Skyglow

Mexico City at night, with a brightly illuminated sky.

Skyglow (or sky glow) is the illumination of the night sky or parts of it. The most common cause of skyglow is artificial light that emits light pollution, which accumulates into a vast glow that can be seen from miles away and from high in the sky. Skyglow from artificial lights is common throughout the world and can be observed over most cities and towns as a glowing dome of the populated area. Skyglow's light domes can be large, as in that over a city, or small, as in that over an over-illuminated shopping center or a stadium.

Although often referring to artificial light, skyglow also includes natural sources of diffuse nighttime light like the zodiacal light, starlight, and airglow emitted high in the upper atmosphere.:^[1][2]

Skyglow can also be caused by natural occurrences, such as the 1908 Tunguska event, in which a meteoroid spanning a few meters in mean radius exploded 5–10 kilometers above the Podkamennaya Tunguska River in the Krasnoyarsk Krai region of Russia. The explosion is estimated to have had released more or less 15 megatons of energy, which is around 1,000 times as powerful as the atomic bomb that exploded over Hiroshima, Japan in 1945 and about one third as powerful as the thermonuclear bomb Tsar Bomba, the most powerful nuclear bomb ever detonated. The light emitted from the Tunguska explosion was so great that it created skyglow as far away as England, where the population experienced a number of weeks of intermittent "bright nights" (a term that is now synonymous with skyglow).

Dependence on distance from source

For relatively small distances between the light source and observer, the intensity of the skyglow contribution from a single source of light is inversely proportional to the distance between the source and the observer (falls off as 1/r). This can be understood as follows: The illumination of any portion of atmosphere visible by the observer falls as 1/r², but the path length of illuminated air along any given line of sight which is illuminated comparably to the brightest illumination grows linearly with distance; together, this gives a 1/r dependence. This is valid when the distance between the source and observer is smaller than the scale height of the atmosphere. At distances much larger, where the path length of illuminated air is limited by the height of the atmosphere itself, this becomes an inverse-square (1/r²) dependence.

Negative effects

Skyglow is mostly unpolarized, and its addition to moonlight results in a decreased polarization signal. Humans cannot perceive this pattern, but some arthropods can.

Skyglow, and more generally light pollution, has various negative effects: from aesthetic diminishment of the beauty of a star-filled sky, through energy and resources wasted in the production of excessive or uncontrolled lighting, to impacts on birds^[3] and other biological systems,^[4] including humans. Skyglow is a prime problem for astronomers, because it reduces contrast in the night sky to the extent where it may become impossible to see all but the brightest stars. It is a widely held misunderstanding that professional astronomical observatories can "filter out" certain wavelengths of light (such as that produced by low-pressure sodium). More accurately, by leaving large portions of the spectrum relatively unpolluted, the narrow-spectrum emission from low-pressure sodium lamps allows more opportunity for astronomers to "work around" the resulting light pollution.^[5] Even when such lighting is widely used, skyglow still interferes with astronomical research as well as everyone's ability to see a natural star-filled sky.

Many nocturnal organisms are believed to navigate using the polarization signal of scattered moonlight.^[6] Because skyglow is mostly unpolarized, it can swamp the weaker signal from the moon, making this type of navigation impossible.^[7]

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.

Causes

In this 10-second exposure photo, facing south toward Sagittarius, three forms of light pollution obscure the stars and faintly visible Milky Way in the suburban night sky over Southern California: skyglow, glare, and light trespass.

The several causes of skyglow mainly differ in source. For example, public lighting provides a different form of light pollution than attention-grabbing strobe lamps, and these differ from commercial lighting installations. Light from electric lamps shines directly upward into the atmosphere from poorly shielded fixtures, and reflects from surfaces like the ground or streets into the sky. On clear nights, some of this light is then scattered in the atmosphere by molecules and aerosols back toward the ground, causing skyglow.

Near urban areas skyglow is made considerably worse when clouds are present,^[8] and when snow is on the ground. This is because both clouds and snow have a very high albedo, preventing the light from escaping Earth's atmosphere, or being absorbed on the ground. In pristine areas snow brightens the sky, but clouds make the sky much darker.

