If you search on XContest, you will find flights using thermals made from Fiesch in every month of the year. However, in November, December and January, it is rare – but not impossible – for conditions to be good enough to clock up much distance. The first substantial XCs of the season usually begin to appear in February. In March, it may be flyable on about one day in three, of which half may provide conditions enabling good pilots to achieve 100km flat triangles. In April, the frequency of days when it’s pleasant to fly is much the same, but a few are likely be superb: the header image was taken in April 2013 as I started the transition from Eggishorn towards Bellwald, showing a cloud street marking a line of regular 2-3m/s thermals releasing from the snow line to base at around 3,000m, all along the valley.
May can be exciting – some say “brutal” – due to the combination of strong solar heating and a brisk lapse rate, but the weather is often quite fickle then, and the uphill transport may be closed for maintenance or on a severely restricted schedule from the closing of the ski season until the end of that month. In that case, you can use the takeoff at Riederalp, about 8km to the west, accessed by the gondola from Mörel. In June, you can usually expect plenty of good days, but the thermals are likely to have sharper edges and the frequency of unsettled weather is higher than later in the season.
In my opinion, the best month to choose to fly in Fiesch is July, due to the likely maximum frequency of good days. The really classic conditions with cloudbase at 4000m tend to occur in August, but there is also a greater chance of stability and inversions. September can be quite mellow, with some days of excellent XC conditions, but by October these are rare, and the tendency to weather disturbances in the Alps in the autumn can produce an unacceptable frequency of non-flyable days.
Here are a few of my flights from outside the normal season: