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 throughout 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.