Why is the sky blue but sunsets red?

At noon on a clear day, the sky is blue in every direction. A few hours later, the same sun turns orange and the clouds near it glow red. The sun has not changed color. What changed is the path its light takes through the air.

▶ Play with this now in Sunset Lab: drag the sun to the horizon and watch the blue drain away.

Why does air pick out blue?

Sunlight is every color mixed together. Each color is a wave, and blue waves are shorter than red ones. The molecules in air are far smaller than any of those waves. As sunlight passes them, they send a little of it off in new directions.

Shorter waves get scattered much more strongly. Halve the wavelength, and the scattering grows sixteen times. Blue light at 440 nanometers is scattered nearly six times more than red at 680 nanometers. The exact ratio depends on which blue and which red you compare.

Physicists call this Rayleigh scattering. Its strength goes as one over the wavelength to the fourth power, written ∝ 1/λ⁴. At noon, that scattered blue reaches you from every part of the sky. That is the blue sky. Wikipedia: Rayleigh scattering.

Violet scatters even more than blue, so why isn't the sky violet? Sunlight carries much less violet than blue. Our eyes also read the scattered mix as blue with some white in it. Exploratorium: Blue Sky.

What changes when the sun is low?

When the sun is high, its light takes a short path down through the air. Near the horizon, it comes in at a low angle and crosses far more air. Astronomers call this amount the air mass. At the horizon, sunlight crosses nearly forty times more air than when the sun is straight overhead. Wikipedia: Air mass.

Along that long path, most of the blue is scattered out of the beam. What's left is orange and red. The blue is not destroyed. It lights up the sky over places along the way, where the sun is still higher.

Clouds can catch the reddened light that remains and glow orange and red. Wikipedia: Sunset.

What does the clean-air model leave out?

Our film and Sunset Lab use a clean-air model with a light haze. It follows sunlight that scatters once, through an even atmosphere, at three wavelengths. Real skies are messier:

• Haze and dust. Particles about the size of a light wave, or bigger, scatter all colors more evenly. They wash the sky toward white. Fine dust, smoke and pollution can strip out more blue and deepen the red. NASA Space Place: Why Is the Sky Blue?
• Volcanoes. After a large eruption, a haze of fine particles can linger high in the air for years. Some of the vivid red skies J. M. W. Turner painted may trace back to the eruption of Mount Tambora in his lifetime.
• Clouds. They can hide the low sun completely, or catch its light and glow.
• More than one bounce. Real light often scatters many times before it reaches you. The model follows a single bounce.

That is why Sunset Lab's sky colors are illustrative. They are not a weather forecast.

Can you make a sunset in a glass?

This small experiment comes from the Exploratorium's Blue Sky activity. These steps are a suggestion, so adapt them to the child beside you. You need a clear container of water, a flashlight, a little milk and a white card. A tall glass works. A clear plastic box or small aquarium works better, because the beam travels farther.

Predict. Darken the room and shine the flashlight through the water from one side. Ask your child what will happen once the water turns slightly cloudy. What color will the beam look from the side? What color will the light be when it reaches the far end?

Try. Add milk a few drops at a time, and stir until you can see the beam. Look at it from the side. Then hold the white card at the far end and look at the spot of light. Too much milk turns everything white, so go slowly.

Explain. From the side, the beam looks bluish white. On the card, the light looks yellow-orange. The tiny particles in milk scatter blue light more than red, much as air does. The side view is your sky, and the spot on the card is your sunset. Milk's particles are far bigger than air molecules, so this is a model of the sky, not a copy. Wikipedia: Tyndall effect.

Next time you watch a sunset together, ask where the missing blue went. Then ask what the sky would look like with no air at all. For the next question, explore Ryan Sael's Prism Room and split white light into its colors yourself.

Sunset Lab is an ExplainerTools Original.