A very important local feature is the cold wind which pushes up the Haslital from the north-west and blows through the Grimsel Pass into the Goms. It’s often referred to as the “Grimselschlange” (“Schlange” meaning “snake”), but some say this term is only correct when it’s actually visible, as in the header image above or the lower picture below. These enable you to see how it spreads cold air over the shoulder at the end of the ridge (which is an excellent place to find a thermal when the Grimselschlange is absent!) and also down the slope above Gletsch, where the road to the Grimsel Pass zigzags.
This wind will nearly always develop on flyable days. However, with south overpressure, or a general meteo wind from between south-east and west, it may well be absent in the morning, and only blow relatively weakly in the afternoon. Conversely, with north overpressure, a general airflow from between north and east, or in Bise conditions, it is likely to arise early and have a significant effect on flying conditions. Furthermore, the same meteo situation will tend to be associated with a much stronger wind in the spring than in late summer. As you fly up the Goms, it may begin to cause a drift on the thermals from the north, or an increasing headwind. Unless you intend to cross the Grimsel Pass, it can be simply avoided, by staying high and heading away from the valley towards the higher slopes as you get near. Otherwise, if you are too far out, over the shoulder, you may find yourself caught in a sudden rough headwind being sunk out into significant leeside turbulence behind one of the spines running up the slope. Once you have reached its zone of influence, you will often find a blue gap or rapidly evaporating clouds, where thermals are suppressed by the general downward airflow which it causes.
As it descends into the Goms, it swerves to the right to blow down the valley from the north-east, and often gusts around 50km/hr in Oberwald. Further down the valley, e.g. at the Ritzingen landing field, the breeze will often reach 30 km/hr in the afternoon, but the flow is usually laminar here. Although generally viewed as a negative feature, it actually helps to trigger thermals by undercutting the warmer air in the valley, and towards the upper end of the Goms it may sometimes be used to slope soar and reconnect with thermals if you get low. It can also produce widespread convergence lift where it meets the southwesterly valley breeze coming up from Brig – a nightmare if you want to get down, but a delight if you succeed in finding it when needing a low save in the afternoon. You can also see a short video version of the picture above (i.e. the Grimselschlange viewed from the east).
When you intend to fly a triangle and start to feel the effects of this wind at the upper end of the Goms, it can be a tricky decision to decide how far to the east to persist for your turnpoint. In the example below, I pushed a bit too hard, and nearly ended up getting decked as a result.
Note how the thermals when approaching the end of the Goms with plenty of height appear to be more or less vertical (though somewhat fragmented), but once I am lower down, my climbs show a substantial drift from the east.
However, the Grimselschlange can also become your friend, e.g. if you have failed to find lift in the Grimsel/Furka area, enabling you to soar up from relatively low, to the south of Oberwald, and continue your flight along the north-facing side of the Goms, rather than bombing out, as in this example:
You can see that I reach the slope about 400m above the valley floor, and start by S turning, but am soon able to 360 in a thermal and climb out. Obviously this is likely to work better later in the day than earlier.