Why is the sky blue?

The sky is blue due to a phenomenon called Rayleigh scattering. Light is scattered off of the molecules in the atmosphere.

When the white light from the sun is scattered, the sky appears blue as the wavelength of blue light is on the shorter side of the visible spectrum. The sun appears yellow as blue light and yellow light combine to make white light.

At sunrise and sunset, the light is also scattered, but since the sun is low on the horizon, the light is scattered over a longer distance and through more of the atmosphere. The scattered blue light at that time falls outside a viewer’s line of site. Yellow, red and orange have longer wavelengths, and those are the colors you will see at sunrise and sunset.