Danger Scale Tutorial
Danger Ratings Explained
Confused by danger ratings? You are not alone. Here is a tutorial and some frequently asked questions. This tutorial begins with basic concepts and it becomes progressively more technical, so read as far as your interest dictates.
This is the scale produced by a joint U.S. Canadian committee, the North American Avalanche Danger Scale Project (NAADSP) published in the fall of 2010. It has also been adopted by all English-speaking countries. The first two columns (the colors, levels and icons) have been adopted as an international standard in 2010. The icons were developed by the Canadians, then used by the Swiss, with some modifications and they proved very popular, so they are now the international standard.
How to Manage Terrain based on Danger Ratings
An easy way too choose appropriate terrain based on danger ratings is to adjust the slope steepness; the higher the danger rating, the gentler the slopes you choose. Remember that especially at higher danger ratings, you have to consider the terrain the slope is attached to, especially the terrain above the slope you plan to cross. In hazardous conditions, you can easily trigger slopes from the bottom, the side and even from flat ridges above the slope.
Courtesy of the Swiss Institute for Snow and Avalanche Research
What Danger Level is most "Dangerous"?
Although avalanche danger rises exponentially with each danger level, most fatalities occur at Level 3 (Considerable) because the most interaction between people and avalanches occurs at Level 3. With Level 4 and 5, (High and Extreme), most people perceive that conditions are too dangerous and they don't go out. (Thus, the information provided by avalanche centers saves many lives.) But at Level 3, people perceive that it's safe enough to venture out, yet conditions are dangerous enough that only well trained and experienced can travel safely. Since the vast majority of people overestimate their avalanche skills most people are killed at Level 3.
What Amount of Avalanche Training and Skills are Appropriate for each Danger Rating?
By "Basic Avalanche Skills" we mean that anyone who ventures into the backcountry--regardless of danger level--needs to have basic avalanche skills and knowledge. For example, everyone needs to have the skills to navigate safely in avalanche terrain and recognize obvious clues such collapsing, cracking, loading and read snow surface textures. Finally, everyone needs to carry basic rescue gear and know how to use it.
By "Experts Only" we mean that people who venture into terrain rated as Level 3 (Considerable) or higher must have a minimum of a multi-day avalanche class, and preferably have a Stage 1 (NZ) avalanche certification, plus have several years of experience. "Experts Only" does NOT mean being an expert in your sport. It refers to avalanche skills, knowledge and experience. The average avalanche victim is very skilled in their sport but their avalanche skills usually lag far behind.
Even the avalanche professionals avoid avalanche terrain at Level 4 and 5 (High and Extreme). Often you need professional-level skills and experience to know how to travel well away from avalanche runnout areas or have intimate local knowledge of terrain and snowpack conditions.
How is Avalanche Danger Determined?
The diagram above is a CONCEPTUAL diagram only. No agreement has been reached on the exact boundaries between danger ratings using size vs. likelihood. But notice that avalanche danger depends on BOTH the probability (or likelihood) of triggering an avalanche and the size of the expected avalanche. In other words, in the above example, Level 3 avalanche danger can have an infinite combination between probability and size yet still be in the same danger level. This is difficult to express in the limited wording in a danger table, so a table like this expresses it best.
Avalanche forecasters estimate the likelihood of triggering using snow profile stability tests as well as observations of avalanche activity. They estimate avalanche size based on the mass of snow above the weak layer as well as from recent avalanche activity. Finally, avalanche forecasters take avalanche character into account. For instance, a shallow, soft slab within the new snow is much less dangerous than a shallow, hard slab on surface hoar with a slick bed surface.
"Scary" Avalanche Conditions
The above diagram is also only a CONCEPTUAL diagram and the danger ratings are filled in only as an example. Notice the importance of knowing both the likelihood of triggering and the size of the avalanche. Smaller avalanches may be "manageable" by experts, but large avalanches are very difficult to survive, so your only choice is to avoid places where they could occur. Sometimes these large avalanches can be difficult to trigger but if you do trigger one, they will be very large and unsurvivable. We sometimes call these "scary" avalanches because there may be many tracks on a slope, yet it's still possible to trigger a very large avalanche.
What is the difference between Hazard, Danger and Risk?
The graphic above is another conceptual diagram developed by the North American Avalanche Danger Scale Project to understand terms like hazard, exposure, vulnerability and risk. For our purposes, we must separate properties of the snowpack from terrain and people. As avalanche forecasters, we only know about the snowpack and have no idea what kind of terrain people will choose, their travel techniques or mitigation measures. Therefore we can only forecast the first circle--hazard. Each of us determines our own risk by our choice of terrain, travel techniques, the number of people we expose and our vulnerability (are we wearing beacons or airbags or in a car or building, etc).
Is the Danger Scale Linear?
No. Although it appears linear, avalanche danger rises exponentially with each danger rating. Therefore your mitigation measures must rise exponentially as well.
The Problem with Words to Describe Avalanche Danger
The term "Considerable" has been very controversial since its international adoption in the 1990's. The English language, especially, does not have a good term for the middle of a 5-level scale. About 15 years ago, after many extended discussions, it was voted to be the best term. Other languages have similar problems with the middle term. Partially because of this, in Europe you almost always hear people refer to the danger level number instead of the word.
At the European Avalanche Warning Services (EAWS) committee meeting in the spring of 2009, there was much discussion about changing the wording on the 5-level scale, not just for English but for other languages as well. After several months of deliberation, we voted to leave the scale in its present form. The general consensus was that especially in Europe, the Danger Scale is used not only by recreationists but by municipalities, highways, railroads and many other segments of daily life in mountainous areas. So it appears that the scale has become so entrenched to make any change of the scale too difficult.
Partially because of the problems with the words to describe avalanche danger, the EAWS voted in the fall of 2009 to standardize on icons with numbers as the graphical way to represent the avalanche danger, which you can see in the European Danger Scale in the first graphic in this tutorial.