Air is composed of molecules, that reflect specific colors depending on the light quality of the day and time of the day. On a sunny day the air color is blue, which reflects on everything it contains. On an overcast day, the air color is a shade of gray, around sunrise and sunset it is orange/red and so on. With color perspective, the more an object recedes in space, the more air color is added to the color of your picture element (i.e. far away mountains on a sunny day will look blue). Color perspective in addition to linear perspective greatly enhances the sense of space.
