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| In [[meteorology]], '''visibility''' is a measure of the [[distance]] at which an object or light can be clearly discerned. It is reported within [[surface weather observation]]s and [[METAR]] code either in [[Metre|meters]] or [[Mile#Statute_mile|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.
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| ==Definition==
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| [[File:Plane flying into clouds.jpg|thumb|right|150px|A commercial aircraft flying into the clouds over Los Angeles]]
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| [[ICAO]] Annex 3 ''Meteorological Service for International Air Navigation'' contains the following definitions and note:
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| :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;
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| :b) the greatest distance at which lights of 1,000 [[candela]]s can be seen and identified against an unlit background.
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| :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).
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| Annex 3 also defines [[Runway visual range|Runway Visual Range (RVR)]] as:
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| :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.
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| [[File:Foggy morning road - visibility at 200 ft.jpg|thumb|250px|[[Fog]]gy morning road]]
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| [[File:Tel Aviv Carmel Panorama.jpg|thumb|240px|On clear days, Tel Aviv's skyline is visible from the Carmel mountains, 80km north]]
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| In extremely clean air in Arctic or mountainous areas, the visibility can be up to {{convert|70|km|mi}} to {{convert|100|km|mi}}. However, visibility is often reduced somewhat by [[air pollution]] and high [[humidity]]. Various [[weather station]]s 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 [[Dust storm|sandstorm]] in and near [[desert]] areas, or with [[forest fire]]s. Heavy [[rain]] (such as from a [[thunderstorm]]) not only causes low visibility, but the inability to [[brake]] quickly due to [[Hydroplaning (tires)|hydroplaning]]. [[Blizzard]]s and ground blizzards (blowing snow) are also defined in part by low visibility.
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| ==Derivation==
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| To define visibility, we examine the case of a perfectly [[Black body|black]] object being viewed against a perfectly white background. The [[Contrast (vision)|visual contrast]], ''C''<sub>V</sub>(x), at a distance ''x'' from the [[Black body|black]] object is defined as the relative difference between the light intensity of the background and the object
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| <math>C_\text{V}(x) = \frac{F_\text{B}(x) - F(x)}{F_\text{B}(x)}</math>
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| where ''F''<sub>B</sub>(x) and ''F''(x) are the intensities of the background and the object, respectively. Because the object is assumed to be perfectly [[Black body|black]], it must absorb all of the light incident on it. Thus when ''x''=0 (at the object), ''F''(0) = 0 and ''C<sub>V</sub>''(0) = 1. Between the object and the observer, ''F''(x) is affected by additional light that is [[scattering|scattered]] into the observer's line of sight and the [[Absorption (electromagnetic radiation)|absorption]] of light by gases and [[Particulate|particles]]. Light scattered by [[Particulate|particles]] outside of a particular beam may ultimately contribute to the [[irradiance]] at the target, a phenomenon known as [[multiple 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 scattering]]'s contribution to the [[irradiance]] at ''x'' is modified by the individual particle scattering coefficient, the number concentration of particles, and the depth of the beam. The intensity change ''dF'' is the result of these effects over a distance ''dx''. Because ''dx'' is a measure of the amount of suspended gases and particles, the fraction of ''F'' that is diminished is assumed to be proportional to the distance, ''dx''. The fractional reduction in ''F'' is
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| <math>dF = -b_\text{ext} F dx</math>
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| where ''b''<sub>ext</sub> is the [[attenuation coefficient]]. The [[scattering]] of background light into the observer's line of sight can increase ''F'' over the distance ''dx''. This increase is defined as ''b' F''<sub>B</sub>(''x'') ''dx'', where ''b''' is a constant. The overall change in intensity is expressed as
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| <math>dF(x) = \left[b' F_\text{b}(x) - b_\text{ext} F(x)\right] dx</math>
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| Since ''F''<sub>B</sub> represents the background intensity, it is independent of ''x'' by definition. Therefore,
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| <math>dF_\text{B}(x) = 0 = \left[b' F_\text{B}(x) - b_\text{ext} F_\text{b}(x)\right] dx</math>
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| It is clear from this expression that ''b''' must be equal to ''b''<sub>ext</sub>. Thus, the [[Contrast (vision)|visual contrast]], ''C''<sub>V</sub>(''x''), obeys the [[Beer–Lambert law]]
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| <math>\frac{dC_\text{V}(x)}{dx} = - b_\text{ext} C_\text{V}(x)</math>
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| which means that the [[Contrast (vision)|visual contrast]] decreases exponentially with the distance from the object:
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| <math>C_\text{V}(x) = \exp(- b_\text{ext} x)</math>
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| Lab experiments have determined that contrast ratios between 0.018 and 0.03 are perceptible under typical daylight viewing conditions. A contrast ratio of 2% (''C''<sub>V</sub> = 0.02) is usually used to calculate visual range. Plugging this value into the above equation and solving for ''x'' produces the following visual range expression (the Koschmeider equation):
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| <math>x_\text{V} = \frac{3.912}{b_\text{ext}}</math>
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| with ''x''<sub>V</sub> in units of length. At sea level, the [[Rayleigh scattering|Rayleigh atmosphere]] has an extinction coefficient of approximately 13.2 × 10<sup>−6</sup> m<sup>−1</sup> at a [[wavelength]] of 520 nm. This means that in the cleanest possible atmosphere, visibility is limited to about 296 km.
