SnowFlurry 1 2018/19 | The old snow problem in the Alps

Observations

While old snow problems with weak layers near the ground are almost never found in certain areas in the Alps, there are others where they form almost every winter. This depends on the climatological conditions: Most strongly on the amount of precipitation in (early) winter. This is because weak layers of angular crystals or deep frost form much faster in a less thick snow cover during phases with clear skies due to the cooling of the snow surface. In other words, facetted crystals.

The connection between the vegetation types, the snowpack structure and the old snow problem, or rather the formation of weak layers near the ground, has been known for a long time. In other words: Where palm trees grow, there is no old snow problem - because there is no snow. Where the forest is largely made up of beech trees, there is less of a problem with old snow because it is milder and wetter there than in the Alps, where there are many more stone pines.

We have investigated this assumption based on experience using tree distribution maps and snow cover data and have been able to confirm it: The worst snow cover structure in the Alpine region is found where the tree line is largely made up of Swiss stone pines (often together with larches).

SnowFlurry

presented by

A schematic overview map of the alpine regions with the most common old snow problem. Schematic, not scientific. The probability of weak layers near the ground in the snowpack is of course continuous and cannot be clearly defined. It will also remain a matter of dispute whether, for example, the Hohe + Niedere Tauern are old snow hotspots or not. The average snowpack structure over several winters is also less good there, but already better than in the Engadin or the Ötztal Alps, for example. — A schematic overview map of the alpine regions with the most common old snow problem. Schematic, not scientific. The course of the probability of weak layers in the snowpack close to the ground is of course continuous and cannot be sharply defined. It will also remain a matter of dispute whether, for example, the Hohe + Niedere Tauern are old snow hotspots or not. The average snowpack build-up over several winters is also less good there, but already better than in the Engadin or the Ötztal Alps, for example.

Background

Of course, one Swiss stone pine or one beech alone, or even a few of them, doesn't say anything at all: Both tree species can be found in all areas of the Alps. This applies to the regions where there are almost only Swiss stone pines and larches in the upper forest area, and in the same regions in the lower forest areas there are almost only spruces, but hardly any beeches and practically no silver firs. Think of Kühtai, Obergurgl, the Engadin - also the Visper valleys or the upper Vinschgau: all areas characterized by stone pines in the upper forest area and at the same time known for their old snow problem winters. This is not because beech or fir trees no longer grow in the valleys at an average altitude - as they also climb to 1800m or even higher in the randalpine areas - but because the stone pine is better adapted to the harsh, cold, low-precipitation climate than the others. In addition, practically all Swiss stone pines die sooner or later due to a fungus that is only an issue in the snowier regions.

In fact, this finding cannot be used operationally. The correlation is simply interesting. The winter of 2017/18 also showed this well: We're talking about probabilities here. There were almost no weak layers close to the ground in the entire Alpine region, even where there are many Swiss stone pines. The avalanche situation report is and remains our planning tool.

Typical Swiss stone pine landscape at the tree line in the areas of the Alps that are plagued by old snow problems.

Output

A small part of the practical application for today is nevertheless in such contexts: In mountains where there is still no or insufficient avalanche information, you can orientate yourself a little during a ski touring trip by the vegetation and possibly sharpen your senses for a potentially increased probability of weak layers in the snowpack: Are the vegetation types changing? Do we suddenly find plant and tree species that did not exist in another part of the mountain range at this altitude?

We can imagine this especially in the Caucasus - where the amount of precipitation decreases continuously from northwest to southeast. If you cross the Caucasus in this direction, the vegetation types also change: For example, the Nordmann fir is only found in the humid climate areas in the west of the mountain range. Whether such a good "indicator community" as Swiss stone pine-larch also exists there is questionable. The change in vegetation type is a tiny, rough additional tool in the sense of a holistic view of avalanche danger. Changes in vegetation type with potentially decreasing precipitation at the same altitudes during a ski touring trip can bring to mind an increased probability of weak layers in the old snow and thus allow an (even) more precise evaluation of the current snowpack structure in different areas to be considered.

Full paper as PDF in English here