The Avalanche Danger Levels - A Simplified Representation Of The Complex Processes In Nature
The European Avalanche Danger Scale is a five-level, ordinarily ascending, categorical scale. Ordinary ascending means that the scale has a ordinary ascending ranking. Categorical means that the scale consists of classes which – even if they are expressed with the numbers 1-5 – may not simply be processed mathematically. For example, the danger level 3-Considerable is not simply higher by one than danger level 2-Moderate, because the avalanche danger probably increases disproportionately (i.e. not linearly). In other words, danger level 3-Considerable is not one level higher than 2-Moderate, but probably twice as high. The danger levels must not be added either.
Definitions In The European Avalanche Danger Scale
In the definition of the danger scale, the characteristics of the avalanche danger are defined for each level with the parameters
- Probability of avalanche release,
- Distribution of hazardous sites and
- Size and frequency of expected avalanches
The changes or the combination of these input variables determine the avalanche danger level. Avalanche forecasters in Europe have agreed to use the EAWS matrix for a more harmonised determination of the danger levels. In order to do so, they take the three parameters probability of avalanche release, distribution of hazardous sites and size and frequency of the expected avalanches into account. In the case of 1-Low avalanche danger, for example, small avalanches should only be triggered in a few places and usually only with a high additional load, whereas in the case of avalanche danger 4-High many and sometimes also very large avalanches are likely to be triggered with to low additional loads (e.g. skiers) or may release spontaneously. The European avalanche danger level scale consists of five levels. In nature, however, the avalanche danger changes continuously and rises disproportionately.
Figure 1: The natural course of avalanche danger (blue line) increases non-linearly. The categorical nature of the danger levels must therefore always reflect a range of natural avalanche danger conditions.
Therefore the avalanche danger can have very different characteristics within a danger level. This range is most apparent at hazard level 3-Considerable. At the upper limit of this danger level many alarm signs are observed (whumphs, shooting cracks, fresh avalanches) and the danger is very present. At the lower limit of this danger level these clear indications are missing. Often – mostly unconsciously – this leads to riskier terrain choices.
Frequency Of Danger Levels
In the assessment area of the avalanche.report, danger level 2-Moderate is forecast in almost half of the days (per warning region). This danger level is most frequently used. It thus describes a day with average avalanche danger. Level 3-Considerable is issued on about one third of the days. This is where most fatal accidents occur (approx. 50%). The danger level 4-High is issued on average only on 2% of winter days, i.e. on approx. 2-4 days per season. Level 5-Very high is reserved for castrophic situations and is therefore rarely used.