Scattering in the atmosphere occurs when light is absorbed by an atom or molecule, and then emitted again, usually in a different direction from the way the light was originally travelling. Light scattering off atoms and molecules in the atmosphere is unpolarized if the light keeps traveling in the same direction, is linearly polarized if it scatters in a direction perpendicular to the way it was traveling, and somewhere between linearly polarized and unpolarized if it scatters at another angle.
If you wear polarized sunglasses you can observe this in the sky. On a clear day look about 90o from the Sun. If you then tilt your head one way and then the other you will see the sky darken and lighten. When it's darker there's not as much light reaching you, and when it's lighter there's more.