TYPES OF Avalanche

A quick look at the three main types of avalanches; windslab avalanches, powder avalanches and wet snow avalanches.

Slab avalanches

Wind is a key indicator to how likely an avalanche is to occur. If there is no wind, snow is distributed evenly. If there is even a moderate wind, snow is deposited in sheltered areas referred to as lee slopes. The greater the wind and the snowfall, the more dangerous these snow deposits become. Unfortunately, these are not the only conditions that allow an avalanche to occur. Where a ski area is subjected to long periods of sun and high winds, large amounts of snow can also be deposited on the lee slopes.

On these lee slopes the wind packs the snow together in a cohesive mass. The situation becomes especially dangerous when the snow fails to bond with the slope. The lack of bond or ‘weak layer’ can exist within the snow pack for a long period of time and may stay dormant long after snow has stopped falling.

Worryingly, windslab avalanches occur most frequently on slopes of between 28 to 45 degrees, the ideal inclination for skiing and boarding. Slab avalanches require very little disturbance to occur. Often the weight of one rider will be enough. Although less frequent, on very steep slopes it takes even less to trigger an avalanche, in some cases all that is required are adverse weather conditions such as a storm or rapid rise in temperature.

As a windslab avalanche begins, the whole slab moves at the same time initially, before breaking up. The deeper the snow, the more dangerous the avalanche is. However, skiers and snowboarders should be aware that a relatively shallow avalanche could be fatal.

Powder avalanches

This is a type of avalanche that could level a city block but fortunately it is very rare. Powder avalanches occur after heavy snowfall and begin on slopes of over 45 degrees. Ironically, powder avalanches may start off small but build up to colossal proportions as they meet other small unstable areas of snow. As this occurs repeatedly, an avalanche will quickly become unstoppable. If they travel over a long distance on a steep slope the avalanche can become airborne and attain speeds of up to 250mph. At the front of the avalanche an awesome vortex will form that can uproot large trees and small buildings.

Wet snow avalanches

During periods of warm weather, water from melting surface snow can permeate through the snowpack to a weak layer. As the layer becomes more saturated it gains weight and cannot support itself. If the snow is on a smooth grassy slope there is little friction and a large-scale ‘full depth’ avalanche may occur. If you get caught in such an avalanche you have practically no chance of survival. The snow is heavy and devoid of air spaces. As it stops moving the snow cements and becomes almost impenetrable. In this scenario, south facing slopes and hot days in spring are the most dangerous places a skier or snowboarder could be.

Note: Even though there is a science to a slide, predicting avalanches can be extremely difficult because you simply don't know what's beneath the snow. A slope can be made unstable from weak layers metres under the surface.

Top mountain safety tips

Our mountain safety tips, which we have compiled with Henry’s Avalanche Talk (HAT), are a succinct reminder of how to prepare yourself before heading off piste.

  • Know where the unsecured areas begin in your resort
  • Read the daily avalanche bulletin and know what the ratings mean
  • Go one at a time in areas exposed to slopes of 25° or more
  • Know what is above you, below you and decide where to go up or down 
  • Stop at ‘islands of safety’ when on or below the slope 
  • Have the right safety equipment and know how to use it to rescue a member of your party - remembers you have less than 15 minutes
  • If buried make an air pocket in front of your mouth (remove pole straps off while riding)
  • Go to an avalanche talk and take a course

Don't forget that the Ski Club offers a transceiver hire scheme to members and non-members.

Avalanche awareness

Skiing and boarding away from the marked pistes can be an unforgettable experience, but it's important to acknowledge the risks.

There are lots of ways you increase your knowledge and get the skills you need to make safe decisions when going off piste, whether it's join one of our Freshtracks Touring Skills holidays, or attending a pre-season avalanche awareness talk. Nothing beats hands-on practice or even taking a practical course with a suitably qualified mountain guide, but here are a few things to bear in mind when it comes to planning an off-piste excursion.

Our friends at Henry’s Avalanche Talk (HAT) have worked with us to provide the following information:

Identifying avalanche terrain

Slope angle

Avalanches in Europe generally don’t release on slopes less than 28° (about where black runs begin
or a very steep part of a red run). In cold continental climates like North America the minimum angle is 25°.

But there is a difference between where the avalanche releases and where you could trigger it. You can trigger a slab avalanche when you are standing on a lower angle slope by triggering a slab to release above you even if you are on a slope of less than 28°. This can happen when there are steeper slopes around you.

So, slope angles are a critical to think about when you’re deciding where you’re going to go. In theory, if you avoid riding on and under steep slopes, then you won’t ever be exposed to an avalanche.

Snow stability

This is important. When the snow is particularly unstable then there is a higher chance that just one person can trigger a slab and more of a chance that it will release above - making the consequences that much worse. A slab that fractures as little as 2 or 3 metres above you is much more dangerous.

You can get information about snow stability from avalanche forecasts. Reading or listening to the avalanche forecast is essential to understand the current risks. It will include a danger rating. You must understand the definition for the rating. You also need to get an idea of how unstable the snow is and where the instability tends to be most acute on that particular day.

