Above ground is the atmosphere, air mass that surrounds the Earth. The composition involving gases such as nitrogen and oxygen in a high percentage, in addition to minimum rates of argon, carbon dioxide and water vapor, among others. The sky is presented before our eyes as a blue dome. Its color is the result of optical effect of the diffusion of sunlight through the atmospheric air molecules. In this nuance involved light waves of low amplitude (blue) and short length, which are dispersed in all directions. The same happens in a dark room where a beam of light penetrates. Looking at the color of that portion of illuminated air, it will be blue because of the dust particles in the air. At the dawn or sunset, the sky becomes redder, because the short wave lengths that produce blue light have a long way to go and lost, giving way to waves of greater length (close to red) .
Saturn's largest moon, Titan, is the only one with snow, rain and a sky blue, because its surface is covered by an atmospheric layer.
The second layer of the atmosphere, the stratosphere, does not refract light rays or disseminates, spreads in the direction of the sun. That is why in this layer the sky appears dark blue.