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. If both the eyes and the object are raised above the reference plane, there are two right-angled triangles. The hight of the elevated point plus the Earth radius form its hypothenuse. 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. When the sky is clear and the meteorological visibility is high, the curvature of the earth limits the maximum possible geodetic visibility. Planes and water surfaces provide a maximum range of vision, but vegetation, buildings and mountains are geographical obstacles that limit the geographical visibility. The geographical visibility depends on the altitude of the observation site and the topology of its surroundings. The effect of air molecules on visibility is minor for short visual ranges but must be taken into account for ranges above 30 km. 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 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. Visibility is reduced by significant scattering from particles between an observer and a distant object. Scattering by particulates impairs visibility much more readily. 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. Visibility degradation is caused by the absorption and scattering of light by particles and gases in the atmosphere. Visibility and air pollution Ī visibility reduction is probably the most apparent symptom of air pollution. 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 the ground in low visibility. These generally advise motorists to avoid travel until the fog dissipates or other conditions improve. 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. 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 these conditions, roads may be closed, or automatic warning lights and signs may be activated to warn drivers. Visibility of less than 100 metres (330 ft) is usually reported as zero. These cold weather events are caused largely by low-lying stratus clouds. The combination of low visibility and ice formation can lead to accidents on roadways. 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. This usually occurs when temperatures are below 0 ☌ (32 ☏). With fog, occasional freezing drizzle and snow can occur. 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 and mist are generally assumed to be composed principally of water droplets, haze and smoke can be of smaller particle size. The international definition of fog 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). 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 sensitivityįunction of the HVS to assess visibility. Visibility perception depends on several physical and visual factors. This means that in the cleanest possible atmosphere, visibility is limited to about 296 km. At sea level, the Rayleigh atmosphere has an extinction coefficient of approximately 13.2 × 10 −6 m −1 at a wavelength of 520 nm. 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 objectĬ V ( x ) = F B ( x ) − F ( x ) F B ( x ) To define visibility the case of a perfectly black object being viewed against a perfectly white background is examined.
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