Skyglow

Skyglow (or sky glow), is a kind of light pollution, visible by the "glowing" effect seen in the skies over many cities and towns as a dome of light. Light domes can also appear above over-illuminated rural shopping centers, stadia, and the like.

Skyglow often refers to man-made light, but the term also includes natural sources of diffuse nighttime light, such as the zodiacal light, light from faint stars, and natural air glow emitted high in the atmospherecite book
title = "The Light of the Night Sky"
author = F.E. Roach and Janet L. Gordon
publisher = D. Reidel (Dordrecht-Holland/Boston-USA)
date = 1973 ] .

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 [http://www.flap.org Fatal Light Awareness Program (FLAP)] ) and other biological systemscite book
title="Ecological Consequences of Artificial Night Lighting"
author = C. Rich, T. Longcore (eds.)
publisher = Island Press (Washington; Covelo; London)
date = 2006 ] ,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 pollutioncite journal
title = "Why Astronomy Needs Low-Pressure Sodium Lighting"
author = C.B. Luginbuhl, in "Preserving the Astronomical Sky," IAU Symposium No. 196, eds. R. J. Cohen and W. T. Sullivan, III , pp. 81-86, 2001
publisher = PASP, San Francisco, USA
date = 2001 ] . 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 skyglow.

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 and fluorescent lamps) 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 articlecite web
url = http://resodance.com/ali/bluskies.html
title = IDA/IESNA Article on Skyglow
accessdate = 2007-12-11
author =
publisher = ] and a 2003 presentation to both the International Dark-Sky Association Conference and the Illuminating Engineering Society of North America,cite web
url = http://www.mindspring.com/~[e-mail]/scatter.pps
title = IDA/IESNA Powerpoint on Skyglow
accessdate = 2007-12-11
author =
publisher = ] 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. [cite news | last=Bortle | first=John E. | title=Observer's Log — Introducing the Bortle Dark-Sky Scale | date=February 2001 | publisher=Sky & Telescope | url=http://skyandtelescope.com/printable/resources/darksky/article_84.asp] 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

ee also

*Campaign for Dark Skies (CfDS)

External links

* [http://www.star.le.ac.uk/~dbl/cfds/skyglow.htm Skyglow: the effect of poor lighting] (CfDS) (examples of skyglow in the UK)
* [http://www.streetlights.us/ Skyglow across the Great Lakes] (examples of skyglow in the US)
* [http://home.earthlink.net/~sac8/SkyglowBits.htm Filtering Skyglow] (from CCD cameras)
* [http://www.star.le.ac.uk/~dbl/cfdsdisk/cfdsdisk/Towns%20and%20skyglow/ Towns and Skyglow] (UK skyglow image collection)
* [http://resodance.com/ali/bluskies.html 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)
* [http://www.mindspring.com/~[e-mail]/scatter.pps 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)