The Föhn effect (also spelled foehn) is the warming and drying of air as it descends the leeward side of a mountain range after having lost moisture on the windward side. The process begins when moist air is forced upward over mountains (orographic lifting), cooling at the dry adiabatic rate (about 10 °C/km) until condensation begins, then cooling at the slower saturated adiabatic rate (about 5–6 °C/km) as it continues rising and producing precipitation.
Once the air crosses the mountain crest, it has lost much of its moisture as rain or snow on the windward side. As it descends the leeward slope, it warms at the faster dry adiabatic rate (10 °C/km) because the moisture is gone. The result: the air arrives at the base of the lee side significantly warmer and drier than at the same elevation on the windward side. Temperature jumps of 10–20 °C in a few hours have been recorded during intense Föhn events.
The Föhn effect takes its name from the warm, dry wind that descends the northern slopes of the Alps into Swiss and Austrian valleys, but analogous winds occur worldwide: the Chinook (Rocky Mountains, North America), the Zonda (Andes, Argentina), the Berg wind (South Africa), and in Spain, Föhn effects occur when southerly flow crosses the Cantabrian Mountains (warming the north coast) or when the cierzo brings subsiding air through the Ebro valley. Föhn winds can trigger rapid snowmelt (avalanche risk), wildfires (extreme dryness and warmth), and health effects (headaches, irritability — the "Föhn sickness" recognised in Alpine cultures). In some valleys, night-time temperatures can exceed summer averages due to intense Föhn warming.