Concise Bible Dictionary:

1. ἀήρ, the fluid that we breathe, and which surrounds the earth. There are two remarkable references to the air: one, that at the Rapture of the saints the raised dead and the living will be caught up into the clouds, and will meet the Lord in the air (1 Thess. 4:17), in the same sphere in which the Lord disappeared at His ascension. The other is that Satan is called the prince of the power of the air (Eph. 2:2), doubtless referring to the permeating character of the air, which we not only breathe, but it fills our houses and enters everywhere. When the air is tainted with malaria it permeates everywhere and poisons everything; so Satan has such power that his poisonous spirit morally contaminates everything in the world, of which he is the god.
2. οὐρανός, literally “heaven,” translated “air” only in reference to the birds and fowls (Matt. 8:20).