A weather phenomenon known as the föhn (with an identical effect to the chinook wind) can occur at all times of the year and is characterised by an unexpectedly warm wind, bringing air of very low relative humidity to the north of the Alps during rainfall periods on the southern face of the Alps. This works both ways across the alps but is more efficient if blowing from the south due to the steeper step for oncoming wind from the south. Valleys running south to north trigger the best effect. The driest conditions persist in all inner alpine valleys that receive less rain because arriving clouds lose a lot of their content while crossing the mountains before reaching these areas. Large alpine areas such as Graubünden remain drier than pre-alpine areas and as in the main valley of the Valais wine grapes are grown there.

What is the weather phenomena characterized by an unexpected warm wind bringing low reletive humidity to the northern Alps during rainfall in the southern Alps?