Tuesday, June 23, 2009

The dreaded "goutte froide"

Right now flying conditions are not good in the Alps. We're in the dreaded "goutte froide" situation. This occurs when cold air sweeps across the Atlantic Ocean and a blob on the end breaks off and floats around the Mediterranean Sea. It's visible on the temperature chart on the right as a big orange "bite" centred over the Adriatic Sea and is a low pressure system.

The winds blow anticlockwise around a low pressure system in the Northern Hemisphere and these winds collect moisture from the Mediterranean Sea and dump it as rain on the Eastern end of the Alps. You can see on the precipitation chart on the right that today is going to be a very wet day in Austria and Northern Italy. No flying there.

At the same time there's a big high pressure system over the British Isles and the North Sea. The winds blow clockwise around these and the air is squeezed between this high pressure and the "goutte froide" and accelerates, i.e., it's windy. At the West end of the Alps (Geneva) this cold NE'ly wind is too cold to flow over the Alps so it flows around them and is further squeezed between the main body of the Alps and the Jura mountains. It's the classic "Bise" wind: cold, dry and strong. Right now in Geneva it's about 40km/h at ground level and 60-70km/h at altitude. Further south, this wind blows down the Rhone Valley past Lyon and becomes the "Mistral" in Marseille.

In these conditions flying is impossible in all but the most protected areas. Plan Joux in Chamonix is one such site as it protected by the enormous cliffs of the Rochers de Fiz behind it. Further south, St Vincent les Forts is well protected by the 4000m+ Ecrins massif. At these sites you can fly comfortably all day long in bubble of light winds, but stray to high or far and you quickly hit the strong winds and things can get interesting. For paragliders, flying cross country is almost impossible.

The bad news is that such unflyable situations - rain in the East, wind in the West - can persist for several days at a time. The current forecast is for an easing of the winds tomorrow and the "goutte froide" to dissolve in to the surrounding air towards the end of the week leaving a fairly even pressure over the Alps. Such flat pressure fields usually mean storms.

The X-Alps has historically had fairly mixed weather, and if a situation like the current one sets up in late July/early August then it could become a war of attrition on the ground. It'll be tough.

Update Tuesday evening: Tomorrow (Wednesday 24 June) is looking like a cracking day in the Southern Alps while everywhere else in the Alps is pretty much unflyable. The hang gliders at the Hang Gliding World Championships in Laragne should have an epic day. It'll be very interesting if such a situation develops in the race: the runners might be able to get ahead in to the good flying conditions while the fast-in-the-air-slow-on-the-ground pilots will be stuck in the bad weather. You really have to be ready for anything in this race.