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Skyglow (or sky glow) is the diffuse luminance of the night sky, apart from discrete light sources such as the Moon and visible individual . It is a commonly noticed aspect of light pollution. While usually referring to luminance arising from artificial lighting, skyglow may also involve any scattered light seen at night, including natural ones like starlight, zodiacal light, and airglow.
In the context of light pollution, skyglow arises from the use of artificial light sources, including electrical (or rarely gas lighting) lighting used for illumination and advertisement and from . Light propagating into the atmosphere directly from upward-directed or incompletely shielded sources, or after reflection from the ground or other surfaces, is partially backscatter toward the ground, producing a photon diffusion that is visible from great distances. Skyglow from artificial lights is most often noticed as a glowing dome of light over cities and towns, yet is pervasive throughout the developed world.
Part of this artificial light at night interacts with the air molecules and aerosols, and it is absorbed and scattered depending on the optical characteristics of the surrounding environment (see ) thus creating skyglow. Whether clouds are present, this effect is amplified by the interaction with water droplets.
Research indicates that when viewed from nearby, about half of skyglow arises from direct upward emissions, and half from reflected, though the ratio varies depending on details of lighting fixtures and usage, and distance of the observation point from the light source. In most communities, direct upward emission averages about 10–15%. Fully shielded lighting (with no light emitted directly upward) decreases skyglow by about half when viewed nearby, but by much greater factors when viewed from a distance.
Skyglow is significantly amplified by the presence of snow, and within and near urban areas when clouds are present. In remote areas, snow brightens the sky, but clouds make the sky darker.
Despite the strong wavelength dependence of Rayleigh scattering, its effect on sky glow for real light sources is small. Though the shorter wavelengths suffer increased scattering, this increased scattering also gives rise to increased extinction: the effects approximately balance when the observation point is near the light source.
For human visual perception of sky glow, generally the assumed context under discussions of sky glow, sources rich in shorter wavelengths produce brighter sky glow, but for a different reason (see ).
which is known as "Walker's Law."
Walker's Law has been verified by observation to describe both the measurements of sky brightness at any given point or direction in the sky caused by a light source (such as a city), as well as to integrated measures such as the brightness of the "light dome" over a city, or the integrated brightness of the entire night sky. At very large distances (over about 50 km) the brightness falls more rapidly, largely due to extinction and geometric effects caused by the curvature of the Earth.
| + Sky Glow brightness ratios for different lamp types |
| 0.4 |
| 0.4 |
| 1.0 |
| 1.0 |
| 1.5 |
| 1.8 |
| 2.1 |
| 2.7 |
| 3.3 |
In detail, the effects are complex, depending both on the distance from the source as well as the viewing direction in the night sky. But the basic results of recent research are unambiguous: assuming equal luminous flux (that is, equal amounts of visible light), and matched optical characteristics of the fixtures (particularly the amount of light allowed to radiate directly upward), white sources rich in shorter (blue and green) wavelengths produce dramatically greater sky glow than sources with little blue and green. The effect of Rayleigh scattering on skyglow impacts of differing light source spectra is small.
Much discussion in the lighting industry and even by some dark-sky advocacy organizations (e.g. International Dark-Sky Association) of the sky glow consequences of replacing the currently prevalent high-pressure sodium roadway lighting systems with white LEDs neglects critical issues of human visual spectral sensitivity, or focuses exclusively on white LED light sources, or focuses concerns narrowly on the blue portion (<500 nm) of the spectrum. All of these deficiencies lead to the incorrect conclusion that increases in sky glow brightness arising from the change in light source spectrum are minimal, or that light-pollution regulations that limit the CCT of white LEDs to so-called "warm white" (i.e. CCT <4000K or 3500K) will prevent sky glow increases. Improved efficiency (efficiency in distributing light onto the target area – such as the roadway – with diminished "waste" falling outside of the target area and more uniform distribution patterns) can allow designers to lower lighting amounts. But efficiency improvement sufficient to overcome sky glow doubling or tripling arising from a switch to even warm-white LED from high-pressure sodium (or a 4–8x increase compared to low-pressure sodium) has not been demonstrated.
Many nocturnal organisms are believed to navigate using the polarization signal of scattered moonlight. Because skyglow is mostly unpolarized, it can swamp the weaker signal from the moon, making this type of navigation impossible. Close to global coastal megacities (e.g. Tokyo, Shanghai), the natural illumination cycles provided by the moon in the marine environment are considerably disrupted by light pollution, with only nights around the full moon providing greater radiances, and over a given month lunar dosages may be a factor of 6 less than light pollution dosage.
Due to skyglow, people who live in or near urban areas see thousands fewer stars than in an unpolluted sky, and commonly cannot see the Milky Way. Fainter sights like the zodiacal light and Andromeda Galaxy are nearly impossible to discern even with telescopes.
Although sky glow can be the result of a natural occurrence, the presence of artificial sky glow has become a detrimental problem as urbanization continues to flourish. The effects of urbanization, commercialization, and consumerism are the result of human development; these developments in turn have ecological consequences. For example, lighted fishing fleets, offshore oil platforms, and cruise ships all bring the disruption of artificial night lighting to the world's oceans.
As a whole, these effects derive from changes in orientation, disorientation, or misorientation, and attraction or repulsion from the altered light environment, which in turn may affect foraging, predator-prey dynamics, reproduction, migration, and communication. These changes can result in the death of some species such as certain migratory birds, sea creatures, and nocturnal predators.
Besides the effect on animals, crops and trees are also susceptible to destruction. The constant exposure to light has an impact of the photosynthesis of a plant, as a plant needs a balance of both sun and darkness in order for it to survive. In turn, the effects of sky glow can affect production rates of agriculture, especially in farming areas that are close to large city centers.
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