Fohn is a warm dry wind that occurs in connection with ridges. The term originates from the Alps but is now used in meteorology to denote this type of wind in general.
When the air is forced up a mountain slope, it carries with it water vapor which partially condenses and is emitted in the form of precipitation. When the air then drops again, the heating becomes greater than the previous cooling (due to the lower water content) and a hot, dry wind results.