Why Is the Sky Blue?

Short answer

Sunlight is a mix of all colors. When it enters the atmosphere, air molecules scatter blue light much more than other colors. So wherever you look in the sky, you see scattered blue light.

A bit more detail

Light is a wave. Each color has its own wavelength. Blue has a short wavelength, red has a long one.

Air molecules are tiny. They’re comparable in size to the wavelength of blue light. So blue light bumps into them more often and scatters in every direction.

This is called Rayleigh scattering. The shorter the wavelength, the stronger the scattering. Blue light is scattered roughly 10 times more than red.

What about sunsets?

At sunset the sun is low, and light travels through a thick layer of atmosphere. Blue gets completely scattered along the way, and only red and orange reach your eyes. That’s why sunsets are red.

Remember

The sky is blue because air scatters short (blue) wavelengths of light much more than long (red) ones.