The Foehn is one of the best known weather phenomena of the Alps, but it is not limited to this region alone. Foehn phenomena occur all over the world when mountains overflow. Foehn is warm, dry and turbulent. Some people are happy about the pleasant temperatures, others complain about headaches. Research into the origin of the balmy wind has a long tradition in the Alpine countries. But how does it originate?
Thermodynamic foehn theory
It is the best known theory and can be found in almost every textbook or popular technical literature. It focuses mainly on the question why the foehn is warmer and drier in the lee of the mountains. The foehn is caused solely by the different behavior of the air in the windward and leeward directions. According to this theory, the air is dammed upwind (the side facing the wind) and forced to rise. This causes the air to cool down, at a rate of 1 degree/100 altitude meters (dry-adiabatic temperature gradient). At some point the dew point is reached (100% humidity), the previously gaseous water vapor begins to condense – and clouds and precipitation form. During condensation, energy is released that previously had to be used for evaporation. This heat of condensation now ensures that the air which continues to rise now cools less strongly, namely only with about 0.6 degrees/100 altitude meters (humid-adiabatic temperature gradient). In the lee, the air sinks and warms up again by 1 degree/100 altitude meters. Due to the rain in the windward direction, however, the air is now drier and thus warmer than it was before the overflow began. This theory is also called the Swiss Foehn type, since it actually rains frequently on the south side of the Alps when the Foehn is blowing south (accumulated precipitation).
Fig. 1: Schematic representation of a foehn, in this case with precipitation upwind of the mountains ; Source: MeteoNews
But now there are also foehns without precipitation upwind, sometimes even without clouds at all. The true foehn capital of the Alps is Innsbruck, and here there is no rain in more than 50% of the foehn cases south of the Brenner. Furthermore, this theory does not explain the high wind speeds and the turbulent behavior of the flow. Also an explanation for the penetration into the valleys is missing. Although people like to call Foehn a warm fall wind, this is physically incorrect. Because here nothing falls. The warm air is actually stably stratified (warm = lighter, cold = heavier), there is no reason for the sinking movement from the point of view of temperatures.
Hydraulic Foehn Theory
This is also called the Austrian Foehn theory. The name "hydraulic" comes from the fact that the air can be considered like a liquid under certain conditions. Mathematically this is described with the shallow water equation, but this only in the margin – we limit ourselves here only to a strongly simplified description. The big difference here is that the air is not forced to rise in the lee, or only to a very small extent. Much more there is a cold/cool and above all rather static air mass (blocked) below the ridge level. And indeed, during the Foehn in Innsbruck, one observes in many cases over South Tyrol and the Po Valley high fog-like cloud cover – and just no precipitation. Due to the lower air pressure on the northern side of the Alps, the air practically "runs" over passes and mountain ridges. You can imagine it like the overflow of a weir.
Fig. 2: Water flowing over a weir. Before the obstacle calm and slow, from the weir significantly faster; Source: Wikipedia
In front of the obstacle the water flows calmly and slowly, at a slightly greater depth it hardly moves. In the area of the edge, the vertical expansion of the flow becomes thinner, it accelerates. This is called "shooting current" (for those who want to go deeper, this is described in terms of flow dynamics with the Froude number ). So the flow accelerates in the lee and interacts there with the topography of the valleys. Other effects come into play here, as the valley in question may become narrower or wider, or at some point join another valley. These are the two-dimensional Venturi effect and the three-dimensional Bernoulli effect. Again, we don't want to get too deep into this. In any case, the flow cannot remain in this accelerated (shooting) state indefinitely. At some point, a"hydraulic jump" occurs, where the kinetic kinetic energy is dissipated turbulently (via turbulence). This sounds complicated, but again, a picture with water can be used to illustrate. In the above picture of the weir, the hydraulic jump would be the bubbling and turbulent water roll. The following picture invites to an experiment at home:
Fig. 3: Shooting flow and the hydraulic jump surrounding it in a circle in a sink; Source: Wikipedia
Why does the foehn rise into the valleys?
Before the foehn breaks through, it is usually cooler in the valley and the relative humidity is higher. Why does the foehn manage to displace the heavier (because colder) air in the valley? There are several approaches to this, each of which can have a greater or lesser part – depending on weather conditions and topography. Each Foehn valley has its very local peculiarities, and no two Foehn cases are exactly alike. The names of these theories may seem daunting, but again we keep it as simple as possible:
- Vertical Aspiration theory: Here the fast Foehn flow digs into the cold air lake from above, it planes it away step by step, so to speak.
- Horizontal Aspiration theory: The cold/cooler air is sucked out of the valley by the sinking air pressure, clearing the way for the foehn.
- Leewell theory: When flowing over a mountain range, the air is sometimes more, sometimes less vertically deflected – it starts to oscillate in waves. It is best to imagine a stone in a stream, still far behind the obstacle the wave movements continue. These waves propagate vertically and downstream. Typically, the crests of the waves become visible in the form of the typical lenticular foehn clouds(altocumulus lenticularis).
- Waterfall theory: the air in the foehn wall is colder and heavier, so this would be a true downdraft wind (the technical term is catabatic downdraft wind – the glacier wind would be another example of this process).
- Hydraulic theory (analogous to above)
- Solenoid theory (would lead here too far)
And which is correct now?
Everything to a certain part! Each of these approaches is a gross simplification, nature is more complicated – especially terrain structures. The thermodynamic theory, although the best known and most popular, actually contributes to warming only to a small extent. Also in Switzerland, the hydraulic aspect usually dominates. As already mentioned, Innsbruck is the real foehn capital in the Alps, here some special features come together. It is located at the confluence of the Wipptal and Inntal valleys, and the Brenner Pass lies to the south. It is the deepest north-south pass in the Alps, and the air flows northward through the Wipptal valley even at low pressure differences. Above ridge level, a completely different flow direction can prevail. This is called "shallow foehn", the exclusive flow through a pass "gap flow". In Switzerland, the north-south connections are usually higher, here a supporting southwest, south or southeast flow is often necessary at altitude ("high reaching Foehn"). This article is only intended to touch on this topic, but from here the interested reader can get further and more in-depth information. Enclosed is an overview of the many foehn valleys in Switzerland:
Fig. 4: Foehn valleys (south and north foehn) in Switzerland and neighboring Vorarlberg; Source: MeteoNews
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