Rayleigh and Mie scattering

Light doesn't just travel in straight lines through air — it also bounces off the molecules it passes, in every direction, and how strongly it bounces depends enormously on colour. Rayleigh scattering, the process responsible when the scattering particle (an air molecule) is much smaller than the light's wavelength, grows as the fourth power of 1/wavelength. Blue light, with roughly half the wavelength of red, is scattered nearly sixteen times more strongly. Look anywhere in a clear daytime sky, away from the Sun, and you're seeing sunlight that has been redirected toward your eye by exactly this process — which is why the sky is blue rather than the black of space with a white Sun in it.

The same effect explains the opposite-looking sunset. At midday, sunlight travels through a relatively thin slice of atmosphere to reach the ground. Near the horizon, the same light travels through a much longer slice, so far more of its blue and violet content gets scattered away before the beam ever reaches your eye — what's left, the light that comes to you directly rather than being scattered in from elsewhere, is dominated by red and orange.

Clouds, fog and haze look white rather than blue precisely because their scattering particles — water droplets, dust — are comparable in size to or larger than the wavelength of visible light. That regime is Mie scattering, and unlike Rayleigh scattering it depends only weakly on wavelength: red, green and blue light all get scattered by roughly the same amount, so the mixture that reaches your eye is still white. The distinction between the two isn't just academic — the sharp wavelength dependence of Rayleigh scattering, and the near-absence of one in Mie scattering, is the entire reason the sky and the clouds in it look like two different colours of the same light source.