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

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.

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!