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45-578: Visibility , in meteorology, is a measure of the distance at which an object or light can be seen. Visibility may also refer to: Visible may also refer to: Visibility The geometric range of vision is limited by the curvature of the Earth and depends on the eye level and the height of the object being viewed. In geodesy , the atmospheric refraction must be taken into account when calculating geodetic visibility. ICAO Annex 3 Meteorological Service for International Air Navigation contains

90-408: 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. To define visibility the case of a perfectly black object being viewed against a perfectly white background

135-521: A contrast ratio of 2% ( C V = 0.02) is used to calculate visual range. Plugging this value into the above equation and solving for x produces the following visual range expression (the Koschmieder equation): with x V in units of length. At sea level, the Rayleigh atmosphere has an extinction coefficient of approximately 13.2 × 10 m at a wavelength of 520 nm. This means that in

180-444: 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 where b ext 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 B ( x ) dx , where b'

225-512: A light wave acts on the charges within a particle, causing them to move at the same frequency. The particle, therefore, becomes a small radiating dipole whose radiation we see as scattered light. The particles may be individual atoms or molecules; it can occur when light travels through transparent solids and liquids, but is most prominently seen in gases . Rayleigh scattering of sunlight in Earth's atmosphere causes diffuse sky radiation , which

270-604: Is a constant. The overall change in intensity is expressed as Since F B represents the background intensity, it is independent of x by definition. Therefore, It is clear from this expression that b' must be equal to b ext . Thus, the visual contrast, C V ( x ), obeys the Beer–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,

315-424: 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) and haze 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 as thermal imagers (TI/ FLIR ) operating in

360-502: Is also an important mechanism of wave scattering in amorphous solids such as glass, and is responsible for acoustic wave damping and phonon damping in glasses and granular matter at low or not too high temperatures. This is because in glasses at higher temperatures the Rayleigh-type scattering regime is obscured by the anharmonic damping (typically with a ~ λ dependence on wavelength), which becomes increasingly more important as

405-399: Is capable of calculating MOR is the optical extinction analyzer (OEA). It actually calculates the optical extinction coefficient (ß) by directly measuring the decay time (aka the ring-down time constant) of injected laser light inside an optical cavity containing an ambient gas sample. The OEA is a cavity enhanced absorption spectroscopy (CEAS) technique. Briefly, the injected laser light into

450-407: Is examined. The visual contrast , C V (x), at a distance x from the black object is defined as the relative difference between the light intensity of the background and the object where F B (x) and F (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 when x =0 (at

495-425: Is fast (1 Hz) and fully automated for an unattended OEA operation in the field. 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

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540-528: Is given by I s = I 0 1 + cos 2 ⁡ θ 2 R 2 ( 2 π λ ) 4 ( n 2 − 1 n 2 + 2 ) 2 r 6 {\displaystyle I_{s}=I_{0}{\frac {1+\cos ^{2}\theta }{2R^{2}}}\left({\frac {2\pi }{\lambda }}\right)^{4}\left({\frac {n^{2}-1}{n^{2}+2}}\right)^{2}r^{6}} where R

585-460: Is high, the curvature 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 the Pythagorean theorem , since the line of sight and the radius of the Earth form the two legs of a right triangle . The height of the elevated point plus the Earth radius form its hypotenuse . If both

630-456: Is minor for short visual ranges but must be taken into account for ranges above 30 km. Meteorological optical range (MOR) is a measurement of visibility in kilometers. MOR is the length of path in the atmosphere required to reduce the luminous flux in a collimated beam from an incandescent lamp to 5% of its original value. There are few analytical approaches available to measure visibility (MOR) directly or indirectly. One novel instrument that

675-428: 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 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

720-417: 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. Airport travel is also often delayed by low visibility, sometimes causing long waits due to approach visibility minimums and the difficulty of safely moving aircraft on

765-455: Is reduced by significant scattering from particles between an observer and a distant object. The particles 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

810-665: Is the distance to the particle and θ is the scattering angle. Averaging this over all angles gives the Rayleigh scattering cross-section of the particles in air: σ s = 8 π 3 ( 2 π λ ) 4 ( n 2 − 1 n 2 + 2 ) 2 r 6 . {\displaystyle \sigma _{\text{s}}={\frac {8\pi }{3}}\left({\frac {2\pi }{\lambda }}\right)^{4}\left({\frac {n^{2}-1}{n^{2}+2}}\right)^{2}r^{6}.} Here n

