Season Diary - Day 10

Henry

Dec 12, 20244 min read

Thursday 12 December, 2024

Avalanche safety course day. I love avalanches. Or rather, I love learning about them, why they happen, and what we can do to avoid them.

I have already completed this part of the course, attending just the avalanche safety element about four years ago. It was what started an absolute spiral down into the wide and fascinating world of avalanche safety, something I have looked to keep developing and proselytising on ever since.

Day one of this two-day course-within-a-course was all about why avalanches happen. I’ve touched upon this at times throughout this blog, but it is one of the most fascinating elements of skiing and backcountry safety.

95% of avalanches that impact humans are triggered by humans themselves – the vast majority of skiers and riders triggering the avalanches that bury them themselves, and small sliver triggered by other people or other groups above or around the avalanche victims. Therefore, it stands to reason that if you can avoid triggering avalanches in the first place, you can do a lot of work to avoid getting avalanched.

So why do avalanches happen? It is often a perfect combination of a lot of factors, far from the random event that many still perceive it to be. You need a slope angle of between 30° and 45°, with a sweet spot of around 38° where most avalanches happen. You then need some form of layering in the snowpack, perhaps where new snow has fallen on a hard slab of windpack or surface hoar. And, on a much much finer level, you these layers to be changing within a very small space – 10mm or less – to create a bed for the avalanche to slide on.

You can still get avalanches where this clear layering hasn’t occurred, but these tend to be soft slab avalanches are of significantly less danger than others.

A persistent weak layer is perhaps the greatest danger, and when this is flagged in an avalanche report, needs to be taken seriously. Morning dew freezes on the surface of the snow, creating a layer of crystals and are then buried by fresh snow; thanks to a temperature differential in the snowpack, where it is warmer at the bottom than at the top, these crystals do not disappear, and instead automatically create the layering system mentioned above. With the right application of pressure, these crystals can shatter, sending the entire snowpack on top down the mountain.

Examining the layers of an avalanche test pit

With all this in mind, after a morning spent in the classroom we set off up the hill to do my favourite pastime – digging avalanche test pits.

Avalanche pits are a really useful tool for getting a rough understanding of the snowpack make up, revealing layers and demonstrating how susceptible these layers are to “popping off”. We headed straight to the top of the Grand Motte, where it was bitterly cold, so we made sure to wrap up warm!

Under a nice sheltered bit of rock, we dig our trench, Graham clearing a nice big space for our entire group to work, probably three metres wide and down to the ground, about 160cm at this point.

When assessing the snowpack in a pit, first try and visually identify the layers. Then do a firmness test, trying first to push a fist, then four fingers, then one finger, than a pencil, then a knife into the snow. This will tell you where the layers are, and therefore where you can expect them to fail.

Once you’ve identified your layers, test them. Cut out a 30x30cm block that extends at least a metre down into the snow. Place your shovel upside down on top, and give it thirty taps – 10 from the wrist, 10 from the elbow, and 10 full force from the shoulder. Keep an eye on the snowpack, and if it drops sharply after a hit, then a layer has collapsed; record the number of taps it took to collapse your layer. Reach behind your isolated column and pull off the layer that has collapsed, and explore the bed surface of what’s left of the column; is it smooth, or fractured and broken? Is there a layer of sugar-like snow on the surface, the hoar or crystal layer that has collapsed?

All this is designed to help inform you of what the snowpack might be doing. If it pops off straight away, leaving behind a smooth bed surface and plenty of snow, then it tells you that, when avalanches do occur, they will be big and powerful. If they don’t, or if they take thirty hard whacks and leave behind a rough bed surface, then the snowpack probably isn’t susceptible to triggering big and hared avalanches.

There’s a lot of nuance to a snowpit that this piece simply can’t get in to. They are not the be all and end all of avalanche safety that some advocates believe them to be, but they can be a powerful tool when used in conjunction with an avalanche and weather forecast, and terrain knowledge.