Visibility
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In meteorology, visibility is a measure of the distance at which an object or light can be seen. It is important to all forms of traffic: roads, sailing and aviation. Meteorological visibility refers to transparency of air: in dark, meteorological visibility is still the same as in the same air in daylight.
For aviation, visibility is defined as the greater of:
- a) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background;
- b) the greatest distance at which lights in the vicinity of 1,000 candelas can be seen and identified against an unlit background.
In aviation another term, Runway Visual Range (RVR) is also defined.
In extremely clean air in Arctic or mountainous areas, the visibility can be up to 70 to 100 km. However, visibility is often reduced somewhat by air pollution and high humidity. Various weather stations report this as haze (dry) or mist (moist). Fog and smoke can reduce visibility to near zero, making driving extremely dangerous. The same can happen in a sandstorm in and near desert areas, or with forest fires. Heavy rain (such as from a thunderstorm) not only causes low visibility, but the inability to brake quickly due to hydroplaning. Blizzards and ground blizzards (blowing snow) are also defined in part by low visibility.
Low-visibility conditions are generally one mile or 1600 meters or less. Visibility of less than 100 meters or 1/16th of a mile are usually reported as zero. In these conditions, roads may be closed, or automatic warning lights and signs may be activated to warn drivers. These have been put in place in certain areas that are prone to repeatedly low visibility, particularly after massive pile-up accidents involving collisions of several (or even dozens) of automobiles have occurred there.
In addition, an advisory is often issued by a government weather agency for low visibility, such as a dense fog advisory from the U.S. National Weather Service. These generally advise motorists to avoid travel until the fog burns off or other conditions improve. Airport travel is also often delayed by low visibility, sometimes causing long waits due to instrument flight rules and wider spacing of aircraft.
Visibility is measured by scattering of light. Either some light is transmitted, and the device measures how much is received a distance away, or the scattered portion is measured directly.
[edit] See also
- Obscuration
- Plasmonic cover
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