Why the sky is blue and the sunset red

It’s the same sun and the same air all day. The reason the sky is blue at noon and burns orange at the horizon comes down to a single idea — scattering — seen from two different angles.

Blue overhead

Sunlight is a mix of every colour. As it crosses the air, molecules of nitrogen and oxygen knock it off course — but not evenly: they scatter short, blue wavelengths far more strongly than long, red ones. So the blue is peeled off the beam and flung across the whole dome of the sky. Look anywhere but straight at the sun and what you see is that scattered blue. (Violet is scattered even more, but there is less of it and our eyes are less sensitive to it, so the sky simply reads blue.)

Red at the horizon

At sunset the light reaches you along a grazing path many times longer than at midday. Over that long journey almost all of the blue has been scattered away before the light arrives — so what is left of the direct beam is red and orange. That surviving red light is exactly what goes on to set high clouds alight in a fire sky. The blue of the daytime sky and the red of a sunset are two sides of one coin: the sky is blue because the blue was removed from the beam that reaches you red.

Haze, dust and smoke

Molecules aren’t the only thing in the air. Larger particles — haze, dust, sea salt, wildfire smoke — scatter light too, but in a different way: more evenly across all colours, which is why a hazy sky looks milky-white rather than deep blue. How much of this the air is carrying, and of what kind, is what makes one evening’s sunset washed-out and another’s saturated. The light is always being filtered by whatever happens to be floating in the air that day.

The maths, briefly

Scattering by molecules — Rayleigh scattering — falls off very steeply with wavelength. The fraction of light scattered goes as

$$ \text{scattering} \;\propto\; \frac{1}{\lambda^{4}}. $$

That fourth power is what makes the effect so lopsided. Compare blue light (~450 nm) with red (~650 nm):

$$ \left(\frac{650}{450}\right)^{4} \approx 4.4. $$

Blue is scattered more than four times as strongly as red. Peel four-plus times as much blue off a beam along a long path and what remains is unmistakably red — which is all a sunset is.

Blue days and red sunsets aren’t two phenomena; they’re one, watched from different directions. A fire sky is simply that surviving red light finding a cloud to land on.