What are the conditions for a slab avalanche?

By Haas on 15 January 2016 · 0

Avalanches come in all shapes and sizes. Big, small, with wet or dry snow, spontaneously or triggered by man. The most spectacular avalanches are the big dust or powder avalanches lile the one below which was triggered in a controlled manner near the French village of Bessans in January 2016.

It's great footage, but these aren't the avalanches which are the most dangerous to skiers. No, we have to fear the slab avalanche ('Schneebrett' of 'Schifahrer Lawine' in German or 'Avalanche du plaque' in French). Because at least 90% of all fatal avalanches are slab avalanches. Avalanches that are triggered by ourselves or by someone in the group. Yes, you read that right, ninety percent. In the video below you can see how a skier triggered his own avalanche. But what are the reasons why a slab starts sliding?

Snow falls in layers

The snow pack does not fall in a single day, but is formed in the course of the winter. One snowstorm brings a lot of snow, others less. In addition, wind, thaw, cold temperatures and the sun influence the snow pack. The snow pack is therefore constantly changing, and consists of several layers. The snowpack is not homogeneous and therein lies the danger.

Layers that don't adhere to the layers below could slide down

A good example to show that danger is the 'trick' with the books. Grab a pile of books, hold on to the bottom one and tilt the stack slowly. At one point, the 'slope' is steep enough and the books that you are not holding start sliding down. If the snow does not consist of layers, there would be no problem. But you rarely ride the perfect loose powder. Snowflakes usually aren't piled on top of each other like feathers. No, the flakes connect with one another, and as a result layers are created which aren't always optimally connected to each other (as in the example of the stack of books). If a layer of snow (a book) can not directly attach to the layer below (the other books) you have a problem. There is a foundation for trouble invisible to the naked eye and not really easy to recognize. Trouble that can be expressed in a slab avalanche. That is, if it is steep enough.

A slope should be steeper than 30 degrees

When is a slope steep enough for a slab avalanche? That's pretty much the case if a slope is steeper than 30 degrees. The majority of avalanches come down on slopes between 35 and 45 degrees. Exactly the slopes that are perfect from skiing or boarding. But how steep is thirty degrees?

What is the steepest run you ever did? Wait, before you answer, how steep do you think the steepest black slope in a resort is? The steepest black slopes are between 33 to 35 degrees. Le Tunnel in Alpe d’Huez, the Harakiri in Mayrhofen, the Mont Fort in Verbier and Le Mûr in Avoriaz are examples of this. But when standing in the powder it becomes clear that the steepness is not that bad. The fun only begins at 27 degrees. For most of us powder snow becomes fun between 30 and 39 degrees. You feel the resistance underneath your feet, you can pick up speed and the snow hits your face. Unfortunately this is also the angle where most deadly avalanches take place.

And with the angle the conditions for an avalanche are completed. After years of study, they found out that there's an increased risk on avalanches if:

  • the angle is greater than 30 degrees;
  • and a bonded layer (in the example above this is the book or books that start sliding) is present;
  • and a weak- or sliding layer is present (in the example above the book you hold on to) is present

These three conditions together guarantee an increased risk. If you only have to deal with only one or two of these conditions, then there is nothing to worry and is the probability of a slab avalanche nil (for example: it won't start sliding when there's no such thing as gravity (like on a 25 degrees slope)). And remember, the shit hits the fan because you're there! Pretty much every deadly slab avalanche is caused by an external trigger, which is you or someone from your group. YOU ARE THE TRIGGER.

The conditions for a slab avalanche
The conditions for a slab avalanche

It sounds a bit like Russian roulette, right? The question arises whether it is still safe to ride off-piste. If you want to reduce the risk getting caught by an avalanche to zero, then the most effective measure is never to go skiing or snowboarding off-piste anymore.

But isn't it possible to minimize the risk? Of course it is. In the Mountain Academy we'll teach you everything you need to know about how avalanches are created and how to anticipate so that you are riding off-piste as safe as possible.

Test your knowledge about avalanches and snow safety for free!

The avalanche danger in the Alps is 'Considerale' (3 on a scale of 5) or 'High' (4 on a scale of 5) at the moment. More and more people venture off-piste, but not everybody has the right knowledge and the right gear. Already 15 people died because of avalanches this winter. Do you have the right knowledge to ride off-piste? Test it yourself and start with the first capital of the Mountain Academy for free! No knowledge? Stay on the marked slopes or hire a mountain guide!


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