Overpressure

“Overpressure” (a term used to refer to differences in atmospheric pressure) has a significant influence on the winds around Fiesch, hence the inclusion of this page here.

Having learned to fly in the flatlands of the south of England, I was taught to expect winds to blow more or less parallel to the isobars because of the coriolis effect, with their speed increasing with height due to the reduction in ground friction.  However, in mountains there may also be an extent to which air moves directly from high to low pressure, i.e. perpendicular to the isobars, and funnels through more strongly low down in narrow valleys than higher up, so it is important to consider overpressure as well as the geostrophic flow here.

If you ask a regular Fiesch pilot for advice about the day’s flying conditions, they are likely to mention this parameter.  Around here, the pressure difference in the north-south axis is considered to be the most important, and the measurements which are being referred to are those from Lugano (to the south) and Zürich (to the north).  So if you hear that there is “2 hPa of south overpressure”, you are being told that the value (adjusted for the different heights of the weather stations) is 2 hPa higher in Lugano than in Zürich.  A chart is available at Profiwetter:

As you may have inferred from the text on the chart, the most significant feature generated by overpressure is föhn.  It is also associated with the tendency of other winds such as the Grimseler to arise from side valleys.  Another potential negative effect is the production of leeside downwashes, often particularly noticeable on south-facing slopes in north overpressure.  Finally, the higher the value, the more likely it is that the air will feel generally choppy and challenge your active flying skills.

However, overpressure is a complex issue.  A chart showing differences between a variety of locations (and hence axes other than north-south) can be found at Wind und Wetter:

Wind und Wetter pressure differences chart (on the web page itself, click on each shaded area for details)

Furthermore, the pressure differences at ground level (which can easily be measured) may well not correspond to those at different heights (which are generally derived by weather models and difficult to ascertain), especially on days with inversions.  In August 2024, there were several days with north overpressure measured between Zürich and Lugano but substantial south overpressure at higher altitudes, which produced turbulent flying conditions and unexpected winds.

Variations of temperature and humidity between different airmasses can also affect overpressure and its consequences, and the time of year seems to influence the intensity of its effect.

Finally, it is estimated that the heat low associated with vigorous thermic activity on a good day may generate up to 3 hPa of negative pressure above the middle of the main valley, boosting the gradient on the lee side whilst inhibiting it on the windward side.  This explains why it is sometimes possible to escape from generally rough air on one side of the valley by crossing to the other.

To fly safely around Fiesch, you must be aware of the expected value of this parameter because of its potential to cause dangerous conditions.  It is often stated (and implied by the dotted lines on the Profiwetter chart) that 4 hPa is some sort of threshold for föhn, but this is a dangerous oversimplification and not borne out by experience.  As with all weather phenomena, predictions often do not correspond to reality, and other factors may combine to reinforce or counteract the effects of individual meteorological factors such as this one.