text: why is the sky blue?   it is called rayleigh scattering, or molecular scattering.  this is primarily caused by oxygen and nitrogen molecules, whose effective diameters are at least 0.1 times smaller than the effected wavelengths.  rayleigh scattering is most influential at the alitudes above 4.5 km, occuring in what is called the pure atmosphere.  the amount of rayleigh scattering is highly selective, being inversely proportional to the fourth power of wavelength.  consequently, invisible ultraviolet radiation, at a wavelength of 0.3 micrometers, is scattered 16 times as readily as red wavelengths at 0.6 micrometers.  in the visible spectrum, blue wavelengths at 0.4 micrometers are scattered about five times as readily as red wavelengths at 0.6 micrometers.  the preferential scattering of blue wavelengths explains why the clear sky appears blue in daylight.  the blue wavelengths reach our eyes from all parts of the sky.