Weather strategy

Right now there's a thunderstorm in Geneva. The lightning is spectacular and the rain torrential. There's just a thin sliver of clear sky sandwiched between the towering clouds and the crests of the Jura mountains. The light has an metallic blue quality, illuminating the city in eerie surgical glow. I've never quite seen anything like it.

Despite the years of preparation, the months spent training, route planning and tuning gear, the winner of the X-Alps race will most likely be decided by the weather. Maybe the strong pilots and aerial tacticians will be able to weave a thread through the mountains, keeping them in the air and covering hundreds of kilometres day after day. But if the rain falls and the wind blows then the runners will be kings. Positions in the race will rise and fall based on each team's abilities to exploit the weather to play to their own strengths.

Alex and I have a whole host of weather information sources at our disposal. For the general long term forecasts we use TopKarten which publishes the output of the American GFS model (click on the link then click on GFS). This gives us forecast pressures and wind speeds at different altitudes, precipiation, cloud cover and more esoteric variables like CAPE which allows us to predict the chance of thunderstorms. You can click on the "+6" and "-6" links to animate through the forecast for the next two weeks. We'll use the output of this model to make the grand strategic decisions, for example do we take the north route through Chur or the south route through Bellinzona?

Here's a quick guide to interpreting the output of the GFS model so you can predict the teams' strategic decisions during the race.

The first thing to look at is the wind speed. Meteorologists prefer to use pressure altitudes rather than elevations. To first approximation, 850hPa corresponds to about 1500m -- the altitude at which we normally fly -- and 500hPa corresponds to about 5000m -- the summit of Mont Blanc. Wind speeds less than 10km/h at 1500m are "go anywhere" days and paraglider pilots can fly in any direction. Up to 20km/h pilots can still fly but it's difficult to make progress against the wind, and above 30km/h we are grounded.

If there are also light winds at 500hPa then expect some very long flights, if you see a "snake" of strong winds (Stromlinien) over the Alps then it means that jet stream is near and flying will be difficult.

The precipitation (Niderschlag) is accumulated over several hours, so a little bit of light blue is not a worry: it means the occasional shower at worse. However, once you start seeing purple on the forecast charts then you can expect a wet day with limited flying opportunities between the rain clouds.

As I'm finishing off this blog post at my kitchen table the sun has set and the cloud above is as black as ink. Out on the road and in the mountains it will be a long and dark night.

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.