Commentary Par XXVIII 79-87

The four main winds were, in Dante's day and for centuries after, portrayed as faces.  Boreas, the north wind, blows straight ahead or from his left (from the northeast) or from his right (from the northwest).  This last was considered the mildest of these three winds, swelling up his right cheek and clearing out the night sky.  However, there is some disagreement on this point.  Those who find Dante's source in the Tresor of Brunetto Latini (I.cvi.14) maintain that the passage refers to the northeast wind.

Tommaseo (DDP Tommaseo.Par.XXVIII.79-81) locates a source in Boethius (Cons. I.m3.1-10).