SnowFlurry 1 2021/22 | The old snow problem again

Current old snow problem on the main Alpine ridge

The winter is still young, but the SLF and the Avalanche Warning Service Tyrol are already reporting weak old snow on shady slopes above 2800m. On the Furgghorn in Valais, there was already an avalanche on the first of November with a 500m wide avalanche edge. There were also several avalanches triggered by winter sports enthusiasts, mainly in the glacier ski areas.

Definition of old snow (problem)

Old snow is by definition only snow that is older than three days. Fresh snow is only referred to as new snow if it is less than three days old. It is therefore not surprising that people are already talking about an old snow problem.

The exact definition of an old snow problem is actually only secondarily dependent on the age of the snow - otherwise all avalanches caused by snow that is older than three days would automatically be avalanches due to an old snow problem. Rather, an old snow problem is a special form of weak layer. This consists of three different types of snow crystals. Two of them are formed by the accumulating transformation within the snowpack: angular crystals and deep frost, better known as floating snow.

The third type of crystals in the old snow problem is the glistening surface frost, which is only later overlaid by snow and thus finds its way into the snowpack and can act as a weak layer underneath a snowboard. This is not formed by the build-up transformation from another snow crystal in the snowpack, but - as the name suggests - on the snow surface. This is caused by water vapor that freezes to the surface of the snow, just as dew drops can form from the moisture in the air on the grass on a summer night. This process is called resublimation or deposition.

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Great snow conditions on the main Alpine ridge and south of it. Staller Sattel. — Great snow conditions on the main Alpine ridge and south of it. Staller Sattel
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Lukas Ruetz

Required formation period of an old snow problem

Surface rime, angular crystals and deep rime usually take several days to several weeks to form and cannot "fall from the sky" - i.e. reach the ground as precipitation or in the form of fresh snow. They only form on or in an existing snow cover. This is why the old snow problem is called the old snow problem. Although, in rare cases, prominent weak layers of angular crystals can even develop in just one or two days.

It's quite confusing overall, as "old" linguistically tends to refer to something that feels significantly older than three days. In English, the old snow problem is referred to as "persistent weak layers". This is a better description of the characteristics, as weak layers of old snow can usually be relevant for the avalanche risk for many days to weeks or even almost the entire winter and are therefore considered to be very persistent. However, there are also exceptions: Every now and then, an old snow problem is only relevant for a few days and then quickly disappears again. This is the case when the crystals are not too large and the layers are rather thin - i.e. only a few centimetres thick.

As a rule of thumb: Thick layers of old snow develop slowly over several weeks and are then usually relevant for much longer. They usually occur close to the ground, i.e. in the lowest part of the snowpack.

Low-lying (= thin) layers of old snow often form within a few days and are often only relevant for a relatively short time. A maximum of two or three weeks. They occur everywhere in the snowpack. They usually form in the top ten to twenty centimetres of the snowpack - most frequently between the surface of the existing snowpack and falling fresh snow.

Why always shaded slopes?

Slopes with little or no sunlight, i.e. in early winter not only northern slopes but also eastern and western slopes, are predestined for an old snow problem close to the ground. The near-surface old snow problem, on the other hand, occurs more frequently on sunny slopes.

In shaded slopes, the temperature gradients in a thin snowpack in the fall and first winter months with the low sun position 24 hours a day are pronounced enough to transform the snowpack in a build-up manner. The situation is quite different on more sunny slopes, at least during the day. Even if temperature gradients are just as strong there at night, the energy of the sun and the associated warming destroys the products of the nocturnal, constructive transformation again or causes the snow cover to melt away completely anyway.

Further north, at the same exposure and altitude, it still looks lean. Kühtai. — Further north in the same exposure and altitude, things are still looking lean. Kühtai
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Lukas Ruetz

A current example using two snow profiles

The difference in the snowpack structure and in the currently active transformation processes within the snowpack can be seen clearly between the north-facing slope and the south-facing slope in the two snow profiles from the Rieserferner group in East Tyrol from 09.11.2021:

Both were recorded at the same time at the same location. Only in a different exposure. While melt forms and round-grained crystals predominate on the south-facing slope, the products of the build-up transformation, i.e. the angular crystals, can already be clearly seen on the shaded side. And that just a few days after the snowfall.

Something was also happening in both snowpacks at the time the photo was taken. On the south-facing slope, the melting transformation is currently taking place in the upper part of the snowpack. Clearly recognizable by the moisture levels 3 and 2 of the first two layers and the measured snow temperature of 0°C. Due to the low temperature gradient, the degrading transformation is currently taking place deeper in the snowpack.

On the north-facing slope a few meters away, things look completely different. The snowpack already consists largely of angular crystals. Especially on the snow surface, where the temperature gradient is usually most pronounced in cloudless skies, the crystals are particularly loose and already a millimeter in size. The current gradient is also much stronger than on the south-facing slope, allowing the accumulating transformation to continue.

The two profiles were recorded at 2400m. The snow comes from the snowfalls at the beginning of November. High in the Alps, where snow from the beginning of October has already been left behind, the processes described above have been able to run riot for much longer on both shady and sunny slopes, forming problematic weak layers.

Conclusion

There will probably never be a winter in the Alps without an old snow problem. Every winter is characterized by several old snow problems in most regions. This is simply due to the conditions under which it forms, which are part of the frequently occurring standard weather pattern. Fortunately, however, there are only a few winters with a very pronounced and persistent old snow problem. Unfortunately, however, it is precisely these situations that are remembered by many winter sports enthusiasts as representative of the old snow problem.

Each avalanche situation should be considered on its own merits and each avalanche problem has a range from "not really relevant for winter sports enthusiasts" to "extremely accident-prone and dangerous".

During the season, the SnowFlurry appears as usual on Saturdays, with regular operation starting at the beginning of December. In the current case, this is a special rummage for the start of the season!