Sky glow is the illumination of the night sky (partially or completely). Its most common cause is the artificial light of the cities which accumulates into a vast glow that can be seen from many kilometers away in any direction. Sky glow from the artificial lights is common througho ut the world, it can be observed as a glowing dome of the populated area.
Other causes include natural sources as the starlight and airglow emitted in the upper utmosphere. Sky glow can also be caused by natural occurences like the meteorits or comets entering in the atmoshpere. One famous event happened in 1908 in Tunguska when an meteoroid spanning a few meters in radius exploded some kilometers above the Podkamennaya Tunguska River in the Krasnoyarsk Krai region of Russia. The light emitted from the Tunguska explosion was so great that it created skyglow as far away as England, the population over there experienced intermittent "bright nights" (a term that is now synonymous with sky glow) for several weeks.
The problem about the sky glow is that it obscures the sky and the astronomers can not identify the stars and their planets, the comets and other meteoroids inside the solar system. The astronomers need clear skies for their telescopes to do their work well - the sky glow adds brightness to a part or all their sky and the light near the ground (the surface of the Earth) blocks the light from the stars (from outside the planet). In other words, all the telescopes except the radio ones become impenetrable because of the sky glow.
The first to disappear when there the sky is glowing are the most less brighter point of light: the distant stars and the ones that do not bright so much. So the constellations and the galaxies will be not complete, some celectial object can disappear completely so the astronomers may lose their orientation.
What you must have noticed is that once the sky glows it never stop glowing except when it's a blackout. When the sky is cloudy we all know that's going to change after a time (several hours, day or weeks), but when the sky glow because the electric light that never changes. It's one of the problems with the technology.
Another name for the sky glow is light pollution - it's that orange colored glaze that can be seen over the cities. To reduse some light pollution there can be installed shades around the lights to direct the loght downward instead letting it to spread into the night sky where it breaks up and scatters.
The escape sky glow astronomers must head for the mountains and rural setting where the cities lights do not cause the glow. The first telescope that searches the Universe from the Earth orbit was set over there a few years ago, it's a very good solution even the maintenance costs are high.
The image with the Portsmouth's light glow is taken from a page about frequent asked questions about light pollution.
