Rayleigh Scattering and Blue Sky

The intensity of Rayleigh scattering is strongly dependent on the wavelength of the light, following an inverse fourth-power law: [latex]I \\propto \\lambda^{-4}[/latex]. This means that blue light (with a shorter wavelength, ~475 nm) is scattered about 16 times more intensely than red light (~700 nm) by the gas molecules in the atmosphere. When we look at the sky away from the direct sun, our eyes perceive this scattered blue light coming from all directions. Conversely, when the sun is near the horizon at sunrise or sunset, its light must travel through a much longer path in the atmosphere to reach the observer. By the time it does, most of the blue and green light has been scattered away from the direct line of sight, leaving the longer-wavelength oranges and reds to be seen. This same principle explains why distant clouds or mountains can appear bluish. Rayleigh scattering is distinct from Mie scattering, which is caused by particles of a size comparable to or larger than the wavelength of light (like water droplets in clouds) and is not strongly wavelength-dependent, which is why clouds appear white.