If there are no clouds, the colour of the sky seems blue during the day; a more intense blue at the zenith and lighter on the horizon. Actually most of the components of the atmosphere are colourless gases. The cause of the colour of the sky is to be found in the phenomenon of the diffusion of different radiations of the solar spectrum, a phenomenon produced by the molecules of air and by the solid particles that are contained in the lower layers of the atmosphere.
The molecules of the atmosphere behave differently in the presence of radiations of different wavelengths (also depending on the size of the molecules and the particles). Some are absorbed, others are reflected, yet others are diffused. Most of the particles of the atmosphere are of a size that allows a diffusion that is greater for light blue and blue wavelengths. For this reason clear skies appear to be this colour. The more the air is clear and rarefied (as at higher mountain altitudes) the more the sky appears to be a dark blue; when molecules and particles are absent, the sky instead appears to be black, as the astronauts orbiting outside the Earth’s atmosphere have noted, because in space diffusion phenomena do not take place