1911 Encyclopædia Britannica/Föhn
FÖHN (Ger., probably derived through Romansch favongn, favoign, from Lat. favonius), a warm dry wind blowing down the valleys of the Alps from high central regions, most frequently in winter. The Föhn wind often blows with great violence. It is caused by the indraft of air from the elevated region to areas of low barometric pressure in the neighbourhood, and the warmth and dryness are due to dynamical compression of the air as it descends to lower levels. Similar local winds occur in many parts of the world, as Greenland, and on the slopes of the Rocky Mountains. In the southern Alpine valleys the Föhn wind is often called sirocco. The belief that the warm dry wind comes from the Sahara dies hard; and still finds expression in some textbooks.
For a full account of these winds see Hann, Lehrbuch der Meteorologie, p. 594.