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You are flying towards an aerodrome west of a mountain ridge. Cumuliform clouds are at the height of the top of the ridge overhead the aerodrome. Wind is coming from the west. Where would you expect the highest risk of aircraft icing?
  • A
    On the Eastern side of the ridge, at the same height as the ridge.
  • B
    On the Eastern side of the ridge, halfway up its slope.
  • C
    On the Western side of the ridge, at the same height as the ridge.
  • D
    On the Western side of the ridge, halfway up its slope.

Refer to figure.
The question text is, most likely, describing the Foehn Effect.

Foehn winds. 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.
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.
  • The greatest risk of aircraft icing is found on the windward side of the mountain in saturated air and thick clouds, such as cumuliform clouds.
Note: In this case, with wind coming from the west, the western side will be the windward side.
Halfway up the slope, the air may not be saturated yet and condensation may not have started yet => risk of icing is, therefore, lower. At the height of the ridge is the most correct option since we don't know the saturation of the air before the ridge.

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