When snow falls with wind, the fresh snow is deposited very unevenly. This creates areas on a slope where the snow cover cannot support its own weight. The weight of the snow masses causes tensions that are much greater than the static friction between the snow layers. In these places, a snow slab spontaneously breaks loose and slides off as an avalanche. Very often, however, the snow slab is still hanging on the sides or on top or is supported from below and does not slide off. The snow slab "hangs? as a taut trap in the slope?
Every unprepared slope resembles a patchwork quilt. Stable slope sections, moderately stable areas and weak spots, so-called hot spots, are directly adjacent to each other. The higher the avalanche warning level stated in the avalanche report, the more weak spots there are in the snow cover.
To be able to assess the risk of a slab, you need more "snow-how? about the snow cover on a steep slope: The snow crystals of a snow cover settle due to the degrading transformation and the melting transformation. In addition, the snow crystals move down the slope. This movement of the snow crystals is called creeping. Each layer of the snow cover moves at a different speed. The uppermost layer is the fastest. With increasing depth, ground obstacles and strong friction slow it down more and more.
The different speeds create tension between the layers of snow, known as shear stress. The shear stress is counteracted by the friction, the static friction, between the layers of snow. The forces in a powder slope are distributed very differently. How resilient a slope is depends on how stable the various sub-surfaces are, i.e. what the ratio of static friction to tension is:
A slope area is stable if the static friction is greater than the tension.
A slope area is in a dangerous equilibrium if static friction and tension are equal.
A slope area is a trap if the friction is less than the tension.
If there is a weak layer in the snowpack, e.g. floating snow, snow-covered surface frost, ice, harsh snow, the static friction is greatly reduced and a potential avalanche slide path is present. Unfortunately, there are potential avalanche paths in [almost] every snowpack.
A weak layer under a bound, less deformable, firmer layer, e.g. wind-blown powder snow over a snow-covered surface, is needed to trigger a snow slab, but even an unstable slope section does not have to break away spontaneously, as other more stable partial surfaces can absorb the stresses. However, if there is an additional load from freeriders or fresh snow, a fracture [= initial fracture] can start from this hot spot. The fracture propagates quickly in all directions until the snow slab is detached and slides off in clods. The slab often breaks away so quickly that it is too late to escape. However, the triggering fracture does not have to happen within a slope, but can also occur from [far] outside. Avalanches are often triggered remotely by groups of freeriders or slope rollers.
When the avalanche danger is high, a single freerider can trigger a break in the flat off-piste terrain. If you pay attention, you may hear a muffled "boom? [This break in the snow cover continues invisibly until it triggers a probably large snow slab on a steep slope.
Only when the slope is so heavily tracked that it is no longer possible to turn in untracked snow can it be considered avalanche safe. [Attention: does not apply in spring when the snow cover is wet!]
Note: The steeper, the steeper - the faster the snow slab release!
Avalanche researcher Bruno Salm discovered that the additional load at which fractures and cracks occur in the snowpack is not always the same. Whether the snowpack breaks or not always depends on the speed at which the additional load acts. When loaded slowly, snow behaves in a similar way to thick honey: it is elastic and deformable. Only when the additional load is applied too quickly - i.e. at the critical deformation speed - does the final break or crack in the snow cover occur. Anyone who has ever walked through deep fresh snow knows that you sink in particularly deeply and therefore use up a lot of energy if you tread quickly and with full force. However, if the snow cover is loaded slowly and carefully, you often don't sink in as deeply. However, if the additional load is applied suddenly - e.g. by jumping over a cornice - the stability can decrease to 1/10 of the original strength. From "considerable? Avalanche danger - or if you have a bad feeling - particularly cautious and prudent behavior is called for. If you run off headlong and in panic, you may fall blindly into a trap.fortunately, unlike bricks, snow is able to absorb and compensate for stresses as it is deformable. But it needs a lot of time to do so. Even thick drift snow deposits can solidify so well over time that you can drive on them.
