Atmospheric scattering
How sunlight is scattered by the air's molecules and particles — the reason the sky is blue by day and red at sunset.
Two kinds of scattering carry most of the look. Rayleigh scattering hits short wavelengths hardest and paints a clear sky blue; when the sun sits low its light travels a longer path through the atmosphere, the blue scatters away and what remains is red. Mie scattering happens off larger particles — haze, dust — and produces the whitish halo around the sun.
Real-time engines compute this either analytically (Preetham's 1999 model) or by precomputing the scattering into lookup tables (Bruneton and Neyret, 2008). The result drives both the sky colour and aerial perspective — the way distant mountains fade toward the horizon. Nishita laid the groundwork in 1993.