The most obvious form of light pollution is the ominous glow in the sky that exterior lighting generates at night.

This is particularly true of urban areas. Generally speaking the larger the urban area the brighter the glow in the sky. It may seem simple, but the proper term for this actually Sky Glow. Sky glow is a distinctly modern issue, given that artificial electric light is just over a century old. Originally the problem seemed to only apply to astronomers, both amateur and professional. The excessive lighting from urban areas spilled into the sky reducing the brightness of the stars. This is because sky glow reduces contrast in the night sky, or the visible difference between light and dark objects. In the case of astronomy, contrast applies to the difference between the brightness of the stars and the abysmal darkness of space making the stars and planets visible through telescopes. Urban lighting schemes tend to produce light in excess of what is actually necessary. In terms of light pollution this excessive light is referred to as Over Illumination. Although considered a separate form of light pollution, sky glow is essentially a side effect of over illumination working in conjunction with natural aspects of the atmosphere. Our needs for light at night essentially work against us by creating the glowing sky. The glow is produced two ways, one natural, one unintentional. The scattering of light within the natural gases of the atmosphere produces the blue glow of the sky during the day, and the yellow glow at night. Earth's atmosphere, a mix of nitrogen and oxygen, will naturally scatter and bounce light around. In some ways, this is similar to a giant halogen or fluorescent bulb. During the day, the scattering of light causes the whole sky to glow blue as we're used to instead of just letting the light of the Sun shine through illuminating a single area of the sky (think of images from the surface of the Moon). Chemical pollution is the second mechanism that causes sky glow. Chemical particulates in the atmosphere, particularly aerosols, reflect and scatter light in much the same way as the natural atmosphere. However, these pollutants are much more reflective. During the day pollution is visible as a thin yellow or sometimes grey haze. At night, this haze will absorb urban lights and produce the same glowing haze, the appearance of which only seems different because it lacks the blue background it has during the day. In fact, these pollutants are also partially responsible for global climate change, as they trap a significant amount of solar and thermal energy inside the Earth's atmosphere. Such energy should normally bounce off the planet harmlessly back into space, but pollutants cause it to scatter in the atmosphere or reflect back down at the Earth. Making use of light fixtures that reduce over illumination will let a home owner contribute to reducing this form of light pollution. In some area, such as California, reduction of over illumination has become a legal issue. Under Californian law this is called Title 24. Internationally, the Dark Sky Movement has led many light fixture manufacturers to produce lighting which reduces over illumination and directs light downward reducing sky glow. Fixture producers like Troy Lighting, UltraLights, Hinkley and Hubbardton Forge are reflective of the worldwide effort to reduce sky glow. Home owners can do their small part by installing fixtures which comply with similar standards.