Aerial perspective is the term used for how atmospheric conditions ("the air") influence our perception of objects in the distance. As objects get closer to the horizon (or further away), they appear lighter in tone, less detailed, and bluer or cooler in color.
Aerial perspective is something that's easier to understand visually than in words. Take a look at the photo towards the right of this page and you'll immediately see how the coastal hills appear lighter in tone as they march down the coast.
Aerial perspective applies to the sky too. Directly above you it'll be much bluer than at the horizon, where it can fade to nearly white.
