Technical information, news, research, and opinion on avalanches, snow safety, and winter backcountry travel.

Tuesday, March 2, 2010

Does Size Matter?

In a big country dreams stay with you—Big Country

Like people, terrain comes in a bewildering variety of size and shapes and we all have different preferences. Sometimes terrain is obviously dangerous, and other times terrain is obviously safe. At the beginning of the season, I wrote a post about perception of instability. Like instability itself, perception of instability is rarely static. This means that your beliefs about instability change with place and time.

According to The Avalanche Handbook, variations in perception are usually very low when instability is widespread. On the other hand, variations in perception are most frequent, and most dangerous, when instability is not widespread, but not exactly scarce, and when the energy required to release an avalanche is relatively high.

The same rules apply to terrain. Perception of hazard is much better when terrain is obviously dangerous. Perception of hazard is less important when the terrain is actually very safe, since accidents are less likely. Serious errors are most likely when people choose "safe" terrain that sits in the middle of the spectrum. This is a fairly strong claim, but I think it's well-supported by records of skier-triggered avalanches accidents in North America.

Figure 1.1. Scale of terrain evaluation. Terrain is either very safe, very dangerous, or somewhere in between. Where does your secret stash fit on this scale?

Evaluating Terrain
Review the following images and think about where the images belong on the terrain scale. Where are you most likely to travel when snow quality and avalanche hazard are high? Where are you most likely to let your guard down?

Figure 1.2. Mt Currie, British Columbia. COMPLEX terrain featuring exposed alpine features and numerous avalanche paths. Many of these avalanche paths fall more than 1000 metres.

Figure 1.3. Twin Sisters, Washington State. COMPLEX terrain with numerous avalanche paths. Many of these avalanche paths fall more than 1000 metres.
Figure 1.4. Mt. Snoqualmie, Washington State. COMPLEX and CHALLENGING terrain with many safe options. This is exactly the type of middle ground terrain where backcountry skiers are at high risk for perceptual errors. There are many options to reduce exposure, and just as many options for small and medium sized avalanches during poor conditions. 
Figure 1.5. Central Cascades, Washington State. CHALLENGING and SIMPLE terrain. As above, there are many options to reduce exposure, and many options for small, but deadly, avalanches.

Why Size Matters
Of all the images, can you take a guess where the most accidents, burials, and fatalities have occurred? The answer is the last image, and it explains why size matters. ( Accessibility is part of the problem as well, but if i can believe what I read on the Internet, many local backcountry skiers believe that the terrain in the last image offers safe options during high avalanche danger. )

Size matters because small avalanches happen much more frequently than large avalanches, especially with respect to skier triggering. Large natural avalanches are relatively rare because enormous slabs can stabilise under their own weight. Large skier-triggered avalanches are rare because the application of dynamic forces from skier influence does not extend much deeper than 1 metre.

However, skiers like to choose terrain in the middle of the spectrum, especially on days when snow quality, and avalanche hazard, are high. Canadian researcher Laura Adams refers to this technique as satisficing: some desirable attributes, such as large, open slopes, are sacrificed while others, such as steepness, are preserved. This technique is definitely valuable, but it has dangerous side effects that contribute to avalanche accidents: choosing terrain in the middle of the spectrum makes it easy to rationalise and let your guard down. The line of thinking is as follows: we sacrificed the big open slopes up in the alpine in favour of steep slopes with trees. The only problem here is that the snowpack and the laws of physics simply don't care about your sacrifices.

The second problem is that most people are killed by avalanches that are, in the big picture, basically tiny. A complete burial requires very little snow, and most slopes that are fun to ski hold enough snow for several bad accidents. These "safe" terrain choices are even more dangerous because most middle ground terrain has at least some tree coverage. A recent study showed that skiers in North America are much more likely to die from traumatic injuries due to trees and rocks, than their counterparts in Europe.

Where I ski in the Washington Cascades and throughout British Columbia, most skiers regard a small slope as ~300 vertical metres, or about 1000 vertical feet. Many popular locations for day trips and yo-yo skiing involve such slopes. At the smallest scale, snowpack information becomes very important: terrain answers the question of is an avalanche possible, and snowpack answers the question of is an avalanche likely. Since it's just as easy to trigger an avalanche on a small, steep slope, it's easy to see why snowpack evaluation can become critically important when small, steep slopes are chosen.

Terrain analysis is one of the best tools for managing uncertainty, but only when slope angles are managed very strictly. Very often, I read trip reports where small, steep slopes are chosen as safe options during times when avalanche hazard is high. This is a fairly poor application of travel technique because small, steep slopes can be just as dangerous as large, steep slopes-especially if you don't seek out specific information about the snowpack.

What are your beliefs about the likelihood of avalanche formation on small, steep slopes? Do you believe small, steep slopes are safer? The record of avalanche accidents in North America provides a clear and resounding answer.

Accidents Involving Small Avalanches


  1. Very interesting read. The frequency of accidents in the middle of the danger spectrum reminds me a lot of rappelling accidents in the rock climbing world. Rappels are fairly simple so a lot of climbers let their guard down and don't acknowledge the risk. As a result, a huge number of climbers die this way. Not because it is the most dangerous activity, but because it is in the middle of that danger spectrum. Dangerous enough to cause death, but not dangerous enough where detailed caution is taken. Great read.

  2. Thanks for the comment.

    I'd never thought of rappelling accidents in that way, but you raise a great point. ( I once rappelled off my rope, but it was a short drop onto snow. Haven't made that mistake again! )

    On Sunday there was an extremely close call in the terrain featured in the fourth photograph. From what I know about the specific area, it was classic middle ground terrain ... a relatively small, steep slope with trees.

    And as you say ... the terrain was dangerous enough to cause death but not dangerous enough where detailed caution was taken.