Processes by which particles of similar size and electrical characteristics separate or disperse different wavelengths (read colors) of light. First described by Gustav Mie in 1808. Since the sun's visible spectrum contains a mixture of (traditionally ordered) red, orange, yellow, green, blue, indigo, and violet colors, these wavelengths are differentially scattered by particles as they travel through the atmosphere. Red--longer wavelength--light is scattered not much and blue--shorted wavelength--light is scattered much more. This is why the sky appears blue: the sun's blue light is scattered back towards your eyes from atmospheric particles so when you look up--that is, away from the sun, the light you see is light scattered to your eye from atmospheric particles. (You might ask yourself why the sky--viewed at an off angle from the sun--isn't black instead of blue.) Mie scattering is also the reason why sunsets appear red: the sun's red light is NOT scattered as much as blue light by atmospheric partricles and so solar blue light is scattered away from your eyes on its way from the sun and red light is scattered less. The result is that more solar red light hits your eyes than blue and sunsets appear red. Notice also that the redness of sunsets increases at the amount of atmospheric particles between you and the sun increase, that is as the sun "goes down" mie scattering increases as the amount of particles between you and the sun increases.
[Webster's New World Dictionary; Third Edition: page 265; 1994.] [Scientific American; v265: pages 80-85; 1991.]
Source: Atmospheric Chemistry Glossary