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Explanation of danger levels

General

The avalanche danger expressed in the avalanche danger scale increases from level to level. At the same time, the snowpack stability decreases (cf. Figure 1) and the prevalence of avalanche prone locations in open terrain increases. Generally speaking, the additional load required to trigger an avalanche decreases at the higher levels. The higher the danger level, the larger the avalanche size and number of avalanches.

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Fig. 1: Occurrence of stability categories with danger levels low (level 1) to considerable (level 3). As the danger level increases, so does the proportion of areas where stability is low. Also note the proportion of areas where the stability is poor when the danger level is moderate (level 2).

The avalanche bulletin indicates any changes in the danger level that are expected to arise during the day covered by the forecast. Examples:

  • "Danger level 3 (considerable) will not be reached until the afternoon."
  • "Danger level 4 (high) will already be reached during the morning."
  • "As the avalanche danger increases during the day, wet avalanches are to be expected again from the middle of the day below approximately 2400 m."

The description of the avalanche danger can differ under the separate headings "danger description" and "additional danger".

  • If the danger level is likely to change during the day, both the level published in the avalanche bulletin and the danger description are guided by the situation in the morning. The outlook (generally an increase in danger) is described under the heading "additional danger".
  • Since most avalanches that cause damage and injury are dry slab avalanches, more space is usually devoted to describing this danger in the avalanche bulletin.
  • In typical "springtime situations", that is on days when the avalanche danger increases significantly during the day, two maps are used to depict both the more favourable morning situation and the less favourable situation prompted by the increasing wet avalanche danger as the day progresses.

If a substantial danger of full-depth avalanches exists and this danger is greater, over a wide area, than the danger of dry slab avalanches, two maps are produced to show both the danger of dry slab avalanches and the danger of full-depth avalanches Note in this case that the danger of full-depth avalanches does not vary during the day. Both dry slab and full-depth avalanches can therefore occur at any time of day (including in the early morning).
The frequency of the published danger levels from winter 1997/98 to winter 2011/12 (serving as a forecast for the following day since the avalanche bulletin was introduced) is depicted in Figure 2 below.

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Fig. 2: Occurrence in percent of individual danger levels in the avalanche bulletin (evening assessment (5 pm) from 01.12. until 30.04.) from winter 1997/88 to 2011/11.

Low avalanche danger (level 1):

The snowpack is generally well bonded or, as a whole, loosely packed. Such conditions frequently occur in mid-winter during long spells of fine weather accompanied by shallow snow cover. When either one of these conditions prevails, a fracture is usually unable to propagate in the snowpack.
The snowpack is therefore generally stable (cf. Fig. 1). In isolated cases in extremely steep terrain, avalanches can be triggered artificially by large additional loads (e.g. explosives or groups of winter sport participants). Human-triggered avalanches can never be ruled out entirely. The dangerous zones are sparse, however, and largely limited to extremely steep terrain. In extremely steep terrain, the danger of avalanches sweeping people along and causing them to fall is often greater than the danger of being buried.
Natural avalanches rarely occur, apart from small slides and small avalanches in steep terrain.
Around five percent of all fatal accidents occur at this danger level.

Moderate avalanche danger (level 2):

The snowpack is only moderately well bonded (cf. Fig. 1) in some places, as generally specified in the avalanche bulletin by reference to the altitude zone, aspect or type of terrain. Provided that routes are selected carefully, the conditions for snow sport activities are favourable in the majority of cases.
If weak layers exist deep in the snowpack, medium-sized avalanches can be released in some places in particular by large additional loads. Especially in places where the snow cover is shallow, the possibility of an avalanche being released even by a small additional load cannot be ruled out. Such triggering occurs more frequently in connection with new snow drift accumulations which, although released easily, are generally small. Alarm signs, such as whumpfing sounds, can exist in isolated cases.
If natural avalanches occur, they are generally wet avalanches. In isolated cases, they can reach medium size. Large natural avalanches are not to be expected. Transportation routes and settlements are rarely exposed to the danger of natural avalanches. Safety measures are also generally unnecessary on marked and open pistes.
Around one-third of all fatal accidents occur at this danger level.

Considerable avalanche danger (level 3):

On many slopes the bonding of the snowpack is only moderate to weak (cf. Fig. 1). Triggering is possible even with small additional loads, especially on the steep slopes in the indicated aspects and altitude zones stated in the avalanche bulletin. Alarm signs typically exist, but not in every case. Isolated slab avalanches can be released even from well outside the starting zone (remote triggering).
The danger of natural avalanches can differ greatly. In case of a weakly bonded snowpack and shallow snow cover, medium-sized avalanches are to be expected only sporadically. If this danger level is forecast after new snow or in connection with (daytime) warming, isolated large avalanches are possible as well. Safety measures, such as the use of explosives (especially in the case of new snow) or temporary closures (especially if temperatures are expected to rise) of exposed parts of transportation routes and, in particular, snow sport runs that are subject to protection by technical measures, are to be recommended. Those engaging in backcountry touring and off-piste activities must dispose over experience and assessment skills acquired through avalanche training. Steep slopes in the indicated aspect and altitude zone are to be avoided as far as possible.
Around one-half of all fatal accidents occur at this danger level.

High avalanche danger (level 4):

The snowpack is weakly bonded on most slopes. Triggering is probable even by small additional loads on numerous steep slopes. Alarm signs often exist. Natural avalanches and remote triggering are typical. In certain cases, when the avalanches are not especially large, snow sport participants are most endangered. Frequently, however, parts of transportation routes and settlements are exposed to danger. Depending on the situation (e.g. snowpack structure, new snow, wind), numerous medium-sized natural avalanches and a greater prevalence of large avalanches, which generally follow familiar paths, are to be expected. Exposed parts of transportation routes and settlements in the areas affected by such avalanches are endangered in the majority of cases. Safety measures, such as the use of explosives or closures, are to be recommended in these places. The conditions outside marked and open pistes are unfavourable. Avalanche runout zones in particular warrant caution.
Around ten percent of all fatal accidents occur at this danger level.

Very high avalanche danger (level 5):

The snowpack is generally weakly bonded and therefore largely unstable (consistent with large quantities of new snow accompanied by a fracture within same, or at transitions between new snow and the old snowpack). Extensive weak layers can also exist deep inside the snowpack; these can collapse if exposed to a heavy burden of overlying snow and give rise to large or very large avalanches. Numerous large and, in many cases, very large natural avalanches are to be expected, including in moderately steep terrain. Avalanches can also occur in the same place several times and open up new paths. Extensive safety measures (closures and, in some circumstances, evacuation etc.) are required. Fortunately, such disaster situations warranting danger level 5 very rarely arise. In such instances, backcountry touring is not recommended and is usually impossible in any case.
Fatal accidents have occurred at this danger level in particular during winters with large avalanches, as in 1951, 1968, 1975, 1984 and 1999, and account for around 1% of the total.