Why is the sky blue?

In the sky, there is no distinction of east and west; 

people create distinctions out of their own minds and then believe them to be true. Buddha 

The sunlight according to our eyes has a high ratio of short, bluish wavelengths compared to medium and long wavelengths, which our eyes perceive as blue. Our eyes see different colors when the scattering of light bounces of molecules, depending on wavelengths.

But what is the sky not always blue? The sky can appear with red or violet variations because larger particles of dust, pollution, and water vapor in the atmosphere reflect and scatter more of the reds and yellows. Pollution at it's finest I suppose. The longer wavelengths make for these colors, while the shorter wavelengths make for blue skies.