16.08.2023
People have been exposed to the danger of avalanches ever since the Alpine habitat was colonized. Avalanches are gravity-driven mass movements that pose a potential danger as a result of meteorological conditions. They are one of the few risks that can be predicted. The most dangerous avalanches begin as dry slab avalanches.
Avalanches are snow masses that move down a slope and occur in all mountain regions of the world. Like floods, mudslides and landslides, for example, they are gravitational mass movements.
Each avalanche can be divided into three areas:
Depending on the type and movement pattern, avalanches can reach different speeds, ranging from 70 km/h to 300 km/h. Typical “skier avalanches”, i.e. avalanches triggered by winter sports enthusiasts, are around 150 m long, with the avalanche face measuring around 50 m by 80 m and an average avalanche thickness of around 50 cm. This corresponds to a starting volume – i.e. the volume of snow that starts to move in the starting zone – of around 2,000 m³ or approx. 400 tons. A distinction is made between five avalanche sizes depending on the damage potential, the range of the avalanche and its extent:
There are several factors that contribute to triggering an avalanche. On the one hand, there are the natural conditions of an area: the shape of the terrain, the geological conditions, the climatic conditions and the vegetation are among the factors that remain constant over a long period of time. Variable parameters, on the other hand, depend on the time of day and season and sometimes change very quickly. These include meteorological conditions, the water balance and radiation intensity.
The snow cover snowpack is very much exposed to such variable processes. Wind, precipitation and temperature changes can lead to a weakening of the snow cover snowpack. An unstable snow cover snowpack and the “right” terrain then lead to an avalanche.
Slab avalanche, loose snow avalanche, gliding sluff gliding avalanche can each be either wet or dry. This distinction refers to the water content of the snow cover snowpack that slides down as an avalanche. If it contains a lot of water (known as the good snowball fight or snowman snow), it is called a wet snow avalanche. If, on the other hand, the snow cover snowpack is very cold and powdery and contains no liquid water, it is a dry snow avalanche. The position of the bed surface describes whether the avalanche has slid along the ground (ground avalanche) or whether a break has occurred within the snow cover (surface layer slab avalanche).
The slab avalanche is the most common type of avalanche. It is also the avalanche with the greatest potential for damage. Slab avalanches have the greatest destructive power and claim the most lives. They are characterized by a linear crack and can be wet or dry. The basic requirements for a slab avalanche are:
With a slope gradient of 50 degrees or more, it can be assumed that slab avalanches can no longer form due to self-discharge.
Weak layers are layers of snow within the snow cover snowpack that cause avalanches to occur. These layers have a particularly porous structure and are therefore unstable. Weak layers have large cavities and little bonding (contact points) between the individual snow crystals. The snow crystals in such weak layers are usually large. A weak layer is either formed by snow metamorphosis within the snow cover snowpack or was originally on the surface and was snowed in.
The snow surface is constantly changing due to meteorological influences such as radiation, wind and precipitation and often forms the future weak layer. Energy exchange with the atmosphere is also an important factor here: whether the snow cover snowpack radiates or absorbs energy depends on atmospheric conditions and is decisive for the nature of the snow surface in particular. If the air is warmer than the snow cover, surface heat is added to the snow cover. If the surface is warmer than the air, heat is lost from the snow cover snowpack (the snow cover radiates). Surface hoar frost, for example, occurs when a cold snow surface is surrounded by moist and warmer air; deposition takes place.
In the above pictures you can see a weak layer of surface hoar frost that has been snowed in. On the left side of the image (purple arrow), a fracture has already occurred, while the weak layer on the right side is still intact because a crack has occurred across the slope. In the second image, the same weak layer can be seen on a CT image. This photo shows for the first time the processes that lead to the release of a slab avalanche:
This snow slab descends as an avalanche from a slope gradient of 30 degrees.
If there is a pronounced weak layer, an additional load, e.g. a skier, can destroy the structure of the weak layer in small areas and trigger the initial fracture. This damage process progresses as more and more connections between the snow crystals break locally from place to place. As the forces are strongest at the edge of the damaged area (the crack), the fracture spreads further and further and once it reaches a certain size, there is no stopping it. The crack continues over large parts of the snow layer – often accompanied by a loud “wham” sound. During fracture propagation, the broken crystals form a small depression in the weak layer, the overlying snow literally sags, bends and sinks. This releases energy, which in turn further drives the fracture process at the edges of the damaged area. After a large-scale fracture of the weak layer, it depends on the friction when and whether an avalanche will occur or not. The steeper the slope, the easier it is to overcome the friction.
Loose snow avalanches occur when the layer of snow near the surface has little or no bond between itself and the layer below. They can also be wet or dry – whereby wet loose snow avalanches can be significantly larger than dry ones. They start at points, spread out in a cone shape in the path of the avalanche and take snow from deeper layers with them. They mainly break loose in terrain with a slope gradient of over 40 degrees.
These are usually triggered by new fallen snow, warming due to solar radiation or rain. Loose snow avalanches usually release spontaneously, but pose little danger to people and infrastructure. For skiers, loose snow avalanches pose a particular danger of falling if they are swept away. Burial is rather unlikely due to insufficient amounts of snow.
Gliding sluff gliding avalanches are full depth slab avalanches, i.e. the entire snow cover slides down to the ground. They have a linear crack. The ideal bed surfaces are smooth surfaces such as deposited grass or rock slabs, on which the entire snow pack above slides off. Gliding sluff gliding avalanches are caused by a loss of friction between the ground and the snow. The friction loss is caused by the presence of water at the interface between the snow and the ground. The entire snow cover begins to slide off the ground and consequently glide cracks – so-called fish mouths – open up in the snow cover snowpack, exposing the ground. It is not possible to predict exactly when this sliding process will occur and at what speed. Gliding sluff gliding avalanches cannot be triggered artificially and therefore in a controlled manner. This means that they always occur spontaneously.
There are cold and warm gliding sluff gliding avalanches, depending on how the water gets into the lowest layer of snow. Cold gliding sluff gliding avalanches are not related to the air temperature. Rather, the water content in the ground and the ground temperature are decisive. The water from the ground rises into the lowest layer of snow due to capillary forces, which leads to a loss of friction with the ground. Warm gliding sluffing avalanches, on the other hand, occur as a result of rain events or warming. The water penetrates through the snow cover snowpack to the ground. In both events, the speed of sliding correlates with the amount of water at the ground/snow interface.
An essential part of any avalanche forecast is an indication of the avalanche problem(s) currently prevailing. These five problem categories are used to describe typical snow situations in the terrain:
Types of avalanche to be expected: dry slab avalanches, dry loose snow avalanches
Expected avalanche types: dry slab avalanches
Expected avalanche types: dry slab avalanches
Expected avalanche types: wet slab avalanches; wet loose snow avalanches
Expected avalanche types: warm or cold gliding sluff gliding avalanches
![Persistent weak layer problem - The problem is caused by existing weak layers [link to description above] within the old snow cover. Typical weak layers are snow-covered surface frost, deep frost (also known as cup crystals or ‘floating snow’) or angular crystals.
Zu erwartende Lawinenarten: trockene Schneebrettlawinen](https://www.snow.institute/wp-content/uploads/2023/10/Lawinenprobleme-4.jpg)
