SnowFlurry 1 2025/26 | "Under the magnifying glass"

The first real snowfall of the season brought winter fun, especially in the Western Alps. But the rest of the Alps were also covered in a white blanket. Around 35-40 cm of fresh snow fell in Davos. Sounds great at first, but the conditions were still not really good. Sharks are lurking everywhere, the base is missing, and with the current snowfall-free period, the snow that has already fallen could become more and more of a problem over the winter. We try to explain the mechanisms behind this here.

The snowfall in mid-November started at -5 °C and cooled down to -12 °C during the two-day precipitation phase. There was a moderate wind, but it was sufficient to transport snow. This snow fell almost everywhere on a melting crust resulting from the snowfall at the end of October and the mild temperatures that followed.

In the last three weeks since the snowfall, we have dug a total of seven profiles at different locations and exposures around Davos and carried out stability tests. We explain two of them in more detail here. If you haven't had much contact with snow profiles yet, or if it's been a while since the last time, you can find a detailed explanation on PowderGuide or on the SLF website.

The formation of angular crystals at the beginning of the winter season is a very common phenomenon. The greater the temperature differences within the snow cover, the more moisture in the air there is.
As a result, the crystals continue to grow (anabolic transformation → angular crystals) instead of settling - as is the case with small temperature differences - and bonding increasingly better with the surrounding crystals (sintering → small, round crystals).

Info: When snow is sintered, snow crystals bond together. The crystals become rounder, lose their sharp edges and adhere better to each other. This makes the snow denser, firmer and harder. In contrast to the build-up transformation, sintering has a stabilizing effect on the snow layer.

This makes it clear that there is a risk of weak, angular layers growing, especially at the beginning of the season, because while the snow cover is still thin, there are often large temperature differences between the ground and the outside temperature over very small distances.

A small example: at the end of November, the snow depth in some places was only around 50 cm, while the ambient temperature was -15 °C. This results in an overall temperature gradient of around 30 °C/m, i.e. the temperature difference per meter of snow cover. Over just 10 cm, the temperature changes by an average of around 3 °C.

The rule of thumb says that snow transforms into angular crystals from a temperature gradient of around 10 °C/m and in this way, weaker and weaker layers form in the snow cover over time. At 30 °C/m in our example, the value is significantly higher and the snow is transformed accordingly quickly. However, you should bear in mind that -15 °C is more of an extreme value that occurs on clear, cold nights.

Particularly when, after the first snowfalls of the season, there is a period of high pressure without precipitation and cold temperatures, this creates the perfect conditions for large areas of weak layers to build up in the snow cover. Without the "right" base, however, this weak snow does not yet cause any problems. In the upper layers, edged snowpacks feel almost like powder when skiing: fluffy, soft layers in which you can really have fun (at least if, unlike this year in Davos, there is at least enough snow underneath to cover the sharks, see snow depth map). However, the dangers of the weak layer tend to lurk in the course of the winter. If better bound layers, such as drift snow accumulations, form above the early-formed, angular crystals over the next few snowfalls, all the conditions for a slab avalanche are present: a bound, i.e. well-connected slab on a weak layer spread over a large area, which has less stability and can easily collapse under additional load from precipitation or from a snow sportsman.

These deeply buried weak layers are identified as an old snow problem in the avalanche bulletin. They are particularly difficult for snow sports enthusiasts to assess, as they can neither be deduced from the weather forecast of the last few days nor recognized on the snow surface. Due to the heterogeneity of the snowpack, it is impossible to predict or delineate exactly where and how extensively such weak layers will occur. This makes them one of the most dangerous avalanche problems. Especially in winters like this one, in which the first snowfall at the beginning of the season is followed by a longer window of low precipitation and thin snow cover is present for long periods of time (see graph of relative snow depths), you should therefore be careful and check the avalanche bulletin carefully for an old snow problem. Digging a snow profile can also help to assess locally how weak the layers close to the ground really are.

Now let's take a closer look at the profiles:

In the first profile (25.11.), the freshly fallen new snow can be clearly distinguished from the slightly older felt (definition: felt). After the snowfall period, the clear nights and low temperatures of up to -14 °C led to strong temperature gradients in the still thin snow cover.
In combination with almost windless conditions (average wind speed 1-10 km/h, peaks 10-20 km/h), surface frost formed, which could be seen in small form in the profile from December 2nd and in more pronounced form in the profiles on December 4th. The crystals grew up to 1-2 cm. In addition to cold, windless conditions, a correspondingly high moisture content in the air was necessary for this development.

Since the profile on December 4th, the temperatures rose and it rained up to the 0°C limit at about 3000 m. The fresh snow that had previously fallen was rained in and transformed into a hard snow cover by the clear nights. In the long term, this could have had a positive effect on snowpack stability, as the angular snowpack stabilizes at warmer temperatures through sintering or even melting processes. However, melt crust and broken snow are not much fun either, of course. There was also the hope that the snow-covered surface frost (snowed in at the weekend of 6/7 December) could be soaked and then frozen into much more stable melt forms.

However, the profile from Tuesday, 09.12., only partially confirmed this hope. The layer of fresh snow above the surface frost remained as a felty, slightly angular layer under a 2 cm thick crust of hard snow. The surface frost itself is also still clearly visible in the profile (translucent profile). In the last two profiles on the Strela Pass at 2440 m, despite positive air temperatures of around +6 °C (i.e. high 0 °C limit), moist snow was observed in the layers close to the ground up to the second melt crust.

Approximately 20 cm below the snow surface, there is a pronounced, snow-covered surface hoar frost with crystals 1-2.5 cm in size. However, the overlying layers do not form a firmly bound "board", but are still loose. This means that a layer is missing that would spread the collapse of the weak layer over a large area. We suspect that this is precisely the reason why no fracture propagation was observed in the profile during the stability tests carried out: No propagation (fracture propagation) occurred either in the slip block stage (RB6, irregular) or in the ECT results (ECTN16, ECTN18 in the surface maturity). Due to the recent warm temperatures, the frost may have become somewhat more "robust", as it only broke under relatively high stress. However, the crystals are still clearly visible and largely unchanged.

Info: Propagation refers to the lateral spread of a fracture within a weak layer after it has been triggered at one point. It shows whether a local fracture is capable of propagating over a larger area and thus fulfills the prerequisite for the triggering of a snow slab. If there is no propagation (e.g. ECTN, irregular fracture in the slide block), the fracture is localized and does not spread independently in the weak layer.