8.32 Mountain Weather

8.32.2 Define the Föhn wind.

A Föhn wind is a warm, dry, and often gusty wind that blows down the lee side of a mountain range.

It develops when moist air is forced to rise on the windward side of the mountains. As the air rises, it expands and cools adiabatically. If the air reaches saturation, cloud and precipitation may form, removing moisture from the air.

Once the air passes over the mountain range and descends on the lee side, it is now drier. The descending air is compressed and warms at the dry adiabatic lapse rate.

Because moisture has been removed on the windward side, the air descending on the lee side becomes warmer and drier than it was before crossing the mountains.

This produces the typical Föhn conditions of:

• Warm air
• Dry air
• Gusty winds
• Turbulence on the lee side

8.32.4 In Föhn wind conditions, describe the typical weather:

(a) to windward of the mountain range;

On the windward side, moist air is forced to rise over the mountains.

Typical weather may include:

• Extensive cloud
• Low cloud base
• Rain or showers
• Reduced visibility
• Strong winds near exposed ridges
• Icing in cloud if the temperature is below freezing