The SLF uses the five-level European avalanche danger scale to denote the avalanche danger. The danger level depends on a range of variables, in particular the snowpack stability, i.e. the probability of an avalanche triggering; the prevalence of avalanche prone locations; and potential avalanche size. The probability of an avalanche triggering increases sharply as the danger level rises.
A danger level always applies to one region and cannot reflect the peculiarities of a specific individual slope. In addition, the danger level stated in the Avalanche Bulletin is always a prediction and should be evaluated on-site.
Recommendations for backcountry recreationists
Extraordinary avalanche situation
Numerous very large and extremely large natural avalanches can be expected. These can reach roads and settlements in the valley.
You are advised not to engage in winter sports beyond open ski runs and trails.
Very rarely forecast.
Around 1 % of avalanche fatalities.
Very critical avalanche situation
Natural and often very large avalanches are likely. Avalanches can easily be triggered on many steep slopes. Remote triggering is typical. Whumpf sounds and shooting cracks occur frequently.
Stay on moderately steep terrain. Heed runout zones of large avalanches. Unexperienced persons should remain on open ski runs and trails.
Forecast only on a few days throughout the winter.
Around 10 % of avalanche fatalities.
Critical avalanche situation
Whumpf sounds and shooting cracks are typical. Avalanches can easily be triggered, particularly on steep slopes with the aspect and elevation indicated in the avalanche bulletin. Natural avalanches and remote triggering can occur.
The most critical situation for backcountry recreationists. Select best possible route and take action to reduce risks. Avoid very steep slopes with the aspect and elevation indicated in the avalanche bulletin. Unexperienced persons are advised to remain on open ski runs and trails.
Forecast for around 30 % of the winter season.
Around 50 % of avalanche fatalities.
Mostly favourable avalanche situation
Warning signs can occur in isolated cases. Avalanches can be triggered in particular on very steep slopes with the aspect and elevation indicated in the avalanche bulletin. Relatively large natural avalanches are not to be expected.
Routes should be selected carefully, especially on slopes with the aspect and elevation indicated in the avalanche bulletin. Travel very steep slopes one person at a time. Pay attention to unfavourable snowpack structure (persistent weak layers, old snow problem).
Forecast for around 50 % of the winter season.
Around 30 % of avalanche fatalities.
Generally favourable avalanche situation
No warning signs present. Avalanches can only be triggered in isolated cases, in particular on extremely steep slopes.
Travel extremely steep slopes one person at a time and be alert to the danger of falling.
Forecast for around 20 % of the winter season.
Around 5 % of avalanche fatalities.
This scale, introduced by the European Avalanche Warning Services on winter 1993/94, defines the danger level based on the snowpack stability and the avalanche triggering probability. The full description of the scale also includes other columns that have not been internationally agreed relating to typical characteristics, recommendations and consequences.
Danger levels – a simplified picture of reality
The avalanche danger does not increase in linear fashion from one level to another, but disproportionately. As the danger level increases,
- snowpack stability, and therefore the additional load required to trigger an avalanche, decreases. In consequence, the probability of an avalanche triggering increases.
- the prevalence of avalanche prone locations increases. In other words, there are more places where avalanches can release naturally or be triggered.
- avalanche size increases as well, in particular when the danger level is at the higher end of the scale.
If the additional load required to trigger an avalanche decreases and there are also more locations where avalanches can be triggered, the avalanche triggering probability increases. This is also shown by the stability distribution in the chart below.
In a typical case, all these influencing variables change as indicated above. This means that in the case of 'low' avalanche danger, usually only small avalanches are triggered in just a few locations and mostly only due to a high additional load, while in the case of 'high' avalanche danger, a lot of avalanches, some of them very large, are triggered or are released naturally (without human influence).
However, there are also atypical situations that do not fit into this picture. These are detailed in the next section.
Also in the case of less typical avalanche situations the danger level is a measure of the order of magnitude of the avalanche danger. These situations are detailed below along with information about their assessment by the avalanche warning service. As these are atypical situations, no list can ever be exhaustive. In atypical situations deviations from the danger level definition are inevitable. These are set out as far as possible in the avalanche bulletin's danger description.
In the case of low avalanche danger (level 1) the snowpack is normally well bonded and stable. However, after long periods of fine weather with shallow snow, another form of snow occurring in mid-winter which makes slab avalanches virtually impossible is the whole snowpack being loose and transformed into faceted crystals. The snowpack is then very weak and there are also weak layers, but the bonded layer on top (the 'slab') is lacking. With no bonded layer a fracture cannot be propagated, meaning that there is no possibility of a slab avalanche forming. This means there is a low avalanche danger (level 1), despite or precisely because of the very loose snowpack. 'Stable snowpack' means 'not liable to trigger slab avalanches' and does not refer to the hardness of the layers.
This situation is not a good omen for the immediate future, given that as soon as it is snowed on, the fresh-fallen snow and fresh snowdrift result in the creation of a 'slab' where one did not exist before. Underneath this is the loose old snowpack in the form of a substantial weak layer. The avalanche danger will increase significantly, generally resulting in a long-lasting old-snow problem.
Small, easily triggered snowdrift accumulations
Fresh snowdrift accumulations can often be triggered by individual winter sports enthusiasts. The size of the snowdrift accumulations depends not only on the wind but also on the supply of fresh and old snow which can be transported. For only little snow which can be transported, the snowdrift accumulations are usually so small that getting buried by them is unlikely. In this case, level 2 (moderate) is often used despite the high probability of an avalanche release. In such situations the snowdrift accumulations can usually be seen in good visibility by the trained eye. They need to be avoided especially on terrain where there is a danger of falling.
