Following the Tignes avalanche there is a lot of confusion, both in the press and amongst some skiers as to how the avalanche was triggered. A number commentators have suggested that the slab is a layer of snow that doesn’t adhere to the base and slides due to the weight of a passing skier or that the slab is “warm” and the base is “cold”.
It should be reasonably obvious that a mass of cohesive snow, weighing hundreds tonnes in many cases, is not going to be sent careering off downhill by the weight of a 80kg skier… okay 85kg skier if he’s using Dynafit Beast bindings, 120mm skis and equipped with an Airbag.
First what do we actually know about the Tignes avalanche? An investigation is underway but Cyril Anceau of the Mountain Rescue Services has said that the group were on foot and possibly triggered the avalanche themselves by punching down to a weak layer.
Given that Captain Anceau probably knows what he’s talking about as he’s already had an expert up looking at the slide, we’ll look at this scenario in detail. There is a weak layer, at or near the base of the snowpack probably formed earlier in the season. The snow crystals are referred to as “goblets” in French - a reference to their structure which vaguely resembles a wine glass - thin fragile stem and wide top.
What happens is that these crystals form in the snowpack due to different temperatures causing sublimation of water vapour. At altitude they are nearly always present at the base of the snowpack as the ground is close to zero degrees and the air above much colder. They may also form in the snowpack between different snow layers, in particular on top of ice crusts. It is not the ice crust that causes the avalanche but a layer of weak snow above.
If the weak layer is disturbed it can collapse locally and the weight of the snow slab above may cause that collapse to propagate over a wide area like dominoes falling. A local collapse may cause a whoumping sound and cracks in the snowpack but if the collapse spreads over a big enough area it can create enough energy to break the bonds the slab has with the surrounding snow and gravity then does the rest.
The video shows graphically that it is only necessary to cause a local collapse by disturbing the weak layer for a much larger area to slide. A hundred skiers can descend a slope before it avalanches, what matters it disturbing the weak layer - maybe where the snowpack is thin, maybe by a fall, or simply walking on foot. The more widespread the weak layers are in the snowpack the larger the potential slide. It is even possible to trigger a slope above a valley where the weak layer extends onto the flat ground below.
Wind, such as that experienced over last weekend, can create new slabs, or increase the size of existing slabs to a critical level by piling snow on to lee slopes. There doesn’t have to be any new snowfall. The majority of backcountry accidents involve slab avalanches. There haven’t been that many incidents this season because there simply hasn’t been much snow depth - fewer, smaller slabs and fewer skiers venturing off piste.
Was yesterday's avalanche avoidable? Well even if we understand the mechanism leading to slab avalanches and why a skied slope doesn't necessarily slide it is impossible to say at the top of slope whether or not it will avalanche. It may the question of one skier skiing a few meters to the left of another skiers line. One can say that an avalanche is probable but 99/100 it doesn't happen. Experience isn't necessarily the best teacher. All you can do is trust the science and carefully read the avalanche bulletin and keep off suspect slopes... hard to do when you know that north face holds a stash of powder that you've already seen a dozen people skiing.
Further Information
http://pistehors.com/backcountry/wiki/Avalanches/Slab-Avalanches