Prevailing visibility in

aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' include fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air aircraft such as h ...

is a measurement of the greatest distance visible throughout at least half of the

horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...

, 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#Load-bearing, structure with a roof, walls and window, windows, usually standing permanently in one place, such as a house or factory. Buildings come in a variety of sizes, shapes, and functions, a ...

hill A hill is a landform that extends above the surrounding terrain. It often has a distinct summit, and is usually applied to peaks which are above elevation compared to the relative landmass, though not as prominent as Mountain, mountains. Hills ...

s, or other

geographic Geography (from Ancient Greek ; combining 'Earth' and 'write', literally 'Earth writing') is the study of the lands, features, inhabitants, and phenomena of Earth. Geography is an all-encompassing discipline that seeks an understanding o ...

features. The value obtained is usually reported on a

METAR METAR is a format for reporting weather information. A METAR weather report is predominantly used by aircraft pilots, and by meteorologists, who use aggregated METAR information to assist in weather forecasting. Raw METAR is highly standardize ...

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 an example of vertical extent, "This basketball player is 7 foot 1 inches in height." For an e ...

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 {{Aviation-stub