In our last article, we defined Foehn wind and the conditions under which it can develop.
Brief summary:
- A warm, dry downslope wind is known as a Foehn in German-speaking countries. This occurs on the leeward side of a mountain range, for example, when air masses flow over a mountain or mountain range due to strong winds.
- The overflow is caused by differences in air pressure on a mountain range's windward and leeward sides.
General weather situation
As already explained, a pressure difference between the north and south sides of the Alps is required for a Foehn wind to develop. One of the factors responsible for the development of pressure differences is the general large-scale weather situation in Europe. For instance, on the 26th of February 2024, the large-scale flow led to the development of a southerly Foehn at that time. Large-scale weather situations are defined as the general atmospheric conditions over a large-scale area and occur over several days. The wind animation screenshot shows a low-pressure area (cyclone) over France. Cyclones always rotate anti-clockwise in the Northern Hemisphere due to the Coriolis force. While cold polar air masses flow into the low-pressure system from the north, warm subtropical air flows in from the south. As the low-pressure area shifts from southwest to northeast throughout the course of several days, a pronounced southerly flow moves towards the Alpine region, causing an accumulation of air masses at the foot of the southern side of the Alps. Situations like these are often associated with heavy precipitation on the southern side of the Alps, while a Foehn wall forms over the Alpine ridge, which can be seen in the left picture showing Jungfraujoch in the Bern Region.
Once the air masses have crossed the Alpine ridge, they descend along the northern side of the Alps, which heats them up and contributes to cloud dissipation (Foehn window). Further north, several side effects of the Foehn wind can be identified. For example, lee waves (turbulent air currents) contribute to the formation of spectacular clouds, so-called lenticularis clouds.
