Föhn Winds

FOEHN WIND/EFFECT

The foehn wind was named for a warm dry wind that occurs in the Alps. There are several other winds in other parts of the world which are caused by the same effect, such as the Chinook, which flows down the east side of the Rocky Mountains.

The foehn wind occurs when air is forced to rise up a mountainside in stable conditions. It cools initially at the DALR until it reaches saturation. At this point, the cloud starts to form and the air continues to rise, but now cools at the SALR.

Once it reaches the top of the mountain it starts to flow down the other side. Initially, it warms at the SALR but quickly becomes unsaturated as much of its moisture has already been lost. It then warms at the DALR.

Since the cloud base is higher on the lee side, the air at the base on that side will be warmer than on the windward side. The difference can be as much as 20°C.

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