You can also ask local professionals especially if in an area that you don’t know very well. Even off-piste and avalanche experts need local knowledge if they are in a new place, or even if they are in a familiar place, but haven’t been there for a few days. All this can be done before you even get out onto the snow, but if there is no avalanche forecast bulletin and/or limited local knowledge that you trust then digging snow pits becomes a priority from the start.

Recent avalanche activity is a great clue. If lots of slopes facing one direction and at the same altitude have recent slab avalanches on them, then that’s a clue that similar slopes probably have instabilities.

When slopes facing one direction have more avalanche activity than others, it is often because of ‘wind loading’. For example of the wind came out of the west during the last storm, then the slopes protected from that wind (east facing slopes) will have been loaded with more snow. More snow can often put additional stress on the snowpack, and if there's instability because of new snow, then a lot of the time recent avalanche activity will show this.

So after a snowfall check the avalanche forecast and speak to friends or professionals about where you are thinking of going. Then, once you are out there, try out some slopes that aren’t very steep. All this time looking around for clues and listening for settling and 'whoomphing' sounds. That’s a sound the snow makes when it settles and it’s another very clear message that the snow is unstable (don’t worry if you’ve never felt or heard it - when it happens you’ll know it). Only if there’s not a lot of recent avalanche activity around and you do not see or hear any other clues of instability, then you can look for steeper and more varied terrain.

Rules of skiing in potential avalanche terrain

Going one at a time on exposed parts of the mountain, where slopes above and/or below are steep enough to avalanche, is one of the golden rules of off-piste skiing and avalanche risk management. Even though unstable slopes won’t avalanche most of the time, you want to treat them as if they could.

When you stop, make sure it is at an island of safety. These are places where you are protected from avalanche risk, such as under a rock, on a ridge and away from loaded slopes.

Ridges can be islands of safety. Riding along a ridge is generally a pretty safe bet as long as you don’t ride out onto a cornice above a big drop-off. A ridge doesn’t have to be a classic knife-edge type of thing. It can be a rounded area, often referred to as a shoulder. The key point for avalanche safety is that it shouldn’t have a significant slope above you that could release and sweep you away.

Also, convexities, where the slope goes from flat to steep abruptly, seem to be where a lot of slabs fracture due to triggering – this is due to a higher amount of traction stress in these areas.

To avoid triggering an avalanche keep your tracks together. The merits of this may not be proven scientifically, but the idea is that if you follow next to the persons track who went in front of you (and they didn’t set off an avalanche) then the chances of you triggering an avalanche are much less.

Think about what is below you, and where you could end up if an avalanche was triggered. Will it take you into a hole, a ravine or a lake? Terrain traps are anything below you that could make the consequences of being swept away even worse – where even a small avalanche could become fatal – for example carrying you over a cliff.

Always have escape routes in mind. It’s very, very difficult to get out of a moving avalanche, but (and this is a very big but) if you’re in front of the avalanche before the snow starts breaking up into blocks, and if you’re a really good skier, you can sometimes ski out in front of the avalanche and then get out to the side.

What to do if you get caught in an avalanche

Since slab avalanche can reach 100 km/h in five seconds, you only have a second or two to get out of it. If you don’t get off the slab in those first couple of seconds then you need to fight to try and allow as much volume of snow to get below you as possible - fight by clinging to the slope or anything around you because the volume of snow becomes more and more brutal, violent and vicious, as you continue to be swept away.

One of the reasons snow is so violent when it’s moving is because it’s actually very heavy – light snow that becomes avalanche debris is very heavy, generally at least 400kg/m3 and that’s dry snow debris. You’d be very lucky if you were taken with only 10m3 above you i.e. you’re lucky if you’re only dealing with 4,000 kg of snow above you.

Whether you stay on top or not depends a lot on physics. According to well known snow and avalanche expert, Dale Atkins, in an interview on pistehors.com, the basic idea is that a human body is denser than snow but he says that an avalanche is a granular flow not liquid and what matters is volume. Small particles settle on the bottom while larger objects remain on the surface. Try shaking a potato chip bag if you don’t believe him, the crumbs and broken crisps can be found at the bottom of the bag.

You can help yourself by shaking off your skis and poles (your pole straps should never be attached to you when you go off piste). If they stay attached, these act as anchors and can drag you down.

What to do if you get buried by an avalanche

Maintain a breathing space around your mouth. In twenty years of investigating incidents and nearly three decades of pulling bodies out of the snow, avalanche expert, Dale Atkins, says the most striking factor was no breathing space and hands a long way from the face.

If victims are frantically flailing with their arms it is hard to make a space in the second when the avalanche comes to a rapid halt and solidifies (even more difficult if the victims poles are attached to their hands). People die quickly from carbon dioxide poisoning because their carbon dioxide builds up in the snow around their mouths. About 90% of avalanche victims can be recovered alive if they are dug out within the first 5 minutes. However, after 45 minutes, only 20-30% are still alive.

The Ski Club works closely with Henry's Avalanche Talk (HAT). They have even more information on avalanche awareness and safety plus comment on current conditions.

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