Beyond the freshly cut hedgerow a blue sky grades to near white towards the horizon. Image ©Les Cowley
Blue skies make the heart soar and poets rush for quill and ink.
The blue dome has subtle variety. Overhead it is darker - noticeably so from mountains or airplanes. Near the horizon it pales almost to white. The best skies are after heavy rain has washed out dust and aerosol saying the colour is conjured from sunlight and pure air alone.
The sun's light is a mix of violet, blues, greens through to reds. Blues and violets have the shortest wavelengths, that of blue is ~450 nanometres or 0.45 thousandths of a mm.
Air molecules, mostly nitrogen and oxygen, are 1000X smaller still. They interact only very weakly with visible light but with their enormous numbers in the atmosphere we see the effects.
Why is the sky whiter near the horizon? Overhead there are only a few miles of dense atmosphere and sunlight photons are scattered once - if at all. Near the horizon the air path is 10X or more longer. Along it, photons are scattered several times and the reds and greens eventually become as strong as blues to yield white. The near horizon sky cannot become arbitrarily bright with the extra scattering because beyond a certain atmospheric path length it effectively becomes opaque. In fact the sky often darkens slightly very close to the horizon for this reason.
Rayleigh scattering requires that there be no coherence between the individual scatterers. In dense gases when molecules are closer together this condition is not satisfied and light is predominantly scattered forwards rather than in all directions. In dense gases and liquids another process can operate, Einstein-Smoluchowski scattering. Molecular motion and collisions produce exceedingly transient local density and refractive index fluctuations that act as scattering centres. The wavelength dependence is the same as for Rayleigh scattering.
Dust, aerosol, moisture:
|Air molecules are 1000X smaller than visible light wavelengths. They act as Rayleigh scatterers and scatter blue light ~4X more strongly than longer wavelength reds.
|Sunlight photons of all colours stream through the air. Its molecules scatter a tiny proportion in every direction. The scattered photons have the same colour and energy.
Blue photons are more strongly scattered than greens and reds.
The scattered light makes the sky appear blue. The sky is not pure blue as it also contains a small proportion of other scattered colours.