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.

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.

If numerous large and, in many cases, very large spontaneous avalanches are expected, the avalanche danger is rated with “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 high’. These situations represent a sever danger for backcountry recreationists, while transport routes remain unaffected or only affected in isolated cases.

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.

Quoted by: EAWS-European Avalanche Warning Service | EURegio Tirol Südtirol Trentin (17.03.2020)