What causes a foehn wind in the Alps?

A foehn wind in the Alps is a warm, dry, and gusty wind caused by adiabatic temperature changes as air flows over the mountain range.12

The process occurs in several stages:

• Windward Ascent: As moist air is forced up the windward slopes, it expands and cools. As it rises, the moisture condenses into rain or snow, a process that releases latent heat and slows the rate at which the air cools.13
• Moisture Loss: By the time the air reaches the mountain crests, it has lost much of its moisture. The upper limit of this precipitation is often marked by a bank of clouds known as a foehn wall.1
• Leeward Descent: As the now-drier air descends the leeward side, it is compressed and warms rapidly. Because the air contains little water to evaporate and absorb heat, it warms at a constant rate through compression. Consequently, the air reaches the foot of the leeward slope significantly warmer and drier than it was at the start of its ascent.132