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Geography
Orographic Rain and Foehn Winds
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Move an air parcel over a mountain and change humidity or mountain height to explore orographic rain and the foehn effect. Compare temperature and humidity on both sides.
Air parcel
28.0 °C Temperature
75%Humidity
Initial conditions
Try a scenario
More conditions
Follow one air parcel
Select Run simulation or drag the air parcel. Follow its temperature and humidity as it crosses the mountain.
Detailed values
- Air parcel altitude
- 0 m
- Pressure
- 1,013 hPa
- Remaining water vapour
- 17.91 g/kg
- Water removed
- 0.00 g/kg
- Condensation level
- 590 m
g/kg dry air
| Landmark | m | °C | Humidity % |
|---|---|---|---|
| Windward foot | 0 | 28.0 | 75 |
| Condensation begins | 590 | 22.0 | 100 |
| Summit | 1,500 | 18.2 | 100 |
| Leeward foot | 0 | 33.6 | 48 |
Follow one air parcel Select Run simulation or drag the air parcel. Follow its temperature and humidity as it crosses the mountain.
Model notes
- The model follows an air parcel forced over one mountain whose two feet share the same altitude. The parcel neither exchanges heat nor mixes with surrounding air.
- All newly condensed water is immediately removed from the parcel. Clouds and rain illustrate condensation; real clouds do not always produce rain. Water removed is measured in g/kg of dry air, not rainfall in mm.
- This represents one mechanism that produces foehn winds. Real foehn winds may also involve air drawn down from higher altitudes, mixing and radiation.
- The parameter range represents water vapour and liquid water only. Ice, snow, re-evaporation and precipitation spilling onto the leeward slope are excluded. Results are not weather forecasts.
- Playback time, horizontal distance and parcel size are illustrative. Relative volume uses an ideal-gas approximation that neglects the small effect of water vapour.