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Chapter 14

Types of Cloud.

The Föhn Wind.

The Föhn Wind can form in a stable atmosphere when the wind blows towards mountains. Air forced to rise over a mountain will begin to cool at the Dry Adiabatic Lapse Rate of 3 degrees per 1,000ft. If it is cooled to the dew point, cloud will form and any further cooling will result in precipitation. This further cooling will take place at the Saturated Adiabatic Lapse Rate (1.5degrees per 1,000ft).

 

When precipitation occurs, moisture is removed from the air which results in drier air on the lee side of the mountain. This has the effect of lowering the dew point and therefore the cloud base will be higher on the lee side of the mountain. When the air starts to descend on the leeward side of the hill, it will warm at the Dry Adiabatic Lapse Rate as it is no longer saturated. This results in warmer, drier air on the leeward side of the hills.

 

This effect is commonly observed in the U.K near the mountains of Snowdonia and the Pennines during a westerly wind. The result is warmer, drier air on the eastern side of the hills.

​That's the end of Chapter 14. Now test yourself on Types of Cloud (requires purchase of practice exams) from the Main Menu.