In pristine areas clouds appear black and blot out the stars. In urban areas skyglow is strongly enhanced by clouds.

Effects on the ecosystem

The effects of sky glow in relation to the ecosystem have observed to be detrimental to a variety of different organisms. The lives of plants and animals alike (especially those which are nocturnal) are affected as their natural environment becomes subjected to unnatural change. It can be assumed that the rate of human development technology exceeds the rate of non-human natural adaptability to their environment, therefore, organisms such as plants and animals are unable to keep up and can suffer as the consequences.^[9] 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. Similar problems of disrupting the environment and its biosphere are also very prevalent in regards to energy resources such as the installation of wind turbines and the interference they cause with not only bird flight paths, but also with human neurology.

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, reproduction, migration, and communication. These changes can even result in the death of certain species such as certain migratory birds, sea creatures, and nocturnal predators.^[10]

Besides the effect on animals, crops and trees are also very susceptible in being destroyed. 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 properly survive. In turn, the effects of sky glow can affect the production and rate of agriculture, especially in regards to farming areas that are close to large city centers.

Mechanism

There are two causes of the light scattering that lead to airglow: scattering from molecules such as N₂ and O₂ (called Rayleigh scattering), and that from aerosols, called Mie scattering. Rayleigh scattering is much stronger for short-wavelength (blue) light, while scattering from aerosols is little affected by wavelength. In most places, most particularly in urban areas, aerosol scattering dominates, due to the heavy aerosol loading caused by modern industrial processes and transportation. Rayleigh scattering makes the sky appear blue in the daytime; the more aerosols there are, the less blue or whiter the sky appears. When the air is clear and relatively free of aerosols, blue or white light (for example from metal halide lamps, fluorescent lamps, and white light-emitting diodes) contributes significantly more to sky-glow than an equal amount of yellow light (for example from high- and low-pressure sodium vapor lamps). Another effect that makes skyglow from white light sources worse than from yellow arises from the Purkinje effect, where the eye becomes more sensitive to bluer/whiter light when adapted to low light levels, as experienced under night time conditions. A simple metric for first effect is the Rayleigh Scatter Index ^[11] , discussed in a brief article^[12] and a 2003 presentation to both the International Dark-Sky Association Conference and the Illuminating Engineering Society of North America,^[13] which indicates that high pressure sodium sources produce roughly one-third to one-half of the skyglow compared to the output of typical metal halide sources, based on the same amount of light entering the atmosphere and pure Rayleigh scattering. When the Purkinje effect is also considered the effect is magnified, to where yellow sources can produce as little as one-eighth the skyglow of an equivalent output white light source, particularly when the observer is located at some distance from the light pollution source, the sky is darker, and the eye more completely dark adapted^[14] .

Measurement

Astronomers have used the Bortle Dark-Sky Scale to measure skyglow ever since it was published in Sky & Telescope magazine in February 2001.^[15] The scale rates the darkness of the night sky inhibited by skyglow with nine classes and provides a detailed description of each position on the scale.

See also

• Light pollution
• Urbanization
• Polarized light pollution
• Over-illumination
• Dark-sky movement
• International Dark-Sky Association (IDA)
• Campaign for Dark Skies (CfDS)

References

External links

• List of peer reviewed research papers about sky glow
• Skyglow: the effect of poor lighting (CfDS) (examples of skyglow in the UK)
• Skyglow across the Great Lakes (examples of skyglow in the US)
• Filtering Skyglow (from CCD cameras)
• Towns and Skyglow (UK skyglow image collection)
• What Blue Skies Tell Us About Light Pollution (an article introducing the Rayleigh Scatter Index which evaluates the potential for contribution to skyglow by different light sources)
• Sources, Surfaces and Scatter (a presentation from 2003 to the IDA Conference and the IESNA Roadway Lighting Committee on atmospheric scatter and the potential for contribution to skyglow by different light sources)
• Loss of the Night an Android app for estimating skyglow by measuring naked eye limiting magnitude
• Dark Sky Meter an iPhone app for measuring skyglow luminance