Due to a physical phenomenon called Rayleigh scattering that causes light to scatter when it passes through particles that have a diameter one-tenth that of the wavelength (color) of the light.
Light of a particular color is characterized by its frequency and wavelenth. The higher the frequency, the more blue it appears.
The earth's atmosphere is filled with minute dust and water particles that act like a filter, scattering the light rays. The rays of light with the longer wavelengths, such as reds and yellows, tend to pass more easily through the atmosphere, while the rays with the shorter wavelengths, like blues and indigos, tend to be randomly scattered more easily. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.
