Light pollution

Regional
Light pollution is the effect of unwanted stray artificial light illuminating the sky. The light scatters off particles and haze in the atmosphere, as well as clouds, and increases the surface brightness of the background sky, thus making faint objects more difficult to identify, as well as accelerating the pace at which imaging sensors saturate. Light pollution varies over time due to different lighting patterns at different times of night, as well as different atmospheric conditions. Snow magnifies the effect of light pollution by reflecting more downwards pointed light back towards the sky. The effect of snow on sky quality is diminished further away from light sources. Clouds in cities tend to be bright, and often orange in coloration due to being lit from the underside by the lights of the city. In a light pollution free area, clouds appear as dark patches of starless sky, and are otherwise very difficult to see.

Some astronomers actively lobby their local politicians and businesses to re-design upwards-facing lighting fixtures in order to reduce the amount of light that is wasted by pointing at the sky, reducing light pollution as well as saving energy.

Amateur astronomers often categorize light pollution into zones of white, red, orange, yellow, green, blue, grey and black, where white areas have the heaviest light pollution, and black zones the least. The color system is borrowed from, and often cites the world light pollution atlas created by italians P. Cinzano, Thiene. http://www.lightpollution.it/worldatlas/pages/fig1.htm.

The bortle dark sky scale is a frequently used system of subjective measures used to categorize night sky brightness. The bortle scale is influenced by various phenomena, including many not related to light pollution, such as latitude, altitude of certain objects, sky transparency and observer experience.

Light pollution is also often measured via Naked Eye Limiting Magnitude, or NELM, which is a naked eye assessment of the faintest stars visible. There are various techniques used to determine the faintest stars, from hunting down known stars, rough estimates based on certain known features, or counting stars in predefined areas of the sky, and matching the number of stars counted to a table which equates that to a limiting magnitude. These measurements are very dependent on the skills and eyes of the observer, as well as sky conditions past light pollution.

Local
While high-altitude light pollution has been mapped globally with a low resolution, significant problems exist from low altitude reflections when in close proximity to strong sources of light pollution. These may be visible as a distinct light dome even at a very small distance from the source.

Line of Sight
Dark-adapted vision is extremely sensitive, and can be reset with only momentary exposure to direct lighting. Ideally, an observing area in an urban / suburban location should be a comfortable area with a good view of the sky, and no line of sight light sources visible in any direction. A fence, screen, or open-roofed building may be ideal in cases where one needs to create such a place.