Saramsha

No. 025  ·  11 October 2026

Why the Sky Is Blue

Air molecules scatter short wavelengths of light far more than long ones — blue gets knocked sideways out of sunlight much more readily than red does. Drag the sun down toward the horizon and watch blue get stripped out of its light first, leaving the sun — and eventually the whole sky near it — red.

gets through right now what got through at midday

About this piece

A molecule that's smaller than the light hitting it

Air is almost entirely nitrogen and oxygen molecules, each one far smaller than even the shortest wavelength of visible light. When light meets something that much smaller than itself, the physics of how it scatters was worked out by Lord Rayleigh in 1871: the amount of scattering goes as one over the wavelength to the fourth power. Halve the wavelength and you don't double the scattering, or even quadruple it — you get sixteen times as much. Violet light, at the short end of what we can see, scatters roughly nine times more than red light at the long end. Blue, a little longer than violet, still scatters several times harder than red.

Sunlight arrives as a mix of every visible wavelength at once, travelling in a dead-straight line until something deflects it. Climb above the atmosphere and that line reaches the ground untouched — which is why astronauts see a white sun in a black sky, and why the Remove the atmosphere button above shows exactly that. Put the air back, and the short wavelengths keep getting knocked sideways out of the beam by one molecule after another, scattering off in every direction — including down, toward you, from every patch of sky you can see. That scattered light, arriving from everywhere at once rather than from the sun's direction, is why the sky has a colour at all instead of just being black between you and a white sun. And because short wavelengths scatter hardest, the light raining down from the whole dome above you is heavily weighted toward the blue end — which is the colour you see when you look at any patch of clear sky that isn't the sun itself.

What the chart is tracking

The band of colour below the scene is the direct beam from the sun to the ground — not the scattered light, the straight-line part that's left over after scattering has taken its cut. At midday the sun is almost overhead, the beam crosses only one thickness of atmosphere, and most of every wavelength still gets through; the band is close to full height all the way across. Drag the elevation slider down and the beam has to cross the air at a shallower and shallower angle, which stretches the distance it travels through the atmosphere enormously — at 2° above the horizon the beam crosses roughly thirty times as much air as it does straight overhead. Thirty-fold is enough for even ordinary Rayleigh scattering to matter a great deal: the blue end of the band collapses almost to nothing while the red end, scattering so much more weakly to begin with, still gets a real fraction of its light through. What's left of the sun's direct light by the time it reaches you is now mostly red and orange — which is exactly the colour the sun and the sky near it turn at sunset.

The sky directly overhead tells a slightly different, gentler version of the same story: it stays some shade of blue the entire time, because the physical preference for scattering short wavelengths never goes away — it just dims as the sun gets lower and there's less total sunlight reaching the ground to be scattered at all. The dramatic reddening is a feature of looking toward the sun specifically, where you're seeing what's left of the direct beam after its long, nearly horizontal crossing.

Why not violet, since it scatters hardest of all?

By the one-over-wavelength-to-the-fourth rule alone, the sky should look more violet than blue. Three things save it from that. First, sunlight itself carries less energy at violet wavelengths than at blue ones to begin with, so there's simply less of it to scatter. Second, the human eye's colour receptors are considerably more sensitive to blue than to violet, so even equal amounts of each would look more blue to us. Third, the upper atmosphere's ozone layer absorbs a share of the violet and ultraviolet light before it ever reaches the altitudes doing most of the scattering we see. All three push the result from violet toward blue — which is why the sky is blue and not the slightly shorter colour the simplest version of the law would predict.

The sky and horizon colours are built from the same thirty-one sample wavelengths as the chart, each converted to RGB and weighted by how much of it survives the beam (for the sun and horizon glow) or by its raw scattering strength and the sun's overall brightness (for the blue overhead, which is modelled as a constant hue that only dims — real multiple scattering, haze and dust add a layer of nuance this keeps to one line). ?angle= and ?atm=0|1 set the scene directly for a given state.