why is the sky blue?

1white light

Sunlight looks white, but it's actually every color mixed together, each traveling as a wave of a different length.

2wave lengths

Red light has long, lazy waves. Blue light has short, tight waves. That difference is the whole secret.

3the atmosphere

That sunlight has to punch through our whole atmosphere, full of tiny nitrogen and oxygen molecules — far smaller than any light wave.

4Rayleigh scattering

Those tiny molecules scatter short blue waves far more than long red ones — about ten times more strongly. This is Rayleigh scattering.

5scattered blue

So blue light gets bounced all over the sky in every direction, reaching your eyes from every patch of sky you look at — that's why it looks blue.