Tuesday, July 7, 2009

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.