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SnowFlurry 5 2019/20 | Early winter firn

Poor snow quality, weak layers near the surface, wet snow

by Lukas Ruetz 12/21/2019
A few days with very poor snow quality and extremely high temperatures due to the foehn are behind us. With the right exposure, slope steepness and altitude, it was already possible to ski a preliminary firn stage.

After the great early winter conditions in the first week of December, the wind has once again made its presence felt. Particularly during a westerly storm on Saturday, December 14, the snow cover in the Stubai Alps was worked on almost everywhere by the wind - eroded, transported, compressed. In the meantime, a "heat wave" lasting several days has left its mark. The resourceful winter sports enthusiast has sought out the first firn instead of the surface littered with wind-created snow. In the meantime, a weak old snow problem has developed near the surface on sunny slopes, which could become more prominent with the coming snowfall due to the formation of a more suitable snowpack above the weak layer.

Profile: Sellrainer Sonnberg, 19.12.2019, S, 34°, 2480m

The profile was taken towards the end of the "heat period" on a steep south-facing slope at just under 2500m in the afternoon. The air temperature during the recording is +1.8°C and the snow depth distribution in the recording area is very irregular due to the wind influence. The snow cover is about 60cm thick and isothermal. This means that it has the same temperature from the very top to the very bottom. Theoretically, the term "isothermal" could be used for any temperature. However, as isothermality practically only occurs at 0°C in a natural snow cover, the word immediately indicates that the snow cover no longer has a "temperature reserve" and is therefore consistently 0°C warm. The energy supplied from now on will only lead to the snow crystals melting instead of further warming of the snow.

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On the surface, we find melt forms with a diameter of 1 - 1.5 mm and a moisture content of 3, hardness 1-2. These lie on a slightly moist melt crust with a hardness of 2-3. The difference between moist to wet, loose melt forms and a melt crust is the connection between the crystals: While in the former there is already so much water between the snow grains that they are de facto no longer connected, in the crust there are still many connections and little to no water between the melt grains (still "frozen together"). This means that they are not yet sulfate, slush, lettuce or whatever you want to call it. They are (still) a crust: harder, crispy, crusty. Scientists speak here of the Liquid Water Content - LWC of a layer of snow. The more water in the molten layer freezes back into ice grains at night, the more it forms something that can be called a crust again.


Between the two molten crusts, we find a thin layer of relatively small, angular crystals that has been transformed by building up. They are again on the path of degradative transformation (rounded edges) and at the same time are also slightly in the direction of enamel transformation (slightly moist). They are part of the near-surface old snow problem that is currently described in the Euregio Avalanche Report on sunny slopes above about 2500m. Here, however, we were only able to initiate a partial fracture (N) in the sixth stroke of the extended column test (ECT) due to the weak development of the overlying snow slab and weak layer.

Below the lower melt crust is a thick layer of crystals that are also angular. They formed during the long period of good weather at the beginning of December. They are no cause for concern because the layer is relatively thick, the crystals are very small and are also currently undergoing isothermal metamorphosis (= decomposing transformation). This means that the crystals are becoming smaller again, the bonds are strengthening and the edges of the crystals are rounding off. And this happens quite quickly because the degradative transformation takes place much faster at 0°C or a few degrees below than at a low temperature gradient at colder snow temperatures.


On the ground, we again see quite moist, very small melt forms. They were primarily moistened so much by the ground heat.


There are spring-like conditions at this location in the afternoon. The snow cover is isothermal, it currently has no more temperature reserves. All snow layers are already moistened. The accumulating, transformed layers (old snow problem) - as described in the avalanche report for sunny slopes above 2500m - are present here, but are too weak to classify an avalanche release as probable. Two or three hundred meters higher up, however, the angular layer should be pronounced enough to consider an avalanche triggering as conceivable. Even if the problem is only very slight.

Note: In early and high winter, after a longer period of fine weather with very high temperatures, firn (= slushy snow on a stable crust) is most likely to be found on the steepest and lowest south-east and south-facing slopes between 12 and 13:30.

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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