855-443: Is the particle's radius, λ is the wavelength of the light and x is a dimensionless parameter that characterizes the particle's interaction with the incident radiation such that: Objects with x ≫ 1 act as geometric shapes, scattering light according to their projected area. At the intermediate x ≃ 1 of Mie scattering , interference effects develop through phase variations over the object's surface. Rayleigh scattering applies to

900-461: Is the reason for the blue color of the daytime and twilight sky , as well as the yellowish to reddish hue of the low Sun . Sunlight is also subject to Raman scattering , which changes the rotational state of the molecules and gives rise to polarization effects. Scattering by particles with a size comparable to, or larger than, the wavelength of the light is typically treated by the Mie theory ,

945-708: Is the refraction index, p is the photoelastic coefficient of the glass, k is the Boltzmann constant , and β is the isothermal compressibility. T f is a fictive temperature , representing the temperature at which the density fluctuations are "frozen" in the material. Rayleigh-type λ scattering can also be exhibited by porous materials. An example is the strong optical scattering by nanoporous materials. The strong contrast in refractive index between pores and solid parts of sintered alumina results in very strong scattering, with light completely changing direction each five micrometers on average. The λ -type scattering

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990-526: Is the refractive index of the spheres that approximate the molecules of the gas; the index of the gas surrounding the spheres is neglected, an approximation that introduces an error of less than 0.05%. The fraction of light scattered by scattering particles over the unit travel length (e.g., meter) is the number of particles per unit volume N times the cross-section. For example, air has a refractive index of 1.0002793 at atmospheric pressure, where there are about 2 × 10 molecules per cubic meter, and therefore

1035-931: The dielectric constant ϵ {\displaystyle \epsilon } of a certain region of volume V {\displaystyle V} is different from the average dielectric constant of the medium ϵ ¯ {\displaystyle {\bar {\epsilon }}} , then any incident light will be scattered according to the following equation I = I 0 π 2 V 2 σ ϵ 2 2 λ 4 R 2 ( 1 + cos 2 ⁡ θ ) {\displaystyle I=I_{0}{\frac {\pi ^{2}V^{2}\sigma _{\epsilon }^{2}}{2\lambda ^{4}R^{2}}}{\left(1+\cos ^{2}\theta \right)}} where σ ϵ 2 {\displaystyle \sigma _{\epsilon }^{2}} represents

1080-532: The discrete dipole approximation and other computational techniques. Rayleigh scattering applies to particles that are small with respect to wavelengths of light, and that are optically "soft" (i.e., with a refractive index close to 1). Anomalous diffraction theory applies to optically soft but larger particles. In 1869, while attempting to determine whether any contaminants remained in the purified air he used for infrared experiments, John Tyndall discovered that bright light scattering off nanoscopic particulates

1125-533: 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. With fog, occasional freezing drizzle and snow 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

1170-437: The variance of the fluctuation in the dielectric constant ϵ {\displaystyle \epsilon } . The blue color of the sky is a consequence of three factors: The strong wavelength dependence of the Rayleigh scattering (~ λ ) means that shorter ( blue ) wavelengths are scattered more strongly than longer ( red ) wavelengths. This results in the indirect blue and violet light coming from all regions of

1215-525: The OEA built-in algorithm. To that end, the amount of light attenuated due to 1)leakage from the high-reflectivity mirrors and 2)absorption by non-aerosol species present in the gas sample is automatically accounted for by flowing the same analyzed gas sample via an aerosol filter into the cavity to measure the light extinction caused by the aerosol-free gas. More details on the OEA principle of operation can be found here . The above-described MOR determination process

1260-409: The amount of scattering is inversely proportional to the fourth power of the wavelength (e.g., a blue color is scattered much more than a red color as light propagates through air). The phenomenon is named after the 19th-century British physicist Lord Rayleigh (John William Strutt). Rayleigh scattering results from the electric polarizability of the particles. The oscillating electric field of

1305-546: The benefit of James Clerk Maxwell 's 1865 proof of the electromagnetic nature of light , he showed that his equations followed from electromagnetism . In 1899, he showed that they applied to individual molecules, with terms containing particulate volumes and refractive indices replaced with terms for molecular polarizability . The size of a scattering particle is often parameterized by the ratio x = 2 π r λ {\displaystyle x={\frac {2\pi r}{\lambda }}} where r

1350-443: The case when the scattering particle is very small (x ≪ 1, with a particle size < 1/10 of wavelength ) and the whole surface re-radiates with the same phase. Because the particles are randomly positioned, the scattered light arrives at a particular point with a random collection of phases; it is incoherent and the resulting intensity is just the sum of the squares of the amplitudes from each particle and therefore proportional to

