Reading the mountain weather before you go

In the mountains, the weather decides almost everything: the route, the start time, sometimes cancelling altogether. A lovely day at Lake Annecy can hide a 3 pm thunderstorm over the Aravis. Here is how I prepare every outing.

Good sources

• Météo-France’s mountain forecast for Haute-Savoie: temperatures at altitude, freezing level, ridge winds, storm risk.
• Several models compared rather than a single app: if they disagree, uncertainty is high.
• In winter, the avalanche risk bulletin (BERA), even for a snowshoe outing.

Reading between the lines

A summer “risk of afternoon thundery showers” really means: start early, reach the summit before noon, be back down or sheltered by mid-afternoon. Cumulus clouds building up late in the morning are the first warning.

Wind and wind chill

A 40 km/h wind at 5 °C feels close to -2 °C. On an exposed ridge like the Semnoz, the wind tires and chills you far more than the cold itself.

On the trail

Keep an eye on the sky: thickening haze, a shifting wind, a sudden drop in temperature often signal a change. Set yourself a turn-around time and stick to it.