Why the Sky Is Blue but Sunsets Are Red

Look up on a clear day and the sky is blue. Look at the same sky an hour before dark and the horizon turns orange and red. Nothing about the air has changed. The question of why the sky is blue, and why the same air gives red sunsets, has a single answer: air scatters short wavelengths of light far more strongly than long ones, and how much of that scattering you see depends on the path the light takes to reach your eye.

Sunlight is every colour at once

Sunlight looks white because it contains all the visible wavelengths mixed together, from violet at about 400 nanometres to deep red at about 700. A nanometre is a millionth of a millimetre, so these waves are tiny. They are still enormous, though, compared with a molecule of nitrogen or oxygen, which is only a fraction of a nanometre across.

That size difference is the whole story. When a light wave meets something much smaller than its own wavelength, the molecule does not block the light the way a wall blocks a ball. Instead the wave sets the electrons in the molecule oscillating, and the molecule re-radiates a little of that energy in every direction. Physicists call this scattering. The version that applies to particles far smaller than the wavelength is named after the British physicist Lord Rayleigh, who published his analysis of the colour and polarisation of skylight in 1871.

Why the sky is blue: the fourth-power rule

Rayleigh's result has one striking feature. The amount of light scattered is inversely proportional to the fourth power of the wavelength. Halve the wavelength and the scattering does not double. It goes up sixteen times.

Put real numbers in. Blue light at around 450 nanometres and red light at around 700 nanometres differ in wavelength by a factor of about 1.55. Raise that to the fourth power and you get roughly 5.8. So a molecule of air scatters blue light between five and six times more strongly than it scatters red.

Now stand under a clear sky and look anywhere except directly at the Sun. The light reaching your eye from that patch of sky did not come straight from the Sun. It was travelling somewhere else, hit air molecules on the way, and was knocked sideways towards you. Because blue is scattered five or six times more than red, that sideways light is heavily weighted to the blue end. The sky is blue because the sky is sunlight that has been bounced out of its path, and blue bounces best.

The same effect explains why the sky is a deeper, more saturated blue when you look well away from the Sun and paler close to it. Near the Sun you are seeing light that has barely been deflected, and larger particles such as dust and haze scatter all colours more evenly, which washes the blue out towards white.

So why is it not violet?

If shorter wavelengths scatter more, violet at 400 nanometres should beat blue. Two things stop that happening.

• The Sun emits less violet than blue. Sunlight is not equally bright at every wavelength, and its output falls away towards the violet end of the visible spectrum, so there is less violet to scatter in the first place.
• Your eyes are not very sensitive to violet. The colour receptors in the retina respond most strongly in the green and yellow-green part of the spectrum, and their response drops off sharply towards violet. Scattered skylight is a mixture of violet, blue and a little of everything else, and the visual system reads that mixture as blue with some white in it.

So the sky is not simply the colour that scatters most. It is the colour your eyes and brain make out of the spectrum the atmosphere hands them. The article on why your screen and your eyes disagree about colour goes deeper into that last step.

Sunsets: the same rule, a much longer path

At midday the Sun is high and its light comes down through the atmosphere by the shortest route. At sunset the light arrives almost horizontally and has to skim through a far greater thickness of air before it reaches you. Astronomers describe this with a quantity called air mass, which is 1 for a source straight overhead and rises to about 38 at the horizon.

Scattering that removed a modest share of the blue over a short vertical path now works on a path dozens of times longer. By the time the direct beam reaches your eye, most of the blue and violet has been scattered out of it and is lighting up the sky somewhere else. What survives the journey is the light that scatters least: yellow, orange and red. The Sun itself, and any clouds it lights from below, take on those colours.

Dust, smoke and other fine particles make sunsets redder still, because they add more scattering on top of what the gas molecules already do. That is why bushfire smoke and volcanic eruptions produce the deepest reds. NASA puts it plainly: aerosols scatter the blue and leave more purely red and yellow light to pass straight through.

Why clouds are white and Mars does it backwards

Rayleigh's rule only holds when the scattering particles are much smaller than the wavelength. A cloud droplet is many times larger than a light wave, so a different regime applies, worked out by the German physicist Gustav Mie. Mie scattering treats all visible wavelengths roughly equally, which is why clouds, fog and steam look white or grey rather than blue.

Mars shows how finely balanced the effect is. Its atmosphere is thin and loaded with very fine dust whose grains sit close to the wavelength of visible light. In daylight that dust gives the Martian sky an orange or reddish tint. Near sunset, though, the dust lets blue light through more efficiently than red and keeps it concentrated near the Sun, so NASA's Curiosity rover photographed a sunset with a blue-grey glow around the Sun and warmer colours further out. Same physics, different particle size, opposite result.

What to look for next time

Three quick observations will show you the whole mechanism from your back step.

• On a clear day, compare the blue overhead with the blue near the horizon. The horizon is paler because that light has come through more air and more haze, so it has picked up every colour, not just blue.
• Hold a sheet of white paper in open shade and then in direct sun. In the shade it looks faintly blue, because shade is lit by skylight alone and skylight is blue light that has been scattered down to you.
• Watch a sunset for its last twenty minutes. The colour shifts from yellow through orange to red as the path through the atmosphere lengthens and the shorter wavelengths are stripped out in turn.

The air above you never changes colour. It only changes how far the light has to travel through it.

To see where all that scattering air sits, read The Atmosphere: Layers of Air. More articles like this one are in the physics category, and every episode of the podcast is at episodes.