The Bise is a north-easterly wind which blows across the Swiss plains on the north side of the Alps. It is composed of cold, dense air, and is stronger at ground level than at higher altitudes. Strictly speaking, the Bise is by definition a regional wind located in the flatlands, but the term is also used in the high Alps to refer to the regime associated with an airmass which has come from the north or north-east. The most obvious feature of this situation is lee side turbulence on south facing slopes created by the general airflow from this direction. However, the Bise may also produce cloud in the flatlands whilst the mountains remain sunny. This suppresses thermic activity in the plains, compared with the mountains, resulting not only in a strengthening of thermal-driven valley breezes, but also an increase in the north>south pressure difference, which boosts the existing north/north-easterly flow. These elements combine to render the flying conditions around Fiesch less pleasant and more dangerous.
This is a typical synoptic chart associated with the Bise:
Although the isobars might not appear excessively close together, the strength of the resultant north-easterly flow was strong enough to cause significant impairment of the flying conditions around Fiesch; all the example charts on this page belong to this date. As the dominant feature, as in this case, is typically a static high pressure system, it can be enormously frustrating for visitors to wake every morning to blue skies followed by sweet cumulus development, but sub-optimal flying conditions.
The main factor determining the strength of the Bise in the flatlands is the difference in atmospheric pressure in the northeast/southwest axis across Switzerland. There is a chart showing a prediction of this parameter on Meteocentrale:
However, values like this will almost always be associated with an even greater pressure difference in the north-south axis, which is considered to be a more important determining factor of the intensity of the unpleasant effects of this regime (and the figure to which pilots will refer when discussing the conditions):
The degree to which the Bise may affect the flying conditions is not easy to predict, but for a given pressure difference it is likely to be more severe earlier in the season than later. I would avoid Fiesch if the figure exceeds 2-3 hPa in the spring, but even 4-5 hPa in August may have only a mild effect.
A weak Bise may be substantially blocked by the high mountains to the north of Fiesch, but it will still encourage the Grimselschlange and produce a strong breeze up the Schöllenen gorge into Andermatt (hence a north-easterly wind in the Urserntal). As its strength increases, so does the risk of this wind “breaking through” down the Fieschertal valley into the Fiesch landing field, or from the Aletschgletscher into the main Rhône valley at Brig. Furthermore, the westerly valley breeze in the main Rhône valley is often particularly strong in this regime, even though it is blowing in more or less the opposite direction to the general airflow.
Although this chart shows a strong north-easterly flow in the upper Goms, note that the westerly breeze around Visp is also quite vigorous.
These diagrams show the typical evolution of the valley winds in Bise conditions in those two locations:
Even though the Bise itself is a low-level wind, it will often be associated with disruptive airflows at higher altitudes, as demonstrated by this view (taken from the ground at Bellwald) towards Eggishorn, showing a typical effect on the clouds in strong northerly conditions:
If you have chosen to fly in Bise conditions, and realise (perhaps from observing pilots below all facing up the valley with no ground speed) that a nasty north-easterly flow has developed in Fiesch, then it will probably be safer to land elsewhere. Around Fiescheralp, options include the Heimat takeoff or the large flat area between Galvera and the cable car station. The club site guide recommends landing in Lax (the village 2km to the southwest of Fiesch) under such circumstances, but there is no official field there. If you can make it to the Goms, the valley breeze at Niederwald or Ritzingen may well be strong, but is likely to be laminar.
In the Surselva (tricky to reach from Fiesch in northerly conditions) you may well find that at any stage after crossing the Oberalppass the Bise can produce a headwind, even at high altitude. North wind can also push through the Panixerpass, washing down the slope from Brigels onwards and killing the thermals in this area. Finally, the northeasterly valley wind from Chur to Disentis is strengthened by the Bise.
Martin Scheel, Meet Director when the PWC last came to Fiesch, suggests that an early XC to the west, landing at Baltscheider, just west of Visp, is a reasonable option in northerly conditions, as the Bise would not be expected to penetrate any further down the main Rhône valley. Here’s an example (which is not one of my flights):
So the Bise can create considerable danger for the unwary around Fiesch, because the conditions throughout the morning may well be benign enough for the local schools to be teaching students, lulling the visitor into a false sense of security, only for a strong valley wind to arrive suddenly in the afternoon. Furthermore, it can even promote a region of shears and strong convergence between the north-easterly wind blowing down from the upper Goms or Fieschertal and the south-westerly valley breeze coming up from the Rhône valley. If the forecast suggests a consistent few days of Bise, I suggest you consider relocating to Verbier, where the flying is likely to be much more pleasant, even if you may be confined to the Val de Bagnes. You won’t find many Swiss pilots choosing to come to Fiesch in such conditions!