Foehn Winds MET

A Foehn wind is a warm, dry wind.

When moist air is forced up a mountain, it reaches its dew point. As it is now saturated, as it continues to rise it cools at the SALRSALR —Saturated Adiabatic Lapse Rate, at 1.8 degrees per 1000'. As it rises it is likely that some of the condensing vapour will fall on the mountain as precipitation. Once the air reaches the top of the mountain, it may start to flow back down. However, as moisture has been lost as precipitation on the windward side, it will become unsaturated more quickly, at a higher level. Because it is now unsaturated, it warms at the DALRDALR —Dry Adiabatic Lapse Rate - 3 degrees per 1000'. Because it has spent a longer distance warming at a higher rate and lost the moisture on the windward side, the wind blowing down the hill is warm and dry.

The side of the mountain on the opposite side from where the wind is blowing gets this warm, dry wind. One example of this is the Chinook wind, which flows over the Rockies in North America.