A real example of Foehn effect

Foehn effect

When relatively cold, moist air ascends a mountain, it may get saturated and precipitate. But when it crosses the mountain and starts dry adiabatic descending, the vapor pressure will decrease, and the air can be much warmer. It can be easily proved by drawing a thermodynamic diagram (see Figure 1).

Figure 1. Foehn effect. AB: dry adiabatic, BCD: moist adiabatic, DE: dry adiabatic.

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A Real example in Norway

Figure 2 shows a real example of Foehn effect, where the polar front zone is affected by topography. Clouds are dissolved in the east of the Scandinavian Mountains, and reappear more to the east. In Figure 3, a large amount of precipitation can be seen in the west of Norway.

Figure 2. Satellite image in 15.12.2021. Left: 01 UTC, right: 04 UTC.

Figure 3. Precipitation amount calculated by GFS model.

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Read these to learn more:

• "Foehn wind" on Wikipedia: https://en.wikipedia.org/wiki/Foehn_wind

Author: ZH. Cheng