Rayleigh Scattering--Why is the Sky Blue?

            Scattering agents smaller than about 1/10^th the wavelength of incoming radiation disperse radiation (both forward and backward) in a manner known as Rayleigh scattering. Rayleigh scattering is performed by individual gas molecules in the atmosphere and primarily affects shorter wavelengths. It is particularly effective for visible light, especially for those colors with the shortest wavelengths (i.e. blue).

Combined with greater effectiveness in scattering shorter wavelengths than longer wavelengths, this characteristic leads to three interesting phenomena:

1.    The blue sky on a clear day

2.    The blue tint of our atmosphere

3.    The redness of sunsets and sunrises

Why is the sky blue?

            Gases and particles in the atmosphere scatter some incoming solar radiation in all directions. Air molecules scatter shorter wavelengths most effectively and blue light is among the shortest (and therefore most readily scattered) of visible wavelengths thus the scatter contains a higher proportion of blue light. And, since Rayleigh scattering occurs at every point in a clear atmosphere and diverts energy towards a viewer from all directions, no matter where you look on a cloudless day, the sky is blue.

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