1395-529: 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 sensitivity function of the HVS to assess visibility. The international definition of fog

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1440-1178: The dependence on refractive index in terms of the molecular polarizability α , proportional to the dipole moment induced by the electric field of the light. In this case, the Rayleigh scattering intensity for a single particle is given in CGS-units by I s = I 0 8 π 4 α 2 λ 4 R 2 ( 1 + cos 2 ⁡ θ ) {\displaystyle I_{s}=I_{0}{\frac {8\pi ^{4}\alpha ^{2}}{\lambda ^{4}R^{2}}}(1+\cos ^{2}\theta )} and in SI-units by I s = I 0 π 2 α 2 ε 0 2 λ 4 R 2 1 + cos 2 ⁡ ( θ ) 2 . {\displaystyle I_{s}=I_{0}{\frac {\pi ^{2}\alpha ^{2}}{{\varepsilon _{0}}^{2}\lambda ^{4}R^{2}}}{\frac {1+\cos ^{2}(\theta )}{2}}.} When

1485-511: The eruption of Mount Tambora in his lifetime. In locations with little light pollution , the moonlit night sky is also blue, because moonlight is reflected sunlight, with a slightly lower color temperature due to the brownish color of the Moon. The moonlit sky is not perceived as blue, however, because at low light levels human vision comes mainly from rod cells that do not produce any color perception ( Purkinje effect ). Rayleigh scattering

1530-440: 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. In geodesy the atmospheric refraction is always taken into account in the calculation, which increases the range of vision, so that even objects behind

1575-596: The following definitions and note: Annex 3 also defines Runway 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 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

1620-518: The ground in low visibility. A visibility reduction is probably 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 impairs visibility much more readily. Visibility

1665-411: The high-finesse optical cavity "bounces" repeatedly, at resonance, between two opposing mirrors for a total pathlength of several kilometers until it completely decays or "rings down", primarily due to its extinction by the ambient gas sample species flowing through the cavity. After accounting for the light extinction caused by non-aerosol species, the aerosol-induced light extinction is readily derived by

1710-399: The horizon can still be seen. Rayleigh scattering Rayleigh scattering ( / ˈ r eɪ l i / RAY -lee ) is the scattering or deflection of light , or other electromagnetic radiation , by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scattering medium (normal dispersion regime),

1755-401: The inverse fourth power of the wavelength and the sixth power of its size. The wavelength dependence is characteristic of dipole scattering and the volume dependence will apply to any scattering mechanism. In detail, the intensity of light scattered by any one of the small spheres of radius r and refractive index n from a beam of unpolarized light of wavelength λ and intensity I 0

1800-647: 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-lying stratus 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 after traffic collisions or pile-ups involving multiple vehicles. In addition, an advisory

1845-493: The major constituent of the atmosphere, nitrogen, has a Rayleigh cross section of 5.1 × 10  m at a wavelength of 532 nm (green light). This means that about a fraction 10 of the light will be scattered for every meter of travel. The strong wavelength dependence of the scattering (~ λ ) means that shorter (blue) wavelengths are scattered more strongly than longer (red) wavelengths. The expression above can also be written in terms of individual molecules by expressing

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1890-435: The object), F (0) = 0 and C V (0) = 1. Between the object and the observer, F (x) is affected by additional light that is scattered into the observer's line of sight and the absorption of light by gases and particles . Light scattered by 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

1935-406: The sky. The human eye responds to this wavelength combination as if it were a combination of blue and white light. Some of the scattering can also be from sulfate particles. For years after large Plinian eruptions , the blue cast of the sky is notably brightened by the persistent sulfate load of the stratospheric gases. Some works of the artist J. M. W. Turner may owe their vivid red colours to

1980-651: The temperature rises. Rayleigh scattering is an important component of the scattering of optical signals in optical fibers . Silica fibers are glasses, disordered materials with microscopic variations of density and refractive index. These give rise to energy losses due to the scattered light, with the following coefficient: α scat = 8 π 3 3 λ 4 n 8 p 2 k T f β {\displaystyle \alpha _{\text{scat}}={\frac {8\pi ^{3}}{3\lambda ^{4}}}n^{8}p^{2}kT_{\text{f}}\beta } where n

2025-457: Was faintly blue-tinted. He conjectured that a similar scattering of sunlight gave the sky its blue hue , but he could not explain the preference for blue light, nor could atmospheric dust explain the intensity of the sky's color. In 1871, Lord Rayleigh published two papers on the color and polarization of skylight to quantify Tyndall's effect in water droplets in terms of the tiny particulates' volumes and refractive indices . In 1881, with

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