Skyglow

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. 

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.

Sky Glow

Sky Glow is a general brightening of the normally black night sky caused by artificial light pollution. Excessive sky glow can almost completely block out views of stars. The main causes are upward-directed light (again from poorly shielded fixtures) and too much light power used for the application. 

These pictures show the sky in Toronto, Ontario in 2003.

The left picture was taken during a blackout before the general use of artificial lighting in this area.

The right picture shows the now-normal night sky, after the blackout. Light pollution has almost completely blocked out the any view of the stars.

These pictures were taken in August, 2003, during and after a blackout which spanned the eastern seaboard. (Photos courtesy of Todd Carlson).