Wind as a master builder of avalanches

Drift snow is one of the five avalanche problems and is one of the most common causes of avalanches. The danger patterns are often very clear to recognize, but are frequently underestimated.

Snow-filled gullies or deep white slopes after an ascent over blown-off terrain are all too tempting. Even small snow slabs (20 x 20 meters) can be life-threatening for winter sports enthusiasts. In addition to avalanche problems with fresh snow, old snow, wet snow and sliding snow, wind action and thus drifting snow is almost always to be expected, especially in the Alps.

THE CAUSER WIND

In contrast to parallel wind streams, turbulent winds often predominate in the mountains, which are much more difficult to assess. In addition to large fronts, especially Föhn, there can also be very local differences. The reasons for this are temperature differences at different altitudes (mountain and valley), between day and night or between shady and sunny sides. Ground and high-altitude winds, for example, can move in different directions to each other. Ground winds are significantly influenced by the terrain and therefore move according to the relief of the terrain. Ground winds can, for example, move into side valleys and therefore often flow at right angles to the high winds. This should always be borne in mind when planning a tour and therefore when assessing drift snow deposits. If the weather forecast mentions a westerly wind, for example, it cannot be assumed that drift snow accumulations can only be found on eastern slopes (the leeward side). A local change in wind direction is not only to be expected from valley to valley, but also between valley and mountain. In addition, drift snow accumulations occur in hollows and gullies, regardless of their exposure.

Snow is already transported in moderate winds (wind force 4, around 20 km/h). At wind force 6 on the Beaufort scale (corresponds to approx. 40 km/h), drifting snow can already become an acute problem.

THE FORMATION OF DRIFT SNOW

Wind forms and transports both falling and already deposited snow. The snow is mechanically crushed and transported from the windward side (windward side) to the leeward side (leeward side). Drift snow is to be expected particularly in areas close to the crest and terrain transitions. Drift snow can be deposited in gullies and hollows in all exposures.

If drift snow is deposited on loose powder snow, these layers can often bond together to form a stable layer within hours or just a few days. However, if the transported snow crystals are deposited on a layer of old snow, there is usually only a weak bond with the layer below. The drift snow is therefore deposited as an unstable layer of snow. When drifting snow is deposited on crystals that have been transformed to build up, such as floating snow (deep rime), surface rime or hard old snow, it can often take months for the individual layers to bond.drifting snow itself has the characteristic property that it is bound within itself and can therefore build up tension. As a result, snow planks can form. The resulting cavities, i.e. the structural break in the snow cover on the angular crystals, form an “ideal” sliding bearing for the layer on top. External loads, such as a winter sports enthusiast, cause the snow layers above a hollow foundation to collapse and, in the worst case, trigger a slab avalanche. A snow slab avalanche has a linear crack and can extend over several hundred meters on a slope.

The prerequisites for a slab avalanche are Bound snow (drift snow), a weak layer, a steep slope and an additional load. Danger signs for instability in the snowpack are crack formation, settling noises (“whump” noises) and avalanche self-triggering.

RECOGNIZE DRIFT SNOW

The bound snow cover can be recognized by its matt surface. In contrast, the glistening snow crystals of freshly fallen powder snow are familiar. The aforementioned tension in the surface can be recognized when ski touring by the sharp-edged edges of the tracks as well as by a “blunt” resistance when tracking or skiing. Particular attention should be paid to cracks in the snow cover, which can often occur next to the track. Drift snow can often be very irregularly distributed in wind shadows and be hard or soft. This is reflected in an irregular sinking depth when making tracks.

RECOGNISING DRIFTING SNOW - ÖAV

READ WIND EFFECTS CORRECTLY

The effect of wind, in particular the wind direction and thus the places where drifting snow deposits are to be expected, can be read from clear signs:

• Blowing snow/snow sweep
• Blown-off ridges: A clear sign that drifting snow is to be expected downwind.
• Cornices: A compacted snow deposit which pushes up towards the leeward side on terrain edges. Caution: The cornice gap (i.e. the predetermined breaking point of a cornice between the snow cover and the windward side) does not run vertically over the edge of the ridge. When choosing a track, it is therefore important to keep a safe distance from the flank.
• Wind gangs/sastrugi: These are wind-induced erosions of loose snow. In contrast to the cornice, the flatter side of the wind gully is the side facing away from the wind. The steeper edge therefore indicates that the wind has acted from this direction.
• Comet streak: The snow accumulates in the lee of obstacles.
• Wind clouds: Open spaces in front of obstacles (e.g. boulders, buildings) caused by strong winds.
• "Anraum": Precipitation that builds up against the wind direction on obstacles such as trees, fences, etc.
• Dunes and waves: The snow surface can be undulating like a dune. The steep side of the dune indicates the side facing away from the wind and is perpendicular to the wind direction.

The handy avalanche guide from the Austrian Board of Trustees for Alpine Safety (Österreichisches Kuratorium für Alpine Sicherheit) is a practical reference work on the subject of avalanches. From planning, avalanche and snow science to strategies in the terrain and the correct behavior in the event of an avalanche accident, the guide provides a good overview of the correct procedure in the open ski area. 

Author: Österreichisches Kuratorium für Alpine Sicherheit
 

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