page_title: Foehn wind
section_title: Condensation and precipitation
hierarchical_section_title: Foehn wind / Causes / Condensation and precipitation
caption_attribution_description: English: Rotor cloud revealing overturning and turbulence above the lee slopes of the Antarctic Peninsula during a westerly foehn event.
context_page_description: A föhn or foehn is a type of dry, warm, down-slope wind that occurs in the lee of a mountain range.
  It is a rain shadow wind that results from the subsequent adiabatic warming of air that has dropped most of its moisture on windward slopes. As a consequence of the different adiabatic lapse rates of moist and dry air, the air on the leeward slopes becomes warmer than equivalent elevations on the windward slopes. Föhn winds can raise temperatures by as much as 14 °C in just a matter of hours. Switzerland, southern Germany and Austria have a warmer climate due to the Föhn, as moist winds off the Mediterranean Sea blow over the Alps.
context_section_description: When winds blow over elevated terrain, air forced upwards expands and cools due to the decrease in pressure with height. Since colder air can hold less water vapour, moisture condenses to form clouds and precipitates as rain or snow on the mountain's upwind slopes. The change of state from vapour to liquid water releases latent heat energy which heats the air, partially countering the cooling that occurs as the air rises. The subsequent removal of moisture as precipitation renders this heat gain by the air irreversible, leading to the warm, dry, foehn conditions as the air descends in the mountain's lee. This mechanism has become a popular textbook example of atmospheric thermodynamics. However, the common occurrence of 'dry' foehn events, where there is no precipitation, implies there must be other mechanisms.
caption: Rotor cloud revealing overturning and turbulence above the lee slopes of the Antarctic Peninsula during a westerly foehn event.
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