In the first approximation the skies on planet Earth are blue and transparent. Why it is sparkly blue, just like our desired clean clothes? To explore this matter one has to understand how the electromagnetic radiation propagates through the atmosphere.
First of all, sunlight is almost fully white, though Sun is classified as a yellow star with its maximum of visual energy set to green visible wavelengths. But in fact, the spectrum emitted by the sun is a mixture of all the colors of the rainbow. So it consists of many various types of light, including very long and low-frequency red light, long orange and yellow waves, medium green rays, and finally increasingly shorter blue and purple light.
This is not all. Apart from visible light our Sun also emits extremely long radio waves as well as short and very energetic UV, X and Gamma radiation. That is why scientists used to say that the spectrum of solar radiation is continuous, and some of its energy travels in short waves while other light moves in long, lazy waves. The secret key to the whole mystery of the blue sky is that blue, more energetic waves of light are shorter than the red ones.
In the presence of an absolute vacuum and without any gravitational distortions every ray of light travels in a straight line. But it gets more interesting when something happens to them along the way. When light beam hits an obstacle, for example in the form of a particle suspended in the Earth's atmosphere, its course may change in one of three possible ways. Light can be reflected (like in a mirror), bent, or just scattered.
Most of the sunlight hitting the atmosphere is scattered in all directions by the gases present in the air, like oxygen, nitrogen, carbon dioxide and some additional molecules. And the blue light is better scattered on the tiny molecules of the air, more than other wavelengths, because it used to travel as shorter waves that are more likely to distort and to change their paths. This is the general reason why we see the blue sky over Earth.