SnowFlurry 11 2020/21 | The delicate weak layer has calmed down

The toxic weak layer, which was present in a certain altitude band and had formed around the rain crust created on 21/22/12/20 by a GM4, claimed several lives at the end of January and beginning of February and was responsible for many avalanches. This weak layer was particularly treacherous because it could not only be triggered on a steep slope, but also as a remote trigger in flatter terrain, but mainly in transitions from little to a lot of snow. This is due to the fact that the weak layer is located in deeper layers and in some cases a thick layer of snow lies above it, which is why the weak layer cannot be disturbed there. In places with less snow, on the other hand, it was only up to one meter below the snow surface (as in profile 1), i.e. at a depth that could still be disturbed. If one caught such a hotspot and initiated a fracture there, it spread over a wide area and the avalanches that occurred sometimes took on a large scale. The slab above was seemingly perfect, due to wind and heat in an altitude range of approx. 1900-2300m. Below this, the avalanches were already coming off by themselves due to the rain, or the rain destroyed weak layers. Above that, this weak layer is hardly or not at all present.

It is all the more pleasing that this treacherous weak layer has calmed down. People seem to have a tendency to suppress or trivialize such an old snow problem, where the danger in the terrain cannot be recognized, perceived or heard through settling noises. If no avalanche has occurred on a tour, many people believe that they have chosen a safe tour. Often, however, it was simply luck that you didn't hit a hotspot where an avalanche could have been triggered. The lure of great skiing fun is obviously too great for many people to put it off until later. Dangerous and beautiful are often very close together.

SnowFlurry

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Spring-like weather favors degrading transformation — Spring-like weather favors degradative transformation

Stefanie Höpperger

Now to the layers of profile 2 and the comparison to profile 1:

The snow profile 2 is from 20.2.21 and was recorded in the Kühtaier Längental, on a 34° steep east-facing slope, at an altitude of 2300m.

Light blue:

Deep rime with angular rounded crystals with a size of 0.5-2mm and a hardness of 1-2.

The snow dates from late November - early December.

Difference to profile 1:

In profile 1 we still had a poor foundation of large and loose floating snow crystals. In profile 2, the cup crystals with their facets are in the process of becoming smaller and rounder again, which is thanks to the degrading transformation and can be seen as positive.

Violet:

Two fusion crusts. The lower one still has angular, slightly larger 0.5-1.5mm crystals and the upper one has angular rounded, slightly smaller 0.5-1mm crystals, with a typical hardness of 4. Both crusts are in the process of "eating away" (becoming looser again), which has formed the somewhat intergrown thin layer of angular and angular rounded crystals in between. The two crusts were probably once connected to each other.

As in profile 1, the crust originates from the rainfall on 21/22/12/20.

Red:

The thin layer of 31-33cm is a rather intergrown layer, not continuous in the profile, of angular and angularly rounded crystals, with a size of 0.5-1.5mm and a hardness of 2. As already mentioned above, it is more likely to have formed because the crust is slowly eating away/degrading.

The layer of 36-50cm consists of angular and angular rounded crystals with a size of 0.5-1mm and a hardness of 2.

It originates from the precipitation including the drop in temperature on 24.12.20 and the subsequent longer cold period until mid-January. This means that it was formed on the one hand by a Gm4 and on the other hand by the cold period during which the snow surface at that time built up.

Difference to profile 1:

The until recently treacherous and toxic weak layer (as in profile 1, red), which was responsible for the many avalanches and avalanche victims, can no longer be disturbed in profile 2.

Green:

The lower green layer consisting of 0.5mm small round crystals with a hardness of 3 originates from the precipitation in mid-January. A compact, well-set layer and the result of the decomposing transformation.

The upper green layer consists of angularly rounded and round crystals, also with a size of 0.5mm. With a hardness of 2-3, it is slightly softer than the one below. This is the heavy snowfall of January 28th and the subsequent snowfall from the end of January - beginning of February.

An approx. 90cm thick, quite well set and compact layer.

Difference to profile 1:

In profile 1, the degrading transformation was already underway, where the initially angular crystals have turned into angular rounded crystals. In today's profile 2, you can already see the final stage of the degradative transformation: round, small grains that have moved close together and transformed into a compact layer. The upper layer of 85-142cm is currently in this process.

Now we find the subsequent weather pattern including precipitation after 24.1.21, when snow profile 1 was recorded. The melt crust present in profile 1 (gray) would be somewhere between the two green layers in profile 2. The crust and the underlying angular layer (profile 1, orange), which was formed by a GM4 in the warm phase from 19-22 Jan, are no longer present in profile 2.

Orange:

It is a thin layer of 0.5mm small, angular crystals and very soft with a hardness of 1. The layer formed as a result of the drop in temperature and precipitation on 7/8/2/21 due to hazard pattern 4 (cold to warm, warm to cold).

Grey:

A fusion crust that has 0.5 mm small angular crystals and is on the softer side with a hardness of 3. It formed during the warm phase with significant plus temperatures up to high altitudes at the beginning of February and the subsequent drop in temperature including precipitation on February 7/8/21.

Dark blue:

A rather soft layer with a hardness of 1-2 consisting of 0.5 mm small, angular crystals and a few angular rounded crystals in between. Like the orange layer, this layer was formed by hazard pattern 4, which was caused by the drop in temperature and the onset of precipitation on February 7/8, 2021 and the resulting large temperature difference that formed between the warm snow surface and the cold fresh snow. This was followed by several days of very frosty sub-zero temperatures, which caused the already very loose snow surface, partly consisting of wild snow, to build up.

Yellow:

The two layers originate from the precipitation around 16/17.2.21 and the spring-like temperatures that began with it. The layers consist of 0.5mm small, round crystals with a hardness of 2. Melt forms up to a size of 1mm formed on the snow surface due to the already quite strong solar radiation and the warm temperatures. Both layers are already slightly moist (2). On the day the profile was taken, there was not yet a usable snow cover for skiing on firn or snow slush.

Left: Profile 1, 24.1.21. Right: Profile 2, 20.2.21.

Stefanie Höpperger

Left: Profile 1, 24.1.21. Right: Profile 2, 20.2.21.

Stefanie Höpperger

Tests

The tests revealed only two partial fractures in the angular layers near the surface, above and below the crust, which are rather negligible in this profile.

The weak layer, which was very easy to disturb in profile 1 and is shown in red, could no longer be disturbed in profile 2 as it lies too deep in the snowpack. However, it could also not be disturbed if the snow was removed down to 90 cm and then the test was carried out. It solidified and calmed down somewhat due to the now lower temperature gradient in the snowpack and the associated degrading transformation.

Already to be noted on this day was the danger of wet and loose snow avalanches from steep and sunny terrain, as well as the very early soaking on steep south-facing slopes, where wet snow slides could already be triggered in the late morning.