Danger Ratings Explained
Confused by danger ratings? You are not alone. Here is a quick tutorial and some frequently asked questions.
This is the current English translation of the European avalanche danger scale. In Europe, most people use Level 1-5 to refer to the avalanche danger, partially because there are so many different languages in Europe and also because words don’t necessarily communicate the actual conditions, especially at Considerable (Level 3). (See discussion below on The Problem With Words.)
A joint U.S. Canadian committee, the North American Avalanche Danger Scale Project (NAADSP) have agreed on an updated danger scale for North America, and it will be introduced in the spring of 2010. In the mean time, it's best to refer to the European Scale above.
What Danger Level is most “Dangerous”?
Although avalanche danger rises exponentially with each danger level, most fatalities occur at Considerable (Level 3) because the most interaction between people and avalanches occurs at Considerable. With High and Extreme dangers, most people perceive that conditions are too dangerous and they don’t go out. But at Considerable, 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 Considerable.
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 be able to recognize avalanche terrain and obvious clues such as being able to recognize wind slabs. 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 Considerable or higher must have a minimum of a multi-day avalanche class, and preferably have a Level II avalanche certification, plus have several years of experience.
Even the avalanche professionals avoid High and Extreme danger conditions. 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, Considerable 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 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, fter 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 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.
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 more dangerous the terrain, 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