The blue color of the sky is caused by the scattering of sunlight off the molecules of the atmosphere. This scattering, called Rayleigh scattering, is more effective at short wavelengths (the blue end of the visible spectrum). Therefore the light scattered down to the earth at a large angle with respect to the direction of the sun's light is predominantly in the blue end of the spectrum.
-
Most of the gasses in the air are slightly blue. If they are packed together for miles like they are in the sky you can see the blue color.
-
Rayleigh Scattering. Blue light from the Sun has the Shortest Wavelength.
-
It might be due to the gases that make up earths atmosphere