• Caitlin Kennedy

Avalanche Science 101

With the heavy dump of snow we had on Monday, there has been much talk of avalanches and many resorts this season in the Western Alps have had deadly slides. In some places in Austria, 3.5m of snow are reported to have fallen in the space of a week. Although Val d’Isere might not have been hit by nearly as much snow, we thought we would take a look at what actually happens when an avalanche occurs.

So what causes a slope to slide or not? Essentially the stability of the snowpack and a trigger. The snowpack is made up of different layers, each of which represents an individual snowfall. Each of these layers has different properties due to the conditions present when the snow fell. Things like the structure of the ice crystals and the surrounding temperature and humidity when they fell impact how well the layer bonds to the one above or below. If the individual snow molecules are 6 branched crystals, i.e what we envisage when we think of a snowflake, they interlock well, creating a more stable layer. On the other hand, when needles form, they don’t gel so well. Similarly, if super-cooled water comes into contact with snow crystals in the air, it creates what is known as rime. Heavy rime deposits can cause pellet-like snow called graupel, which creates a very unstable layer.

6 point snowflake Depth hoar crystal Snowflakes with rime

Once the snow is on the ground, other changes can take place which decrease the layer’s stability:

  • If the top of the snowpack melts and re-freezes, it can form a layer of slick ice, which is obviously not going to hold onto any future snow that falls very well.

  • If air just above the snowpack is dewy, the snowpack can develop hoar, which is a light, feathery crystal that does not bond well to snow.

  • Big differences in temperature between the layers also contributes to instability.

An avalanche can either be triggered naturally, for instance by a sudden change in the weather, a falling tree or a collapsing cornice, or by an external load, like a skier. 90% of fatal avalanches are caused by the latter. Wind is the most common natural cause of avalanches. Wind can deposit snow 10 times faster than snow falling from storms. Wind erodes snow from the upwind side of obstacles and deposits snow on the downwind. This is called wind loading. Of course massive snowstorms can also cause avalanches. If the weight of new snow is added faster than the buried weak-layer can adjust to its load, then it fractures and forms an avalanche.

Sluff avalanche Slab avalanche

If a weak layer in the snowpack gives way, the type of avalanche that occurs will depend on how close to the surface the “failure” layer was. If it was near the top, a sluff avalanche occurs. This is a cascade of loose, powdery snow in an inverted “V” shape down the mountain and usually causes minimal damage to people and property. The dangerous ones are slab avalanches, which happen when the failure layer is deep down. In a slab avalanche, a strong, cohesive layer of the snowpack slides down over a bed layer of snow. These can carry up to a million tonnes of snow and travel at speeds of over 320 km/h. When they eventually come to a halt, the snow sets like concrete, which is what makes them so dangerous. It is impossible to move if you are under the snow once it comes to a standstill.

This article simply covers the basics of how and why avalanches occur. To learn how to avoid getting caught in a slide, and what to do if you or someone in your group is caught, head to Henry’s Avalanche Talk online where you can find detailed Off-Piste safety information. The ESF also run free weekly talks on a Monday evening and this week, the Vie Val d’Is has an Avalanche prevention talk also on Monday, which is free for Vie Val d’Is card holders.