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SnowFlurry 7 2021/22 | When will old snow become an old snow problem?

Explanation of terms and definitions

by Stefanie Höpperger • 02/05/2022
There are repeated reports of an old snow problem, but exactly what this means can be confusing. First of all, you should know that an avalanche problem (old, new, drifting, wet and sliding snow problem) is not a hazard pattern, but can consist of different hazard patterns. An avalanche problem describes the type of avalanche and its mechanisms, whereas a hazard pattern describes the responsible weak layers and their processes.

You should also be aware that the term "old snow" does not initially describe a problem, but simply snow that is a few days old and has not been affected by precipitation, wind or melting processes. The grain shape in old snow can range from small, round grains to cup crystals and says nothing about a hazard.

It is not uncommon for an old snow problem to be just a weak layer of floating snow (cup crystals) on the ground. However, the term "old snow problem" is much more comprehensive. It not only includes several hazard patterns and weak layers, but also describes the age of the snow and the avalanche mechanism. The handling of an old snow problem is also notoriously difficult. Fracture propagation plays a major role here, because if a so-called "hotspot" is caught and a fracture is initiated, it can spread over long distances. The reason for this is the weak layers present over large areas. Avalanches can also be triggered in flat terrain.

With an old snow problem, weak layers develop within the snowpack or on the snow surface. They can continue to form throughout the winter. These are therefore not weak layers that are created, for example, by a changing wind force in the new snow layers, or weak layers of sleet, but weak layers that are created by snow processes (metamorphosis).

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Old snow problem and GM 1 (weak layer close to the ground)

Early snowfall in the fall and early winter is often the first trigger for an old snow problem. This is usually followed by a high-pressure weather situation with sunshine and clear nights. While steep sunny slopes snow out again during mild periods of fine weather, the snow remains on shady slopes and at higher altitudes and begins to transform. Especially on wind-protected slopes, where there is no direct sunlight, the snow cover can radiate well and cool down. The build-up transformation begins. Depending on the depth of the snow, angular crystals form on the snow surface or in the entire snow cover, up to cup crystals (floating snow) at an advanced stage. If it then snows on top of this, the first old snow problem is already present due to hazard pattern 1 (weak layer close to the ground). It can accompany us throughout the winter, especially if the snow cover is not very thick. If this is the case, hazard pattern 7 (little snow next to lots of snow) is also present. In places with little snow, a break can be initiated by individual winter sports enthusiasts, as the weak layer is at a depth that can still be disturbed (approx. up to one meter). In areas with lots of snow, on the other hand, the penetration depth is not sufficient to disturb the weak layer.

Winters with lots of snow are therefore an advantage! This is because the deeper the weak layer is in the snowpack, the less likely it is to initiate a fracture. In such a case, weak layers close to the ground can pose little to no problem throughout the winter. However, caution is required again in spring, because when the first complete soaking of the snowpack takes place, the remaining weak layers can be reactivated by the water ingress and sometimes lead to very large avalanches.

It would be best if an old snow problem close to the ground consisting of floating snow or angular crystals stabilizes again over time. This happens primarily at lower altitudes through rainfall, warmth and/or regular precipitation with minor temperature fluctuations - all processes that set the degradation process in motion. The old snow is then no longer a problem.

Old snow problem and GM 4 & 5

In all other winter months, an old snow problem can form again and again. Constructive transformation does not always just mean deep rime on the ground:

In good weather phases with cold temperatures and clear nights, angular crystals form on the snow surface. If they are subsequently overlaid by new or drifting snow, this is hazard pattern 5 (snow after a long cold period). Even in high winter, when the snowpack is not very thick, the entire snowpack can build up, or weak layers can form on the ground (ground flow), which in turn leads to a GM1 (weak layer near the ground).

Danger pattern 4 (cold to warm, warm to cold) can also occur with an old snow problem. The only difference is that angular crystals do not form on the snow surface but in the snowpack, primarily above or below crusts (crust sandwich).

Old snow problem and GM 8 (surface frost)

The process of deposition, which creates surface frost (hazard pattern 8), can also play a role. If it is snowed in or covered over, it represents an old snow problem. Surface frost is usually formed over several days, is not affected by wind and, if no melting process takes place due to solar radiation and heat input, is referred to as old snow. If it is still on the surface, it does not pose a problem.

Old snow problem and GM 7

The hazard pattern 7 (little snow next to lots of snow) usually occurs in combination with the hazard patterns mentioned above.

Not recognizable in the terrain

The avalanche mechanism should also be mentioned. An old snow problem is always a slab avalanche. Three ingredients are necessary for such an avalanche: A pronounced weak layer, a board of bound snow lying on top and sufficient steepness for an avalanche to occur. A rupture is mainly initiated by an additional load (e.g. winter sports enthusiasts), but spontaneous avalanches still occur time and again.

As you can see, an old snow problem can occur in different situations and through different processes, as well as involving several coupled hazard patterns. It can occur again and again throughout the winter anywhere in the snowpack, lead to fracture propagation over long distances and is often responsible for large avalanches.

The handling is relatively difficult, because the danger cannot be visually recognized in the terrain! It may be possible to perceive and observe settling noises, cracks or fresh avalanches. However, the weak layer and where it is located in the snowpack can only be seen in a snow profile. If you have seen and noticed weak layers on the snow surface before they were covered, still have them on your radar in the best powder conditions and also practice the right risk management, this is clearly an advantage. However, many people can do little with an imperceptible danger and are tempted by the seemingly safe powder descent.

Tips for handling an old snow problem:

Read the text of the situation report/bulletin (no matter what the danger level is!), where an existing old snow problem is reported and the problem is also narrowed down quite precisely in a certain altitude band and exposure sector. Avoid the specified areas as much as possible! Play with exposure and slope steepness when planning your tour to avoid danger spots. Also watch out for terrain traps and transitions from little to lots of snow. In regions without a reliable situation report, handling an old snow problem requires a lot of experience and generally a very defensive approach.

To read more about snow processes and behavior in the event of old snow problems, we recommend the somewhat older linked below, as well as this article about the human factor in connection with the old snow problem.


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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