Avalanche Safety – Travelling in avalanche terrain

Among all the numerous skills and incredible amount of knowledge required to correctly forecast, assess and deal with avalanche hazard, being able to read terrain features and therefore recognize avalanche terrain is certainly the most important one. In other words, if I had to choose go riding with a snowpack geek scientist with a PhD and a ski bum with an outstanding understanding of terrain features, I would chose the ski bum. Besides, I think it’s also much more fun being a ski bum than being a scientist. I don’t think I have to worry about backcountry skiers scientists reading my blog, do I?

Read and understand terrain features will allow you to do the homework and pick the safest route on the way up and on the way down, reducing the risk to the minimum. One simple rule to remember is that avalanches usually occur where they have already run in the past. Therefore, having a clear idea of how an avalanche path looks like is essential. The sections are: a starting zone at the top where in case of a slab avalanche the unstable snow breaks and the slab starts sliding; a track, where the moving snow gains momentum and slides down the mountain and a run out zone, where the snow slows down and eventually stops. And it looks like this:

Identifying avalanche patterns is not always a straightforward exercise. When there are trees around though, it becomes a bit easier. Missing trees, broken-off trees or trees with missing or broken branches represent all evidence of previous avalanche activity.

Observing trees to identify a possible avalanche pattern is a perfect example of how important is observing the terrain around you. Observation is definitely the key to safe winter travel. After you spend some time hiking around you will discover that the great majority of information needed to recognize and assess avalanche hazard is right there in front of you, you just have to pay attention and more importantly know what signs to look for. Sometimes the mountains will almost scream at you and some other times they will just whisper something. Regardless how obvious or difficult to see a sign might be, the secret is to stay focused and try to get as many pieces of the puzzle as possible, even the smallest ones counts. Sometimes, especially the smallest ones counts.

The most important elements of the terrain to consider when evaluating avalanche danger are: slope angle, orientation to wind, orientation to sun, the shape of the slope and some terrain features called terrain traps.

Slab avalanches release most frequently on slopes with an angle between 30° and 45°. 37° seems to be avalanche’s favorite angle. On slopes of less than 25°, generally snow tends to fracture and settle without sliding. The risk of a slab releasing on a slope with an angle of more than 50° is generally much lower than for a slope in the 30°-45° range. On steeper terrain, say in the 45°- 60° range there are usually frequent small loose snow avalanches and slopes with angle greater than 60° generally don’t produce avalanches as the snow sloughs off them continuously during a snowfall.

The terrain exposure to wind and sun is also an extremely important aspect to consider. Lee slopes collect more snow due to drifting and snow being carried from the windward to the leeward side of ridges often creating windslab, one of the trickiest types of avalanche to identify. The top of the mountain is not the only place where you will find a lee slope. Similar terrain exists also on ridges running down the fall line which might have been cross loaded so watch out for those too.

These wind deposits on lee slopes are very important for two reasons: snow will be deposited at several times the overall snowfall rate which often overloads the buried weak layers and increase the likelihood of avalanches. The wind packed layers have the potential to release as slab avalanches which can be large in size and triggered from below.

After storms, the new snow layers tend to remain unstable longer on shady slopes than on sunny ones. This is because the snow settles and stabilizes more slowly favoring the development of weak layers within the snowpack. Therefore, as a very general rule (to be considered together with ALL the other elements and relevant considerations the situation may require, precedent avalanche activity, snow accumulation due to wind, recent snowfall, etc.) south facing slopes tend to be safer in the middle of the winter and more risky when the temperatures rise during spring. Conversely, north facing slopes tend to be safer during spring and more risky during the colder months of the winter.

Regardless the aspect of the slope, south or north facing, another important consideration is that avalanche danger is generally higher at higher elevations. I guess you’ll find this fairly intuitive – more snow and more wind than at lower elevations. Also, at higher elevations there are less trees and bushes that, although do not necessarily prevent avalanches from being triggered, tend to anchor the snowpack. Another fairly intuitive consideration is therefore that when approaching avalanche terrain from lower elevation, once above the tree line the avalanche danger should be reassessed.

