landform that extends above the surrounding terrain. It often has a distinct summit.
Terminology
The distinction between a hill and a mountain is unclear and largely subjective, but a hill is universally considered to be not a ...
s, or other
geographic
Geography (from Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘Graphien’ (to describe), literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and ...
features. The value obtained is usually reported on a
METAR and recorded on the
ATIS.
In Canada, it is the maximum visibility value common to sectors comprising one-half or more of the horizon circle. The horizon circle may be divided into as many sections as required based on differing visibilities.
When the prevailing visibility is less than 4 miles, or goes from less than 4 miles to four miles or greater, tower personnel are required to take a prevailing visibility reading. If this reading differs from the ground reading (i.e., different readings for different
height
Height is measure of vertical distance, either vertical extent (how "tall" something or someone is) or vertical position (how "high" a point is).
For example, "The height of that building is 50 m" or "The height of an airplane in-flight is ab ...
s), the lower value of the two is used for aircraft operations. Also visibility is defined as greatest distance at which an object of specified characteristics can be seen and identified by an observer with normal flight under normal condition in daylight.
References
Aviation meteorology
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