sky blue

An expression referring to the diffuse blue light that emanates from the sky when it is free of clouds. Sunlight scatters as it passes through air molecules, but it does not do so equally at all the wavelengths that make up the visible spectrum. The intensity of the diffuse light corresponding to the shortest wavelengths is much greater than that of the rest, which is why we see the sky as blue, rather than red or yellow; provided that the light, on its atmospheric path, passes through dry air with a low density of aerosols. In the gaseous molecules that make up the air, the so-called Rayleigh scattering takes place, named in honor of the English physicist John William Strutt (1842-1919), better known as Lord Rayleigh.