Sunlight looks white, but it is a mix of all colours. On its way through the atmosphere it bumps into gas molecules that are far smaller than the wavelength of light. These molecules scatter light in all directions — and they scatter short wavelengths much more strongly than long ones. This is called Rayleigh scattering: the effect grows with the inverse fourth power of wavelength, so blue light (around 450 nm) is scattered several times more than red light (around 700 nm).

When you look at any part of the sky away from the Sun, you are seeing that scattered light arriving from every direction — and it is dominated by blue.

Why not violet?

Violet has an even shorter wavelength, so it is scattered more still. But the Sun emits less violet than blue, some violet is absorbed high in the atmosphere, and our eyes are much less sensitive to violet. The mix our eyes receive is perceived as sky blue.

Why are sunsets red and orange?

At sunset, sunlight travels through far more atmosphere to reach you. Most of the blue is scattered out of the direct beam along the way, so the light that arrives directly from the Sun is left rich in orange and red. Dust, smoke and pollution scatter light too, which is why sunsets after dusty days or fires can be especially vivid.

And clouds are white because…

Cloud droplets are much larger than gas molecules. They scatter all visible wavelengths roughly equally (Mie scattering), so clouds look white — or grey when they are thick enough to block the light behind them.

More everyday science in the Wacky Facts collection.