In Rayleigh scattering, the efficiency of scattering is inversely proportional to the fourth power of the wavelength. Red light has double the wavelength of blue light, and is therefore scattered 16 times less. Rayleigh discovered that scattering is strongest in the direction light travels, becoming half at right angles to the incident direction of light. At noon, the sun is directly above. At this time, the sky looks blue because the blue light scattering is strongest to the human eye. Another phenomenon worth knowing about is Mie scattering. This occurs when light hits a water drop, aerosol, or other particle in the atmosphere whose diameter is around the same as the wavelength of the light. In that case, there is no relationship between scattering strength and wavelength, which is why clouds look white and the sky also seems whitish when polluted.