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SnowFlurry 8 2021/22 | Snow profile review

Toxic weak layers in old snow

by Stefanie Höpperger • 02/19/2022
An avalanche-rich time lies behind us. The old snow problem, which was responsible for many of the avalanches in recent weeks, unfortunately remains with us.

A period of fine weather with dry air and good night-time radiation prevailed at the end of January. The snow surface cooled down considerably and snow surface temperatures of below -20°C were measured. There was a large temperature difference in the layers near the surface, which allowed the building transformation to work properly. Angular crystals developed and the snow surface became increasingly loose. There was also a lot of surface frost, especially in the forest areas.

The fine weather days were accompanied in part by winds, which picked up again around January 27th, ranging from strong to stormy. It swept ridges and crests free, whirled up the loose snow surface and surface frost and deposited it again as drift snow in the lee. This had one advantage: the surface frost above the tree line was partially destroyed by the wind.

In addition, a thin but extensive crust formed on the snow surface on January 23 due to freezing high fog. In the days that followed, a toxic weak layer of angular crystals formed under the crust. It is also quite possible that a hazard pattern 4 - cold to warm - formed on January 28 with the onset of precipitation and the associated drop in temperature. Both (weak layer on the crust and GM4) are known to develop over large areas. In combination with a suitable slab, this results in remote triggering and large fracture propagation.

Luckily, the snow variability was high before the precipitation, otherwise the weak layers formed would have been even more extensive and avalanches would probably have been even larger.

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On January 28, the precipitation totals were quite low, but enough for a further temperature difference to form near the surface. The weak layers around the crust continued to develop. On January 31st to February 3rd in the morning, Frau Holle, or the NW current, brought us plenty of white gold, but at the same time the missing board for slab avalanches. As the precipitation was very stormy, fresh drift snow formed over a large area and was prone to disruption. There was also a fresh snow problem in the regions with high precipitation.

With the start of the fine weather days, temperatures also rose, which favored the binding of the snow slab and ensured good break propagation. A perfect constellation of weak layer and slab was created. The number of avalanches in the 3 days was remarkable and at the same time frightening! It seemed as if the whole winter consisted only of these three powder days. The winter is long and only a few days in demand absolute restraint, but with such a constellation is not good cherries eat!

Snow profile review

Profile 1: Loose snow surface of angular crystals was snowed in and now acts as a weak layer.

The snow profile was recorded on 08.02.2022 at an altitude of 1800m, in a NE exposed, 20 degree steep slope. The profile location is in a rather wind-protected area.

Light blue:

The three lower layers originate from the precipitation from the beginning of November to the end of December 2021. They consist of quite large deep ripe crystals, which can be easily recognized by their facets. The corners and edges of the crystals are already rounded, which is an indication of the decomposing transformation. There are also frozen water columns in the snow layers, which have a stabilizing effect. On the one hand, this makes the loose layers uneven and, on the other hand, the crystals are solidified where the columns are.

Orange:

A thin layer of 1 - 2.5 mm deep crystals, with rounded corners and edges. With a hardness of 1 (fist), it is very soft and would still be suitable as a weak layer. However, since there is still a thick, loose layer (yellow) of angular crystals above it, the penetration depth is somewhat reduced.

Violet:

The fusion crust of angular, rounded crystals is the rain crust from 29/30/12/21. With a hardness of 3-4 (a finger - pencil), it is on the softer side for a crust and from the underside it is already somewhat eaten away.

Yellow: The culprit in the snowpack.

A 13 cm thick, very loose layer of angular crystals, with a size of 0.5 - 1 mm. The second symbol "angularly rounded" is mainly for accuracy: some crystals are rounded, but the first are angular crystals.

This layer forms the old snow surface before the snowfall at the end of January - beginning of February. The surface at that time was strongly cooled and transformed during the fine weather phase, including clear nights, which led to the formation of the angular crystals.

An ECTP 1 was achieved here, which means that a fracture was initiated in the weak layer at the first blow from the wrist and propagated across the entire block. This indicates a high susceptibility to failure of the weak layer in combination with an overlying board. At the profile location, however, the weak snow cover poses no danger, as the slope here is largely below 30 degrees.

Green: The board. Bound snow consisting of fresh snow, felt and/or round grains.

The layers originate from the precipitation of 28.1.2022, as well as from the heavy snowfalls of 31.1.-3.2.

Profile 2 + 3 (at the bottom of the gallery):

Show the crust formed by the freezing high fog on 23.1. and the underlying angular crystals, which form the main weak layer here.

Photo gallery

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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