Short blue waves bounce off air molecules more than long red waves.

Our atmosphere is packed with tiny gases like nitrogen and oxygen. Because blue light waves are so small, they hit these tiny gas particles and bounce off them in every direction. Scientists call this Rayleigh scattering. The longer red and yellow waves are too big to care about the tiny gas particles, so they pass right through them without bouncing. This leaves the blue light bouncing around the sky for us to see.