Posted on: 2021-01-04 23:05:57 by davidof

Danger! persistent weak layers at altitude

If you’ve been paying attention to the avalanche bulletins, particularly those published in the Southern Alps, you’ll notice a new phrase: Situations avalancheuses typiques, what we in the English speaking world would call “avalanche problems”.

What we are seeing repeatedly in the bulletins is this phrase: sous-couche fragile persistante en altitude. Persistent weak layer at altitude. It is something we’ve been talking about since the start of the season and results in HRLF events. High Risk (like very dangerous with big consequences) Low Frequency (they don’t happen that often, so even experts like guides don’t have that much experience of them and often ignore the dangers).

The team from Data Avalanche has been up in the mountains over the last week and has posted a couple of interesting videos that highlight the issue.

The first is from Tignes on the 29th December on a West face at 2415 meters. A block has been separated from the snow-pack. A saw cuts into the weak layer and the block goes. This is exactly what happens when you trigger a slab avalanche skiing. You cut the weak layer that collapses like dominoes and in your case, the whole slope goes. It is caused by a thin snow-pack and a big temperature difference between the ground (around zero degrees) and the cold night air (-25°C at 2800 meters altitude). This causes the snow-pack to rot out from the ground up (which is why it is often called an “old snow” problem).

The second video is to the south in the Galibier area. New Year’s day on a South-West slope at 2560 meters. Yes, that’s right, a south sector slope that sees some sunshine has these instabilities, something that is fairly rare.

Don’t be fooled. These kinds of problems can be present even when the overall risk is 3 (Considerable) or even 2 (Moderate) because they are relatively infrequent. Below 2300 meters the snow-pack was stabilized to a certain extent by high altitude rain before Christmas. There is an ice crust below the recent fresh snow. These avalanches are also susceptible to remote triggering so keep well clear of slopes > 30° at altitude at the moment.