is a measure of the distance
at which an object or light can be clearly discerned. It is reported within surface weather observations
code either in meters or statute miles, depending upon the country. Visibility affects 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 daylight for the same air.
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)
- 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 to . 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.
Fog, mist, and haze
The international definition of fog is a visibility of less than ; mist is a visibility of between and and haze from to . Fog and mist are generally assumed to be composed principally of water droplets, haze and smoke can be of smaller particle size; this has implications for sensors such as Thermal Imagers (TI/FLIR) operating in the far-IR at wavelengths of about 10 μm which are better able to penetrate haze and some smokes because their particle size is smaller than the wavelength; the IR radiation is therefore not significantly deflected or absorbed by the particles
Very low visibility
Visibility of less than 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
of several (or even dozens
) of automobiles
have occurred there.
Low visibility warnings
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