sky glow

Mexico City at night, with a brightly illuminated sky.

Skyglow (or sky glow), is widescale illumination of the sky or parts of the sky at night. The most common cause of skyglow is man-made lights that give off light pollution that accumulates into a vast glow that can be seen from miles away and from high in the sky. However, 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 1000 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"). Skyglow from man-made 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.

Skyglow often refers to man-made light, but the term also includes natural sources of diffuse nighttime light, such as:

• the zodiacal light

• starlight, and
• airglow emitted high in the upper atmosphere.^[1][2]

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, and more generally light pollution, has many diverse 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 (see Fatal Light Awareness Program (FLAP)) and other biological systems^[3], 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) - it is more accurate to say that 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^[4]. 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.

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 the Andromeda Galaxy are nearly impossible to discern even with telescopes.

Causes

There are several causes of sky glow. These causes 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. Some of this light is then scattered in the atmosphere by molecules and aerosols back toward the ground, causing sky glow.

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, discussed in a brief article^[5] and a 2003 presentation to both the International Dark-Sky Association Conference and the Illuminating Engineering Society of North America,^[6] 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.

Measuring sky glow

Astronomers have used the Bortle Dark-Sky Scale to measure sky glow ever since it was published in Sky & Telescope magazine.^[7] The Bortle Scale rates the darkness of the sky, inhibited by sky glow, on a scale of one to nine, providing a detailed description of each position on the scale.

References

See also

• Campaign for Dark Skies (CfDS)

External links

• 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)