Definition
A warm, dry, downslope wind that flows down the leeward side of a mountain range. As moist air is forced up the windward side, it cools and drops most of its moisture as precipitation. The now-dry air descends on the leeward side, warming through compression as it loses altitude, producing unusually warm and dry conditions at the base of the mountains.
Plain English
A warm, dry wind that comes down the back side of a mountain range after the air has lost its moisture climbing up the front side.
Context Anchor
Seen in weather discussions for mountain flying, especially when planning routes near ridges, valleys, and airports downwind of mountains.
Derivation
From the German Föhn, the local name for this wind in the Alps. It traces back through Old High German to the Latin favonius, meaning 'a warm west wind.' The German term became the standard meteorological label worldwide.
Why Pilots Care
Foehn winds can produce strong turbulence, sudden temperature rises, reduced air density, and rapid changes in cloud cover that affect climb performance and icing risk.
Grounding Statement
Picture moist air being shoved up one side of a mountain range, raining itself out near the top, then sliding down the other side as warm, dry, gusty wind by the time it reaches the valley floor.
Intuition Check
A foehn is not just any warm wind. It is specifically a warm, dry wind caused by air descending after crossing mountains.
Example Sentence 1
The pilot expected turbulence on the leeward side of the ridge, where a strong foehn was forecast that afternoon.
Example Sentence 2
Foehn winds cleared the sky on the leeward side but created moderate turbulence that required a lower airspeed during descent.