Mountain weather hazards

The essentials in two minutes

1. A thunderstorm is a cumulus that doesn’t stop: towers taller than they are wide, then a smooth top, then an anvil. The signal to act is the tower, not the lightning.
2. A cell lives 30 to 60 min, but air-mass thunderstorms take over from one another from about 14:00 to 18:00; frontal and cut-off-low thunderstorms come at any time.
3. Under a congestus, cloud suck (5 to 10 m/s) can exceed what the wing can descend (big ears and speed bar: 2 to 4 m/s): get out sideways, on speed bar, towards clear sky — and leave before.
4. The gust front runs several kilometres, sometimes tens of kilometres, ahead of the storm, under a sky that is still blue, at 50–100 km/h: land early, in a big field, and pack the wing away.
5. Lightning: 3 s = 1 km; at 30 s or less, take shelter; 30 min after the last thunder before setting off again. Leave high points and cables, no isolated tree, crouch with feet together on your rucksack, spread out from one another.
6. A thunderstorm upstream can make the torrent rise within minutes: no bivouac in the stream bed, no crossing in flood.
7. Wind (gusts 15 km/h or more above the mean: choppy air), foehn, low cloud, whiteout, wet and windy cold (at 5 °C and 40 km/h: feels like −1 °C), heat, refrozen snowfields: each hazard has its signs and its timescales.
8. The timetable is built backwards from the time of the thunderstorm (a margin of at least an hour, then the time to descend); the cut-off time is set before you leave and is not renegotiated.