The foehn effect (or Föhn) is one of the most important mountain weather phenomena. When moist air rises on the windward slope, it cools at the moist adiabatic rate (~6 °C/km) and precipitates. Crossing the summit and descending, now dry, it warms at the dry adiabatic rate (~10 °C/km), arriving in the lee valley warmer and drier than it started.
Worldwide examples
The name comes from the Alps, where the Föhn can raise temperatures by 15–20 °C in a few hours. Other famous examples are the chinook (Rockies) and the zonda (Andes). In Spain, foehn effects are common south of the Cantabrian Mountains and on the southern slopes of the Pyrenees.