Relief rainfall and the rain shadow – föhn effect over a mountain range
GeographyWeather & ClimateAges 15–16
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Sign in to playMoist air from the sea meets a mountain range and is forced up the windward slope: it cools at the dry adiabatic rate (about 1 °C per 100 m) until it reaches the condensation level, where clouds form, it cools more slowly (about 0.6 °C per 100 m) and the condensed water falls as rain. Beyond the crest the dried-out air sinks down the leeward slope, warming at 1 °C per 100 m, and arrives as a hot, dry föhn wind. Change the mountain height, the height of the leeward land, the humidity and the starting temperature to see the condensation level, the rainfall along the cross-section and the temperature–height graph change; try the real examples Andes – Atacama, Himalaya – Tibet and the Cascades.
Lesson: Relief (orographic) rainfall, rain shadows and föhn winds; factors affecting rainfall
What it shows
When wind blows moist air against a mountain range, the air is forced to rise. Rising air expands and cools; once it cools to its dew point, water vapour condenses into cloud and rain falls on the windward slope. Condensation releases latent heat, so cloudy air cools more slowly than dry air. On the far side the air sinks, warms at the faster dry rate and ends up warmer and much drier than before: a föhn wind and a rain shadow, like the Atacama Desert behind the Andes or Tibet behind the Himalaya.
How to use
Move the Mountain crest height, Leeward land height, Relative humidity of the wind and Temperature at the windward foot sliders and watch the air parcel, the clouds and the rainfall strip. Read the temperature–height graph and the results below. Switch Cooling in cloud between the textbook rate and the realistic one, pick an Example, and use Release parcel again or drag the parcel along the mountain.
Parameters you can change
- Mountain crest height 500–6000 m
- Height of the land on the leeward side 0–5000 m
- Relative humidity of the sea wind 20–100 %
- Air temperature at the windward foot 0–35 °C
- Cooling rate inside the cloud Constant 0.6 °C per 100 m (textbook), Depends on temperature (more realistic)
- Real example Custom, Andes – Atacama Desert, Himalaya – Tibetan Plateau, Cascade Range (North America)
Questions to explore
- Why is the air on the leeward side warmer than the windward side at the same height?
- How do the condensation level and the rainfall change when the wind is more humid?
- If the crest is below the condensation level, is there still a föhn effect, and why?
