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Inmeteorology,visibility is the measure of thedistance at which an object or light can be clearly discerned. It depends on thetransparency of the surroundingair and as such, it is unchanging no matter the ambient light level or time of day. It is reported withinsurface weather observations andMETAR code either inmeters orstatute miles, depending upon the country. Visibility affects all forms of traffic:roads,railways, sailing andaviation.
The geometric range of vision is limited by thecurvature of the Earth and depends on the eye level and the height of the object being viewed. Ingeodesy, theatmospheric refraction must be taken into account when calculating geodetic visibility.
ICAO Annex 3Meteorological Service for International Air Navigation[1] contains the following definitions and note:
Annex 3[1] also definesRunway Visual Range (RVR) as:
In extremely clean air in Arctic or mountainous areas, the visibility can be up to 240 km (150 miles) where there are large markers such as mountains or high ridges. However, visibility is often reduced somewhat byair pollution and highhumidity. Variousweather stations report this ashaze (dry) ormist (moist).Fog andsmoke can reduce visibility to near zero, makingdriving extremely dangerous. The same can happen in asandstorm in and neardesert areas, or withforest fires. Heavyrain (such as from athunderstorm) not only causes low visibility, but the inability tobrake quickly due tohydroplaning.Blizzards and ground blizzards (blowing snow) are also defined in part by low visibility.
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To define visibility the case of aperfectly black object being viewed against a perfectly white background is examined. Thevisual contrast,CV(x), at a distancex from the black object is defined as the relative difference between the light intensity of the background and the object
whereFB(x) andF(x) are the intensities of the background and the object, respectively. Because the object is assumed to be perfectly black, it must absorb all of the light incident on it. Thus whenx=0 (at the object),F(0) = 0 andCV(0) = 1.
Between the object and the observer,F(x) is affected by additional light that isscattered into the observer's line of sight and theabsorption of light by gases andparticles. Light scattered by particles outside of a particular beam may ultimately contribute to theirradiance at the target, a phenomenon known asmultiple scattering. Unlike absorbed light, scattered light is not lost from a system. Rather, it can change directions and contribute to other directions. It is only lost from the original beam traveling in one particular direction. The multiple scatterings' contribution to the irradiance atx is modified by the individual particle scattering coefficient, the number concentration of particles, and the depth of the beam. The intensity changedF is the result of these effects over a distancedx. Becausedx is a measure of the amount of suspended gases and particles, the fraction ofF that is diminished is assumed to be proportional to the distance,dx. The fractional reduction inF is
wherebext is theattenuation coefficient. The scattering of background light into the observer's line of sight can increaseF over the distancedx. This increase is defined asb' FB(x)dx, whereb' is a constant. The overall change in intensity is expressed as
SinceFB represents the background intensity, it is independent ofx by definition. Therefore,
It is clear from this expression thatb' must be equal tobext. Thus, the visual contrast,CV(x), obeys theBeer–Lambert law
which means that the contrast decreases exponentially with the distance from the object:
Lab experiments have determined that contrast ratios between 0.018 and 0.03 are perceptible under typical daylight viewing conditions. Usually, a contrast ratio of 2% (CV = 0.02) is used to calculate visual range. Plugging this value into the above equation and solving forx produces the following visual range expression (the Koschmieder equation):
withxV in units of length. At sea level, theRayleigh atmosphere has an extinction coefficient of approximately 13.2 × 10−6 m−1 at awavelength of 520 nm. This means that in the cleanest possible atmosphere, visibility is limited to about 296 km.
Visibility perception depends on several physical and visual factors. A realistic definition should consider the fact that the human visual system (HVS) is highly sensitive to spatial frequencies, and then to use the Fourier transform and the contrast sensitivityfunction of the HVS to assess visibility.[2]
The international definition offog is a visibility of less than 1 km (3,300 ft);mist is a visibility of between 1 km (0.62 mi) and 2 km (1.2 mi) andhaze from 2 km (1.2 mi) to 5 km (3.1 mi). 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 asthermal imagers (TI/FLIR) operating in thefar-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.[citation needed]
With fog, occasionalfreezing drizzle andsnow can occur. This usually occurs when temperatures are below 0 °C (32 °F). These conditions are hazardous due to ice formation, which can be deadly, particularly so because of the low visibility, which usually accompanies these conditions at under 1,000 yards. The combination of low visibility and ice formation can lead to accidents on roadways. These cold weather events are caused largely by low-lyingstratus clouds.
Visibility of less than 100 metres (330 ft) 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 aftertraffic collisions orpile-ups involving multiple vehicles.
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 avoidtravel until the fog dissipates or other conditions improve.Airport travel is also often delayed by low visibility, sometimes causing long waits due toapproach visibility minimums and the difficulty of safely moving aircraft on the ground in low visibility.[3][4]
A visibility reduction is probably the most apparent symptom ofair pollution. Visibility degradation is caused by theabsorption andscattering of light by particles and gases in theatmosphere. Absorption ofelectromagnetic 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 impairs visibility much more readily. Visibility is reduced by significant scattering from particles between an observer and a distant object. The particles scatter light from thesun 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 unitaerosol 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 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.[5]
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The geographical visibility depends on the altitude of the observation site and the topology of its surroundings. Planes and water surfaces provide a maximum range of vision, but vegetation, buildings and mountains are geographical obstacles that limit the geographical visibility. When the sky is clear and the meteorological visibility is high, thecurvature of the earth limits the maximum possible geodetic visibility. The visibility from an elevated observation point down to the surface of the sea can be calculated using thePythagorean theorem, since theline of sight and theradius of the Earth form the two legs of aright triangle. The height of the elevated point plus the Earth radius form itshypotenuse. If both the eyes and the object are raised above the reference plane, there are two right-angled triangles. The tangent touching the surface of the Earth or water consists of the two short legs of the two right triangles, which are added together to calculate the geometric range of vision.
Ingeodesy theatmospheric refraction is always taken into account in the calculation, which increases the range of vision, so that even objects behind the horizon can still be seen.
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