The sun appears red at night because the light it emits must travel a farther distance. Most of the shorter wavelengths have already scattered upon hitting particles in the air, and the only wavelengths left to view are the longer red wavelengths.
The process of selectively scattering wavelengths of light is known as Rayleigh scattering, named after English physicist Lord Rayleigh. Red sunsets do not appear as vibrant if dust and haze fill the air. Instead they turn pale yellow or pink. Late fall and winter are the best times for viewing red sunsets. During these times, air circulation is better and fog and haze are less prevalent.