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
Fog, mist, haze, and freezing drizzle 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. Fog occasionally occurs along with
freezing drizzle and
snow. These cold weather events are caused largely by low-lying
stratus clouds. This usually happens when temperatures are below . These conditions are hazardous due to ice formation, which can be deadly because of the low visibility (i.e., less than 1,000 yards) usually accompanying these conditions. The combination of low visibility and ice formation can lead to accidents on roadways.
Very low visibility Visibility of less than is 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 subject to repeatedly low visibility, particularly after
traffic collisions or
pile-ups involving multiple vehicles.
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 dissipates or other conditions improve. Air travel is also often delayed by low visibility, sometimes causing long delays due to
approach visibility minimums and the difficulty of safely moving aircraft on the ground in low visibility.
Visibility and air pollution Reduced visibility is often the most apparent symptom of
air pollution. Visibility degradation is caused by the
absorption and
scattering of light by particles and gases in the
atmosphere. Absorption of
electromagnetic radiation by gases and particles is sometimes the cause of discolorations in the atmosphere, but usually does not contribute very significantly to visibility degradation. Scattering by particulates between an observer and a distant object impairs visibility much more readily. They scatter light from the
sun and the rest of the sky through the line of sight of the observer, thereby decreasing the contrast between the object and the background sky. Particles that are the most effective at reducing visibility (per unit
aerosol mass) have diameters in the range of 0.1-1.0 μm. The effect of air molecules on visibility is minor for short visual ranges but must be taken into account for ranges above 30 km.
Prevailing visibility Prevailing visibility is the greatest horizontal visibility equaled or exceeded throughout at least half the horizon circle, which need not necessarily be continuous. Prevailing visibility is reported in statute miles or fractions of miles. == Geodetic visibility ==