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| ==Fog, mist, and haze==
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| The international definition of fog is a visibility of less than {{convert|1|km|ft}}; mist is a visibility of between {{convert|1|km|mi}} and {{convert|2|km|mi}} and haze from {{convert|2|km|mi}} to {{convert|5|km|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 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.{{Citation needed|date=March 2010}}
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| ==Very low visibility==
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| Visibility of less than {{convert|100|m|ft}} is usually reported as zero. In these conditions, [[road]]s 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 collision]]s or [[pile-up]]s involving multiple vehicles.
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| ==Low visibility warnings==
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| 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]].{{Citation needed|date=March 2010}} | |
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| ==Visibility and air pollution==
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| A visibility reduction is probably the most apparent symptom of air pollution. Visibility degradation is caused by the [[Absorption (electromagnetic radiation)|absorption]] and [[scattering]] of light by [[particulate|particles]] and gases in the [[Atmosphere of Earth|atmosphere]]. [[Absorption (electromagnetic radiation)|Absorption]] of [[electromagnetic radiation]] by gases and [[Particulate|particles]] is sometimes the cause of discolorations in the atmosphere but usually does not contribute very significantly to visibility degradation. [[Scattering]] by [[particulate]], on the other hand, impairs visibility much more readily. Visibility is reduced by significant [[scattering]] from [[Particulate|particles]] between an observer and a distant object. The [[Particulate|particles]] [[scattering|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.
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| ==See also==
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| *[[Coefficient of haze]]
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| ==References==
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| * {{cite book|last1=Seinfeld|first1=John H.| author1-link=John H. Seinfeld|last2=Pandis|first2=Spyros N. |year=2006|title= Atmospheric Chemistry and Physics - From Air Pollution to Climate Change |edition=2nd |publisher=John Wiley and Sons, Inc. |isbn=0-471-82857-2 }}
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| {{Meteorological variables}}
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| [[Category:Visibility| ]]
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| [[Category:Basic meteorological concepts and phenomena]] | |
Luke Bryan can be a celebrity from the creating plus the vocation growth initial second to his third hotel record, & , is definitely the resistant. He burst to the picture in 2004 along with his amazing combination of down-residence convenience, motion picture star good seems and lines, is defined t within a key way. The brand new recording Top around the luke bryan tickets sacramento - simply click the following internet page - country chart and #2 on the take charts, building it the next maximum debut during that time of 2003 for the land artist.
The kid of any , is aware of persistence and dedication are key elements with regards to an effective job- . His initial album, Keep Me, made the very best hits “All My Pals Say” and “Country Gentleman,” when his hard work, Doin’ Factor, discovered the vocalist-three direct No. 7 single people: In addition Calling Is really a Great Point.”
Inside the tumble of 2011, Tour: Luke And which had an outstanding listing of , which include Downtown. “It’s almost like you are acquiring a authorization to visit one stage further, claims those performers which were a part of the Concertsaround into a larger sized level of performers.” It covered as one of the most successful trips where can i buy luke bryan tickets in their ten-year historical past.
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