In our last two reports, we explained what exactly a Foehn wind is and how it influences the general weather situation.
Brief summary of previous articles:
- 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.
- The general weather situation in Europe has a decisive influence on the formation mechanisms of Foehn wind.
Case Study for the Swiss Alps
In Switzerland, four major large-scale weather situations can be identified depending on the wind flow: West wind, Bise, north and south Foehn. During typical south Foehn situations, there is an area of high pressure (H) southeast of the Alps (e.g. Lugano) and an area of low pressure (L) north of Switzerland (e.g. Zurich). This results in a south-to-south-westerly high-altitude current over the Alps. The air pressure on the Central Plateau is significantly lower than that on the southern side of the Alps. The greater the pressure difference, the greater the pressure gradient force and the stronger the equalising wind flow.
The effect of Foehn
The effect of Foehn is exemplarily shown on the 26th of February 2024, when a southerly Foehn prevailed. The screenshots show the meteograms for Altdorf (North Side of the Alps) and Belinzona (South Side of the Alps). For Altdorf, it shows high wind speeds, no precipitation, cloud cover in the upper troposphere and the highest daytime temperature of the week (14°C). For Bellinzona, however, the situation is almost the opposite: medium wind speeds, precipitation throughout the whole day, cloud cover with a cloud base in the lower troposphere, and significantly lower temperatures compared to the rest of the week. One of the reasons for these contrasts is the southerly Foehn. As a warm and dry downslope wind, it causes gale-force gusts in the northern Alpine valleys and is responsible for a drastic temperature rise when it arrives.
The following illustration explains why the Foehn wind can contribute to localised warming on the leeward side of a mountain foot. The image shows six steps:
1) Imagine an air parcel flowing over the Alps from Bellinzona in the south (high pressure) to Altdorf in the north (low pressure). As the air parcel rises, it initially experiences a dry adiabatic cooling of -0.98°C per 100 metres on average. It is defined as dry because the air parcel is unsaturated, which means the relative humidity is below 100%, and the air parcel can continue to absorb water. Adiabatic refers to the thermodynamic process in which there is no exchange of thermal energy between the air parcel and its surroundings.
2) The higher an air parcel ascends, the colder it gets and the less water it can store. The air parcel is saturated at some point (RH = 100%). At this altitude, dew point and clouds start to form. The height at which this occurs is also known as the condensation level (LCL). From this point onwards, the air no longer rises in a dry but humid adiabatic manner, meaning that the average cooling temperature is now reduced to -0.65°C per 100 metres. This is because latent heat is released during the humid adiabatic ascent, and the air cools down less strongly. In this case, latent heat is the energy released into the environment during a phase change from gaseous (water vapour) to liquid (water).
3) On the windward side of the Alps, there is an uphill rainfall before the air parcel reaches the mountain ridge. From then on, an impressive Foehn wall in Altdorf can be recognized.
4) Once the air mass reaches the leeward side (north side of the Alps), the air parcel is forced to descend. As it has already released precipitation and is warming up due to the descent, the condensation level is reached earlier, meaning at a higher level than on the luv-side of the mountain. From this point onwards, the relative humidity is below 100% again. As the air parcel becomes unsaturated, it returns to a lapse rate by an average of +0.98°C per 100 metres according to the dry adiabatic gradient.
5) Due to the shape of the Alps, the air in the middle troposphere is strongly swirled. So-called lee waves form, which are responsible for the formation of typical clouds (e.g. rotor clouds, foehn fish or stratocumulus lenticularis).
6) Overall, the descending air now heats up more than it previously cooled down during the ascent. The descending air masses (subsidence) lead to the dissolution of the clouds and clear skies. Because, practically, the entire descent is now dry adiabatic, the resulting warm air in Altdorf is responsible for the increasing temperatures in the area.
