In German-speaking countries, the term ‘Foehn’ refers to a warm and dry downslope wind. This article deals with the development of these winds. In two more articles, we will look at the relevance and accompanying phenomena of the Foehn wind.
What is Foehn?
Foehn occurs when air masses flow over a mountain range due to strong winds. On the front side of the mountain range (Luv side), air masses are forced to ascend, causing them to cool down. After passing the mountain ridge, the air masses descend on the rear side of the mountain range (leeward side), where they warm up again and are perceived as dry and warm downdraughts. These downdraughts are known as Foehn or Foehn winds.
There are different terms for this type of mountain wind system around the world. In the alpine region, the term ‘North Foehn’ or ‘South Foehn’ is used depending on the direction of the Foehn wind. In the USA, the warm downslope wind in the Rocky Mountains is known as Chinook. In southern Chile, the term Puelche is used, and in the Andes region of Argentina, they call it Zonda. All terms refer to regional wind systems.
How does Foehn wind form?
In simple terms, a Foehn occurs when there is different air pressure on either side of a mountain range. The air endeavours to equalise the different pressure conditions. This equalisation is perceived as wind. The wind is, therefore, nothing more than equalising air pressure, flowing from places with higher air pressure (Town A) to places with lower air pressure (Town B). The corresponding force is known as the pressure gradient force. If there is an orographic obstacle between the high and low-pressure areas (e.g. the Alps), the air masses are either forced to flow around or over this obstacle. How an obstacle is overcome depends on a variety of factors. The height and width of the mountain range, as well as the stability of the atmosphere, are important.
However, this is a very simplified perspective, as winds rarely blow directly from high to low pressure. Friction at the ground and the Coriolis force are factors that make describing the wind flow more complex.
This phenomenon can be visualized with our wind animation. The video shows the wind for the 26th of February 2024. Regions with wind speeds (10 metres above ground) of between 25 and 35 km/h are coloured green. The wind is significantly weaker in the regions coloured blue. The black isobars (lines of equal pressure) show significantly higher pressure on the southern side of the Alps than on the northern side. The pressure difference between Bern (north side of the Alps, 998hPa) and Brig (south side of the Alps, 1004hPa) is 6hPa. The greater the pressure difference between the two sides of the mountains, the more pronounced the Foehn wind. To equalise the pressure difference, the wind flows from southeast to northwest across the Alps (see video). Orographic precipitation occurs on the southern side of the Alps, whereas a dry downslope wind (=Foehn) is observed on the northern side of the Alps, bringing higher temperatures in the northern Alpine region.
The following two articles deal with the influence of the general weather situation on the Foehn, show impressive pictures of the Foehn effects and explain the physical properties of the effect in more detail.
