Light · interactive
Why is the sky blue but sunsets red?
Top: the view across the lake. Bottom: sunlight’s path through the air, and how much of each color gets through.
1 See
Same sun, same air. So why does the sky go from blue to orange?
2 Change
3 Understand
Model notes and sources
Single-scattering sky in a flat, uniform atmosphere, computed at three wavelengths (680, 550 and 440 nm). Rayleigh optical depth τ ≈ 0.0086 λ−4 (λ in µm), plus a light aerosol haze and ozone absorption (Chappuis band, about 300 Dobson units). Air mass comes from the Kasten–Young formula, which gives about 38× more air at the horizon than straight overhead. Real skies add multiple scattering and variable dust and cloud, so the colors here are illustrative.
Rayleigh scattering grows as 1/λ⁴, so 440 nm blue scatters about 5.7× more than 680 nm red. Violet scatters even more, but sunlight has less of it and our eyes are less sensitive to it. The side view exaggerates the thickness of the atmosphere.
Sources: Rayleigh scattering; Air mass (astronomy); Bucholtz (1995), Rayleigh-scattering calculations for the terrestrial atmosphere.
An ExplainerTools Original.