Why is the sky blue?

Hook

Why and how is the sky blue? Sunlight looks white, and space is black. Yet every clear day, the air over your head glows blue, and at sunset it turns orange and red. The answer is a tiny game of pinball between light and air.

White

White sunlight is really every color mixed together. Isaac Newton showed this in the sixteen sixties by splitting it with a prism. Each color is light with a different wavelength, from long red waves to short blue and violet ones.

Scatter

Air is mostly nitrogen and oxygen molecules, far smaller than a wave of light. When sunlight hits them, some of it bounces off in every direction. This is called Rayleigh scattering, and it hits short wavelengths much harder than long ones.

Math

How much harder? Scattering grows with the fourth power of how short the wave is. Blue light gets scattered several times more than red. So blue light ricochets all over the sky, and reaches your eyes from every direction.

Violet

Violet is even shorter, so why isn't the sky purple? Sunlight contains less violet to begin with, and our eyes are much less sensitive to it. Add it all up, and our brains see sky blue.

Sunset

At sunset, sunlight travels through far more air to reach you. Most of the blue gets scattered away along the trip. What's left is the long wavelengths, the oranges and reds. Dust and smoke can make sunsets even more vivid.

Mars

On other worlds the rules play out differently. On Mars, fine dust scatters light, so the daytime sky looks butterscotch, and NASA's rovers have photographed blue sunsets.

Close

So the sky is blue because air molecules scatter short blue wavelengths far more than red ones. ThatsWhyHow.