SnowFlurry 5 2016/17 | Fair weather phases and the snow cover

Initial situation

The Alps are under the influence of an area of high pressure that will last at least several days. There is hardly any cloud cover, clear nights and no significant precipitation is forecast for the foreseeable future. At low altitudes (< 1000m) there is usually no snow in the Eastern Alps, at medium altitudes (1000m - 2000m) there is mainly snow on the slopes and in higher and high alpine areas (> 2000m and > 3000m) there is only a sparse snow cover. In the former, the best you can do is go hiking or grass skiing, in the latter you can go snow stomping and in the latter you can either keep fit in ski resorts with artificial snow tours or go rock and crevasse slaloming. The snow cover already contains some weak layers and melt crusts, the surface is partly hardened by wind crusts and partly by the warm spell at the end of November and is hard or crumbly, not loose.

How it works

Combined with the current position of the sun and not too warm temperatures, the snow cover is now beginning to change. The nights are primarily responsible: the snow surface cools down massively due to the outgoing heat radiation caused by the clear sky - far below the prevailing air temperature. The large temperature difference between the relatively warm snow near the ground and the snow on the surface causes the entire snow cover to build up. In addition, surface frost forms. This is not caused by free water vapor from the snow cover itself, but from the moisture in the air. The process is very similar to that within the snowpack, only the moisture comes from the surrounding air: the water vapor from the relatively warmer air "freezes" on the snow surface (deposition). The crystal forms of the surface frost are similar in appearance and in their effect on the risk of avalanches (when snowed in) to the products of the build-up transformation within the snowpack. The uppermost layer can simultaneously transform into angular crystals and form surface frost at its boundary with the air. The more continental the climate is (cold, low precipitation), the more likely it is that weak layers will form in this way. As inner alpine mountain groups have a drier climate and are cooler due to the higher mountains, weak layers also form more strongly and more frequently there than in marginal alpine areas. In some Asian mountains, for example, the transformation of the entire snow cover into floating snow, not just in individual layers, can be observed much more frequently than in the Alps.

SnowFlurry

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What happens next

The longer the precipitation-free fair weather continues, the more the entire snow cover will turn into angular crystals or floating snow. The snow cover becomes looser and it becomes easier to sink through. From the point of view of board sliders, this is somewhat similar to the rotten snow from spring, where you sink deep into the snow - sometimes even down to the ground. You will be able to observe massive surface rime, especially in higher, windless locations. The tensions (basically the differences in hardness between different layers, but not completely equivalent to this) decrease daily because harder layers become looser and therefore softer due to the build-up process. They can no longer transfer stresses. As soon as it snows or stronger winds follow (even without precipitation), the risk of avalanches increases - at least for a few days.

In the winter of 2015/16, the build-up processes took place most strongly throughout December, which was a beautiful month in terms of weather. The sparse snow cover became looser and looser, in some places it was possible to ski on a more or less stable snow cover at the beginning of December and from mid-December it was possible to ski through to the ground in the same place. In January 2016, the long-awaited fresh snow plus wind arrived. This brought the necessary build-up of tension to trigger avalanches. The situation was dramatic in a certain altitude band in shady exposures due to the high tendency of the existing weak layers to propagate fractures. The situation worsened with greater snow cover until the weak layers were so deep within the snowpack that it was difficult to disturb them. Gradually, the crystals reconnected somewhat due to a now lower temperature gradient.

Deep rime = floating snow — Deep ripe = floating snow

Lukas Ruetz

While the degradative transformation that is now taking place again is only happening very slowly and forms from the constructive transformation are only becoming smaller and rounder very slowly, they are nevertheless usually joining together so well that at least the probability of fracture propagation is significantly reduced. It was only towards mid-February that the situation slowly eased as a result of these processes. In most cases, however, the weak layers only disappeared completely after the first moisture penetration in spring and were transformed into melt forms. If we are facing another high-pressure early winter this year, if the weather continues to produce artificial snow and then the "real" winter arrives, the signs are good that a massive old snow problem will accompany us for a while - perhaps even a large part of the winter - primarily in shady, higher altitudes.

Note: During long-lasting and windless periods of good weather in early winter, the danger tends to decrease - new snow is usually followed by a sharp increase in avalanche danger, which can last for weeks.