Refer to figure.

FOEHN EFFECT

Foehn winds are warm and dry, gusty winds that periodically descends the leeward slopes of nearly all mountains and mountain ranges (warm katabatic wind). This is a change from wet and cold conditions one side of a mountain, to warmer and drier conditions on the other (leeward) side.

- How does it form?

When air is forced upwards over elevated terrain, it expands and cools adiabatically (at the DALR) until it saturates. After saturation, it will continue to cool at the SALR. Since colder air can hold less water vapour, moisture condenses to form clouds and precipitates as rain/snow above the mountain’s upwind slopes. By the time it reaches the peak and stops climbing, the air is quite dry.
As the air makes its leeward descent, it is compressed and warms rapidly all the way downslope – because there is little water left to evaporate. Thus, the air is warmer and drier when it reaches the leeward slope than when it begins its windward ascent.

• On the windward side we can expect low cloud and precipitation whilst on the leeward side we will see clear turbulent conditions.