Safe glacier travel on the Monte Rosa Massif with Neil McNab (McNabSnowboarding.com)

Doing your homework the day before or the same morning you set off for your tour is just not enough. As the day goes by and you move through the mountains on different terrain, the conditions might also be changing. Due to changes mainly in temperature, sunlight and wind effect, what it was a reasonably safe slope (always better to be conservative) at 10am could very well be a dangerous one only a few hours later. You’ll find out pretty soon that a big part of ski touring is the continuous gathering and processing different kinds of information which it can be a bit stressful at the beginning. That’s why is SO important that at least during your first touring season you choose easy, short tours, where you can easily read and understand the features on the map and you stick to those. If you have the time to do the same tours throughout the season, that is ideal. It will help you gaining confidence and provide your brain with an excellent opportunity to practice the gathering and processing information exercise I was mentioning above.

If you want to go ski touring but you just want to ski and you don’t feel like challenging your brain thinking too much about it, then I suggest you hire a local guide. If you wanted to ski La Grave for instance, I would definitely recommend a Swedish guide call Per As. He’s a great guy and a great guide. He has been living there over 20 years, he’s the best. Local knowledge is something I haven’t mentioned yet but if you are still reading this post by now you can probably guess that, in the complex and articulated process of observing the mountain environment, gathering information, process it trying to assess the likelihood of an avalanche occurring, local knowledge plays the most important role.

And if you want to explore the area I currently live in, the Gran Massif, then let me know. I would be happy to do the homework for you. I know that it might sound a bit strange but, although I am not a geek, I have to confess that I do enjoy doing the homework almost as much as skinning up and riding down.. almost, it depends on how good and deep the snow is.

Before moving over to terrain traps, I must briefly mention something about the shape of the slope. Steep and straight slopes are obviously not only avalanche prone slopes but also slopes for which it may be difficult to predict where a starting zone might be. Statistics show that a good number (no idea about the %, I am not that geeky!) of avalanches start where there is some change in the slope profile. This is the case of probably the most difficult slope profile to read in terms of snowpack stability, a convexity. The rounded top of a ridge or a peak can often be a trigger zone for slab avalanches. I can think of at least 3 reasons – although I am sure there are more – why they present more complex safe travel issues than other slopes:

1)      Comparatively to concave slopes, convex slopes have less compressive support at the bottom,   which makes a difference for small avalanche paths, some difference on medium sized avalanche paths but has little effect of large avalanches.

2)      The wind has a particular effect on convex slopes. When it slows down as it goes around the convexity it drops its load of snow, loading them more than other slopes.

3)      Convex slopes can be more difficult to descend because each turn slowly adds a degree of steepness until you find yourself quite quickly on steeper terrain that the one you were on a few seconds earlier.

Ok, I think we are ready to talk about the terrain traps. Statistics show that about 25% (my geeky side trying to shine again) of avalanches fatalities are caused by injuries. Again, it’s fairly intuitive why they are called traps. We refer to terrain features that make injuries or burial more likely or make escape from an approaching avalanche more difficult. In other words, a small avalanche combined with a terrain trap can have similar consequences of a larger avalanche. Good examples of terrain traps are bowls, river beds, cliffs and crevasses, flats at the bottom of a steep slope, trees and big rocks in slide paths. You get the idea. Identifying terrain traps is therefore one of the essential elements to take into consideration in order to select a safe route on the way up and on the way down.

Are you still reading? If you are, you will be pleased to know this is all I wanted to mention about avalanche terrain, at least for the time being. In the next post I will try to come up with a few smart winter travel habits which should help those of you who would like to start venturing off the beaten track and make their own hazard evaluation and forecasting and consequent route selection.

IMPORTANT DISCLAIMER: Please remember the information on this blog only aim to inspire you to learn and understand more about the risks involved in hiking and sliding away from the ski resorts. It does not represent professional advice. The best way to get professional training is to attend a proper avalanche course. That is your first baby step to start developing the necessary skills to forecast and deal with avalanche hazard.

The European Avalanche School is one of the best places to start from. The British guides run one of the best courses in Europe. All the relevant info on their website:


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One Response to Avalanche Safety – Travelling in avalanche terrain

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