Why is the sky blue, daddy?
Finally, somthing I feel qualified to comment on. White light arriving from the sun contains all of the visible wavelengths. As this light moves through the atmosphere it undergoes elastic (Rayleigh) scattering. If all wavelengths were scattered equally then the sky would be simply a bright white; instead the scattering goes as the frequency of the light to the fourth power. What this means is that the blue end of the visible spectrum is scattered more than the red end resulting in a blue sky.
Cheers, Joe Moryl