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SnowFlurry 15 2019/20 | Hazard pattern 4: A treacherous weak layer

What is hazard pattern 4 and what processes take place in the snowpack?

by Stefanie Höpperger 03/07/2020
Hazard pattern 4 - cold on warm, warm on cold - occurs when there is a large temperature difference (more than 5°C) during the snowing process or between the snow surface and the fresh snow. As the name suggests, it can occur in both temperature directions, regardless of whether it snows cold on a warm surface or vice versa.

The large temperature difference within a few centimetres in the snow cover promotes the build-up transformation. As a result, thin, angular layers can form, which are predominantly present over large areas. The incipient weak layer can form in all exposures, but is increasingly found in sun-exposed terrain, as was the case in the winter of 2018-2019.

Furthermore, the angular crystals do not form immediately during or after the snowfall, but only in the following days. When and to what extent the weak layer forms becomes apparent over time.

In combination with the gm. 4, a melting hard layer usually also forms. Either a relatively warm snow surface is hardened by a drop in temperature, or a cold snow surface is moistened by the onset of warming (e.g. during wet snowfall or rain), which can also lead to the formation of a thin layer of melted snow.

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Depending on whether it is snowing warm to cold or vice versa, the weak layer forms below or above the melting hard cap:

Warm to cold, it usually forms above the hard cap, as the water vapor migrates downwards.

Warm to cold, the weak layer usually forms below the hard cap, as the water vapor migrates upwards.

The reason for this is the difference in vapor pressure. This is higher in the rather moist, warmer snow than in the cold, dry snow, which causes the water vapor to migrate from the warm to the cold layers (process as explained in Gestöber 6 in the case of the build-up transformation.)

There is still a widespread opinion that the cold fresh snow does not bond well with the snowpack (cannot "hold" on it) and therefore slides off the snowpack, which is the reason for the increase in avalanche danger. However, this is not the case, as it is well known that a weak layer is required for a slab avalanche, and the process of constructive transformation is responsible for this.

A fracture occurs in the weak layer of angular crystals and the snow slab above it slides down on the next harder layer, i.e. either on top of the snow cover or - if the weak layer is under the snow cover - together with the snow cover on the next layer.

How to recognize it

Weather observations and measuring stations can be used to determine the time at which a hazard pattern 4 may develop. Watch out for large temperature jumps in connection with precipitation. The competent teams at the LWDs keep a close eye on this and can narrow down the formed weak layer quite well in terms of exposure and altitude based on previous weather events, countless snowpack investigations and feedback from their own observers.

Why is hazard pattern 4 so treacherous?

The treacherous thing about gm. 4 is that the weak layer only develops in the days following the precipitation (usually 2 days) and the avalanche danger does not increase immediately. The weak layer is usually present over a large area and so a break can spread easily.

It therefore becomes an old snow problem that is not visible in the terrain unless you take a look at the snowpack or notice settling noises or similar danger signs.

We are therefore struggling with two problems with gm 4 that make it difficult for us to resist the deceptively safe powder dream. Firstly, the fact that the avalanche risk does not increase immediately after a weather event or due to external circumstances, but only days later, when you may no longer think about it. Secondly, it occurs more frequently in sun-exposed terrain. There, the snowpack structure is largely more favorable compared to shaded slopes and winter sports enthusiasts are often less concerned about the risk of avalanches.

A few days after the precipitation, there is bright sunshine, perfect powder slopes and the temperatures are ideal, so that drift snow has already bonded well and is only a minor problem. It looks like a perfect touring day is just around the corner. But if you let yourself be tempted here and don't know where gm 4 might prevail, this day could prove to be your undoing. If you don't have the deceptive gm 4 on your radar, you will find it very difficult to recognize it in the terrain and thus feel safe. It is always important to read the text of the avalanche report carefully and to take any specified exposures and altitude ranges into account when planning your tour!

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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