Why is the sky blue in the daytime?

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This is the all-time aged physics question. A simple answer would be "Rayleigh scattering." A better question(s) to be asked would be:

• What part of the sky is bluest, and why isn't the entire sky a uniform color?
• Does the daytime sky color actually follow the Rayleigh prediction?
• Why isn't the sky blue on nights with a full moon? What is scattering the sunlight to produce the daytime sky color?
• Would you get a blue sky if the scatters were much larger or much smaller?
  Finally, (and this is my favorite one, especially with the latest landings by the NASA Pathfinder mission to Mars)
• Why is the sky on Mars blue only within a few degrees of the horizon, and black overhead?

The basic color determination of the sky is in the wavelength dependence of the scattering of sunlight by the atmospheric molecules according to the Rayleigh scattering model. The electric field of the incident sunlight oscillates the electrons in these molecules, which in turn radiate light.

The overall effect is to scatter the sunlight. Light with shorter wavelengths (the blue end of the visible range) is deviated more from its longer wavelengths (the red end). When the sun is near the horizon the sky above an observer is therefore largely blue. The sky more than 90 degrees from the sun is less blue because it is illuminated with sunlight which must traverse a long path through the atmosphere and is therefore somewhat depleted in the blue.

The sky near the horizon is red or yellow because it too is illuminated with light whose long traversal of the atmosphere depletes the blue. Dust from a variety of sources (e.g., volcanos, forest fires) can not only scatter additional light, but can also display a different wavelength dependence than the Rayleigh scattering. Sunsets and sunrises after a major volcanic eruption can be brilliant (and also lead to one observing a blue sun and moon). The particular hues seen in any particular sunset are due to a combination of the normal Rayleigh scattering and the dust scattering.

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