Rayleigh Scattering

The coloration of the sky is primarily a result of Rayleigh scattering. Rayleigh scattering describes the scattering of light by particles much smaller than the wavelength of the light. In the case of the sky, light is scattered by the molecules of air themselves. The amount of scattering is inversely related to the fourth power of the wavelength of the light. This means that bluer light is scattered much more than redder light, which results in the blue color of the sky. And when the sun is near the horizon, its light needs to pass through more atmosphere to reach you, so even more blue light is scattered out of the path, and mostly just longer wavelengths remain, giving the sun it's vivid orangish color.

I implemented Rayleigh scattering from the ground up, including scattering cross sections, scattering coefficients, and phase functions. I am aiming for physical accuracy (and trying to learn about the physics) so I even included subtle factors such as dispersion of both the index of refraction and depolarization factor of air. The main resource I used in implementing Rayleigh scattering was this paper.