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Why does the night sky glow? February 19, 2009

Posted by linta in Causes.

glowLight pollution can be simply described as the presence of light where it is not wanted. This gives our sky a pinkish orange haze which is visible especially at night. And guess what, there are no more stars visible in the sky. Why or how does it happen?

 It is obvious that light causes light pollution. However, for convenience of study, the light that escapes to the night sky and causes light pollution can be classified into different categories. Here, I would like to adopt the classification method employed by Muskoka Watershed Council in their so called Technical Bulletin about light pollution  as it seems easy to understand and covers most of the main points. According to them,there are five major categories of light pollution- Light trespass, over-illumination, glare, cutter and sky glow. Now let us see what these are. Light trespass refers to the spilling out of light where it is not supposed to be present. Obviously, this trespassing light is from the earth and we humans allow it to happen. Over-illumination also results from human activities such as the “excessive use of light”. Glare is an other type of light pollution. If there is a large contrast between bright and dark areas, it can be called glare. Clutter is another type of light pollution which usually results from badly designed street lights which makes an “excessive grouping of lights.” This often lead to accidents. The last one,  sky glow, is the most obvious sign of light pollution especially in cities. As the name indicates, the sky appears to be glowing due to the reflection and scattering of light by atmospheric particles. All the above categories of light pollution are present around us, but we do not notice them. It is shocking to realize that we are living in world sunk in light. 

source: http://www.muskokaheritage.org/watershed/PDFs/DarkSkyLighting.pdf   

Image URL: http://media.photobucket.com/image/sky%20glow/curstkate/glow.jpg?o=3