Gliding snow is defined as the slow downslope movement of the entire snowpack over the ground. The rate of slide ranges from a few millimetres to a few metres per day. For snow to glide, the following conditions have to be fulfilled:
- The ground surface must be smooth; examples include flattened grass or a slab of rock.
- The slope must be sufficiently steep, but gliding can occur at a slope angle of just 15°.
- The base of the snowpack – the boundary layer between the snowpack and the ground – must be moist or wet.
This final condition is satisfied in particular if the ground is at a sufficient temperature to warm and moisten the snowpack from underneath. A high incidence of glide-snow avalanches in wintertime therefore depends on snow falling on warm ground and the existence of a deep snowpack, which provides more effective insulation than a light covering of snow. In winters when the snow is plentiful, moreover, in view of the greater snow depths, gliding avalanches are larger than in winters with little snow. At high altitudes the snowpack generally glides more readily on south-facing slopes, where the ground is warmer, than on shady slopes.
Gaps in the snowpack, also known as glide cracks, are tensile fractures penetrating the entire snowpack. They arise at interfaces where the snow is gliding at disparate velocities – faster below the crack than above it. A glide crack that propagates over a period spanning a few days to several weeks can suddenly accelerate and release a gliding avalanche.
Often, but not in every case, gliding avalanches are released below glide cracks which open. The entire snowpack slides over the ground. Gliding avalanches can be released at any time of day or night and generally are naturally triggered. They are frequently preceded by the accelerated opening of a glide crack, but unfortunately this cannot be detected by simple human observation. It is virtually impossible to trigger gliding avalanches with explosives, and they are very seldom released by people.
A distinction is often made between “cold” and “warm” gliding avalanche events, as described below.
Cold gliding avalanches occur predominantly in midwinter. In these cases the entire snowpack, except at its base, is dry and cold (below 0° C). The snowpack’s temperature is 0° C and it is moist only directly at the interface with the ground. With these avalanches, the processes taking place in the transitional area between the ground to the snow play a major role. Researchers at the SLF are currently investigating the exact nature of these processes (see below). Daily temperature fluctuations are known to have an impact on only the uppermost few decimetres of the snow, so that, for example, a melt-freeze crust on the surface of the snowpack does not exert any influence on conditions at ground level. Gliding avalanches therefore occur even when the air is very cold or the surface of the snowpack is frozen. The threat of cold glide-snow avalanches remains more or less consistent throughout the day.
Warm gliding avalanches are released when the snowpack becomes moist all the way through from the uppermost surface downwards and water penetrates through it, thereby reducing the friction at its interface with the ground. It is difficult to clearly differentiate between these avalanches and full depth slab avalanches consisting of wet snow; similar processes appear to unleash in each case. Much like wet snow avalanches, warm gliding avalanches are thought to occur more frequently in the latter half of the day than in the early morning.
Given that releases can occur at any time, and that the precise timing of a release is unpredictable, areas where glide cracks have opened are to be avoided if possible. In any event, you should never linger below glide cracks for any longer than absolutely necessary.
In connection with gliding snow, the occurrence of moisture at the base of the snowpack appears to be influenced by another process as well. Dry snow falls mostly on ground that is substantially moister than itself. In the presence of strong capillary forces at the snow/ground interface, the fine-grained snow draws water from the coarse-grained substrate. It is not clear when is the successive melting of the deepest layers of the snowpack by the warm ground, and when the key factor stems from the capillary forces between the ground and the snowpack are decisive.