Why is the sky blue?

EVER WONDERED, WHY IS THE SKY BLUE?

Blue Sky

Why the sky isn't of any other color? Why is the sky blue?

Well, the color of the sky changes from time to time but most of the times on a clear cloudless day, the color of the sky is blue.

This is because molecules in the air scatter blue light more from the sun more than they scatter red light. The first step towards correctly explaining the color of the sky was taken by John Tyndall in 1859. He discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than red.  This can be demonstrated by shining a beam of white light through a tank of water with a little milk or soap mixed in. From the side, the beam can be seen by the blue light it scatters; but the light seen directly from the end is reddened after it has passed through the tank. The scattered light can also be shown to be polarized using a filter of polarized light, just as the sky appears a deeper blue through polaroid sunglasses.

This is most correctly called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scattering - after Lord Rayleigh, who studied it in more details a few years later. He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles. It follows that blue light is scattered more than red light by a factor of (700/400)4 ~=10.
Tyndall and Rayleigh thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere.  Even today, people sometimes incorrectly say that this is the case.  Later scientists realised that if this were true, there would be more variation of sky colour with humidity or haze conditions than was actually observed, so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering.  The case was finally settled by Einstein in 1911, who calculated the detailed formula for the scattering of light from molecules; and this was found to be in agreement with experiment.  He was even able to use the calculation as a further verification of Avogadro's number when compared with observation.  The molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules.

source : http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html