Skyglow

Skyglow (or sky glow) is the diffuse luminance of the night sky, apart from discrete light sources such as the moon and visible individual stars. It is a commonly noticed aspect of light pollution. Although usually referring to sky luminance arising from the use of artificial lighting, skyglow can also refer to all diffuse sources of night sky luminance, including artificial as well as natural sources of diffuse nighttime light like the zodiacal light, starlight, and airglow.

In the context of light pollution, skyglow arises from the use of artificial light sources, including electrical (or rarely gas) lighting used for illumination and advertisement, and from gas flares. Light propagating into the atmosphere directly from upward-directed or incompletely shielded sources, or after reflection from the ground or other surfaces, is partially scattered back toward the ground, producing a diffuse glow that can be seen from large distances. Skyglow from artificial lights is most often noticed as a glowing dome of light over cities and towns, yet is pervasive throughout the developed world.

Light used for all purposes in the outdoor environment contributes to skyglow, by sometimes avoidable aspects such as poor shielding of fixtures, and through at least partially unavoidable aspects such as unshielded signage and reflection from intentionally illuminated surfaces. Some of this light is then scattered in the atmosphere back toward the ground by molecules and aerosols (see § Mechanism), and (if present) clouds, causing skyglow.

Research indicates that when viewed from nearby about half of skyglow arises from direct upward emissions, and half from reflected, though the ratio varies depending on details of lighting fixtures and usage, and distance of the observation point from the light source. In most communities direct upward emission averages about 10% - 15%. Fully shielded lighting (with no light emitted directly upward) decreases skyglow by about half when viewed nearby, but by much greater factors when viewed from a distance.

Skyglow is significantly amplified by the presence of snow, and within and near urban areas when clouds are present. In remote areas snow brightens the sky, but clouds make the sky darker.

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