What causes a red sky at night?

It has been theorized that street lamps based on solid-state lighting technology cause a form of light pollution known as "skyglow." Clouds increase the radiance of red light found in the skyglow by a factor of 17.6, giving cloudy nights in urban areas their reddish color.

A "red sky at night" is also an old weather saying, denoting the red tinge the setting sun has on clear skies, signifying good weather. Dust and small particles get trapped in the atmosphere due to high pressure, which scatters the blue light and leaves only the red light, giving the sky its appearance.