Definition
ICAO Annex 3 Meteorological Service for International Air Navigation contains the following definitions and note:
- 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 of 1,000 candelas can be seen and identified against an unlit background.
- Note.— The two distances have different values in air of a given extinction coefficient, and the latter b) varies with the background illumination. The former a) is represented by the meteorological optical range (MOR).
Annex 3 also defines Runway Visual Range (RVR) as:
- The range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line.
In extremely clean air in Arctic or mountainous areas, the visibility can be up to 70 kilometres (43 mi) to 100 kilometres (62 mi). 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.
Read more about this topic: Visibility
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