Old-snow problem with a significant weak layer deep down in the snowpack
The more thickly a weak layer is covered, the more difficult it becomes for a fracture to form there. This is most likely to occur in locations where the snow is relatively shallow or in transitions from shallow to deep snow. In the case of an old-snow problem with a significant weak layer deep down in the snowpack, avalanche prone locations are usually relatively rare. However, avalanches often reach large size, and are therefore particularly dangerous for winter sports enthusiasts. Therefore, the avalanche danger in the case of an old snow problem can sometimes be considerable (level 3) even if the avalanche prone locations are fairly rare. An aggravating factor is that the avalanche prone locations are barely recognisable in old snow situations, even to the trained eye. In the case of weak old snow, more fatalities can be expected than in the case of the other avalanche problems (at the same danger level).
If numerous large and, in many cases, very large natural avalanches can be expected, the avalanche danger is classified as 'high’ (level 4). In these circumstances, exposed objects (mostly sections of transportation routes, but also buildings in isolated cases) can be endangered. Parallel to this classic 'road high' situation there is a further variant of this danger level when, even though very large avalanches are unlikely to occur, there are a lot of places where people can very easily trigger medium and large-sized avalanches. In many such cases, avalanches are released naturally as well. In the case of this 'skier high', winter sports enthusiasts are in severe danger away from secured ski runs. Transportation routes, in contrast, are either unaffected or affected only in isolated cases.
Avalanche activity in case of wet and gliding snow
Wet-snow avalanches are seldom triggered by people, and in the case of gliding avalanches this is virtually impossible. Therefore, natural triggering is the main cause of such avalanche types even in the when lower danger levels prevail. The maximum possible naturally triggered avalanche activity, based on the defined danger level, relates mainly to wet-snow and gliding-snow conditions and less to conditions involving dry avalanches. In this context large natural avalanches are possible when there is a moderate danger of wet-snow or gliding avalanches. In the case of a situation with dry-snow avalanches, such naturally triggered avalanche activity normally corresponds to a considerable avalanche danger, as then avalanches are also expected to be triggered by individuals.
The avalanche danger changes over time and may not reach or overshoot the boundary between one danger level and another within the period of validity of the avalanche bulletin. Ordinarily the danger increases, for example because snowfall or wind, significantly faster than it recedes again.
If the danger level is likely to change during the day, the level published in the avalanche bulletin and the danger description normally show the situation in the morning. The change (often an increase) is indicated in the danger description. Here are some examples:
- "The avalanche danger will increase and reach level 3 (considerable) in the afternoon." In this case, level 2 (moderate) is marked on the hazard map.
- "Danger level 4 (high) will be reached during the morning." In this case, level 4 (high) is marked on the hazard map.
- "As a consequence of daytime warming and solar radiation, wet-snow avalanches can be expected again from the middle of the day below approximately 2400 m." (With no danger level indicated)
If the avalanche danger is assessed to be level 4 (high) or even level 5 (very high) at night, and then one level lower during the day, the morning rule is deviated from. As when transport routes are endangered, the avalanche danger is also significant at night, the higher avalanche level that applies during the night is given in the evening edition. In the morning edition this is then reduced to the level applying in the morning.
In typical springtime conditions, the danger of wet-snow avalanches along with daytime warming and solar radiation will increase significantly as the day progresses. During the day the danger of dry-snow avalanches will usually only change slightly in these conditions. Thus, while in the morning dry-snow avalanches are the main danger, in the afternoon the main danger is posed by wet-snow avalanches. In this situation, two maps show both the more favourable morning situation (dry-snow avalanches) and the unfavourable situation in the afternoon (wet-snow avalanches as the day progresses). The transition from one map to another cannot be pinned down to a specific time. It depends on the conditions and the altitude and also in particular the aspect. Whereas on east-facing slopes the danger of wet-snow avalanches will already increase during the morning, this usually occurs only later on west-facing slopes.
In special cases a double map is also used if there is a significant danger of gliding avalanches, and this danger is greater, over a wide area, than the danger of dry slab avalanches. It is worth noting in this case that these two dangers do not vary significantly during the day. Therefore, the two avalanche types are possible throughout the day.
Avalanche danger and risk
The avalanche bulletin describes the avalanche danger, i.e. the likelihood of a release and the possible size of avalanches in a specific region, but the exact timing of an avalanche being triggered and the actual length of its starting zone and fracture depth cannot be determined.
An avalanche danger only becomes a risk (i.e. a probability that damage will occur) if endangered individuals, animals, forest, infrastructure etc. are in the area of the potential avalanche. In the avalanche bulletin, the avalanche danger is assessed regardless of the presence of endangered individuals or objects, i.e. on weekends with fine weather and weekdays with poor weather alike, and for populated areas and outlying terrain alike.
If an avalanche is released somewhere on a glacier during a snowstorm, there would obviously be an avalanche danger, but there would be no risk if there were no people in the surrounding area. If the same avalanche is released on a popular touring route on a sunny weekend, the risk would be much greater. The danger given in the avalanche bulletin may be the same in both cases, but the risk will be different.
Avalanches are a very special natural hazard, since unlike flash floods or earthquakes the 'perilous process' of an avalanche can be initiated by human activity. If anyone comes onto a dangerous slope, the artificial additional load can trigger an avalanche. In the case of more than 90% of the snow sports enthusiasts buried by a slab avalanche, they triggered the avalanche themselves or it was triggered by another member of their group.
Frequency of danger levels
The most frequently forecasted danger level in the Alps is level 2 (moderate), accounting for almost half of days (per warning region). It thus captures a day with 'average' avalanche danger. Level 3 (considerable) is forecasted on about one third of days. Danger level 4 (high) is only forecasted on 1% of winter days on average, while level 5 (very high) is forecasted even more rarely. In the Jura, the lower danger levels are more frequent.