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Avalanche Danger Scale

The avalanche danger describes the likelihood of occurrence and the possible size of avalanches in a specific region of at least 100 km².

Background on the European avalanche danger scale

In 1993, the European Avalanche Warning Services agreed to introduce a uniform avalanche danger scale. The common scale defines the danger level based on the likelihood and size of avalanches. The complete description of the scale also contains additional, not internationally agreed columns on typical characteristics, recommendations and effects.

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

  • snowpack stability,
  • its spatial distribution and
  • avalanche size.

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 snowpack stability, its spatial distribution and avalanche size 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.

ava_danger_en ava_danger_en

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, 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.

Various characteristics of avalanche danger

Small, easy to trigger wind slabs

A single backcountry recreationist can often trigger fresh drifting-snow accumulations. The size of these so-called wind slabs depends not only on the wind, but also on the supply of fresh snow or a loosely bonded old snow surface. The size of the wind slabs depends on the amount of wind-drifted snow and the wind speed itself. In case only small amounts of snow can be transported, wind slabs remain rather small. If skier trigger these small wind slabs, avalanches tend to remain rather small. Consequently, deep burials are unlikely. Although release probability in this example may be quite high, forecaster assesses these conditions to a danger level 2-Moderate. With a certain amount of training accumulations with wind-drifted snow can usually be assessed fairly easy when good visibility prevails. In general, wind slabs should be avoided especially on terrain where slipping and falling may have fatal consequences.

Deep persistent weak layers

Deeply buried persistent weak layers (e.g. buried surface hoar, basal facets formed with cold on warm / warm on cold) are hard to trigger, since it is very difficult to initiate a fracture. Triggering of deep persistent weak layers may most likely occur where the snow is relatively shallow or at transitions from shallow to deep snow (see also shallow snow next to deep snow). In the case a persistent weak layer problem prevails with a prominent persistent weak layer deep down in the snowpack. Hazardous sites, where a single skier may initiate a fracture are relatively rare. However, in case of fracture initiation, avalanches may often become dangerously large for backcountry recreationists – with other words often fatal. Therefore, avalanche forecaster rate these situations often with avalanche danger 3-Considerable, even though locations for triggering an avalanche are rarely distributed. The situation is fairly complected to assess for backcountry recreationists since triggering locations are often very hard to detect, even with advanced skills and trained eye. The persistent weak layer problem produces more avalanche fatalities than any other typical avalanche problem.

High with many human-triggered avalanches

If numerous large and, in many cases, very large spontaneous avalanches are expected, the avalanche danger is rated with 4-High. Under such conditions, exposed locations (usually sections of transport routes, and in isolated cases also buildings) are at risk. In addition to this classical characteristics of avalanche danger level 4-High, there is a further variant where large avalanches are hardly expected (e.g. because there is still too little snow at the start of the winter) but a lot of medium and large avalanches are naturally released and/or can be triggered very easily at multiple locations. In this case, avalanche forecasters speak about a ‘skier 4-High’. These situations represent a sever danger for backcountry recreationists, while transport routes remain unaffected or only affected in isolated cases.

Wet and glide-snow avalanche activity

Wet-snow avalanches are hardly triggered by skiers, while in the case of glide-snow avalanches this is virtually impossible. Therefore, natural triggering is the main cause of these avalanche types – even in the case of the lower danger levels. In fact, the definitions of the maximum possible spontaneous avalanche activity at the lower end of the avalanche danger scale relates mainly to wet-snow and glide-snow avalanches. In this context large spontaneous avalanches are possible when there is a 2-Moderate avalanche danger for conditions causing wet-snow or glide-snow avalanches. In the case of a conditions causing dry-snow avalanches, such naturally triggered avalanche activity normally corresponds to a 3-Considerable avalanche danger, as then avalanches are also expected to be triggered by individuals.