The Prose Edda 1.18

Then said Gangleri: "Whence comes the wind? It is strong, so that it stirs great seas, and it swells fire; but, strong as it is, none may see it, for it is wonderfully shapen." Then said Hárr: "That I am well able to tell thee. At the northward end of heaven sits the giant called Hræsvelgr: he has the plumes of an eagle, and when he stretches his wings for flight, then the wind rises from under his wings, as is here said:

Hræsvelgr hight he who sits at heaven's ending, Giant in eagle's coat; From his wings, they say, the wind cometh All men-folk over."