Mountain weather hazards

The essentials in two minutes

The first danger of summer is the thunderstorm. It grows from a cumulus that will not stop growing: the towers become taller than they are wide, the top ends up spreading out into an anvil, and in two or three hours, sometimes less, the well-behaved cloud of the morning has become a cumulonimbus. This brings everything at once: a violent downpour, hail, lightning, fierce gusts, and sometimes a flood in the mountain stream below.

For the pilot, two pitfalls dominate. Under the base of a big, growing cumulus, cloud suck can exceed what the wing can descend: you must leave beforehand, to the side, towards clear sky. And the cold air falling from the downpour spreads along the ground as a gust front that runs several kilometres, sometimes tens of kilometres, ahead of the storm, under a sky that is still blue: you must have landed well before it arrives.

For the hiker, the enemy is lightning. Count the seconds between the flash and the thunder: three seconds, one kilometre. At thirty seconds or less, you are within range: leave ridges, summits, arêtes and cables, move away from isolated trees, crouch with your feet together on your rucksack, and spread out from one another. Wait thirty minutes after the last thunder before setting off again. Above all, build your schedule backwards from the forecast time of the storm: it’s the turnaround time that saves you, not the crouching position.

Then there are the other hazards — strong wind, foehn, turbulence, low cloud and whiteout, cold even in July, heat, summer snow: for each one, what you see and what you do.