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SnowFlurry 7 2016/17 | Repetition and clarification

The weather bores more than just the WeatherBlog

by Lukas Ruetz 12/15/2016
The Alpine region is still under the influence of an Omega high. The snow cover continues to build up at higher altitudes and on the shaded side, while there is something similar to firn on steep, lower southern slopes. Surface frost can also be found, but only rarely due to the low humidity in relation to the amount of radiant nights.

The powder apocalypse continues to fail to materialize and the snow pusher is amused by various internet reports flirting with a change in the weather in the crystal ball range. Time is better spent sitting back and relaxing, waiting for the onset of winter and in the meantime looking at the current situation and other interesting things. And to clarify a few things from the first SnowFlurry articles:

To repeat:

The SnowFlurry knows that this has already been mentioned in the last SnowFlurry. However, thanks to his studies with a pedagogical-didactic background, he also knows that building knowledge and developing an understanding of such processes does not happen overnight. Abstract learning in itself is a slow process that can be supported by repetition and the constant reappearance of various descriptions.

One process is still currently shaping the snow transformation: the constructive transformation. Where two weeks ago you would have encountered a load-bearing snow cover due to wind and melting crusts, the ground is breaking through more and more easily or feels more and more "powdery". The build-up transformation first forms crystals with visible edges and facets from each original crystal form, later even entire cups (like a hollowed-out pyramid) up to 1 cm in diameter. These crystals have little bond to each other and trickle through the hands like sugar. The build-up transformation starts at a temperature gradient of around 15°C/m. In simple terms: the temperature difference between the surface and the ground of a 1m thick layer of snow must be about 15°C. With a snow cover only half a meter thick, half the temperature difference between the layer close to the ground and the surface layer is enough.

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The degradative transformation can form tiny, tiny (less than 0.5 mm) roundish grains from any grain shape. Although it always takes place in the snowpack - even while the constructive transformation is in progress - it loses its supremacy over the constructive transformation from a temperature gradient of about 15°C per meter.

The names are derived from the crystals formed: "constructive" because the crystals become larger in the process, "degradative" because they become smaller. Then there is the melting transformation, which takes place due to water or heat input - the snow simply melts. The layers from the melting transformation and the constructive transformation become more glassy, the products from the degradative transformation remain pure white like fresh snow. The formation of new snow itself can also be categorized, at least conceptually, in a similar way to the constructive transformation. Although the formation of snow crystals in the atmosphere takes place in a different way than within the snow cover, it also involves an enlargement of the crystals. All crystal forms in the atmospheric formation and constructive transformation category are (when snowed in) "more fragile" than products of degradative transformation and melting transformation.

And here we have the salad that explains why winters with little snow tend to be more dangerous than winters with lots of snow and why such weather conditions in early winter or with sparse snow cover unfortunately make the snow looser: The cloudless nights cause the snow cover to cool massively on the surface, and the temperature difference towards the ground becomes greater. The surface temperature is usually between 5°C and 15°C below the air temperature. The less snow there is, the smaller the space in which the snow exhibits the temperature difference.

Why "snowy" is not inherently dangerous

To understand this, you need a basic understanding of the term "snow cover": Snow is porous, relatively "hot" (as it is always relatively close to the melting point) and thus readily transformable. Moreover, it consists not only of ice, but also of ice and air - possibly also with a water content. Imagine the snow cover as polystyrene: An ice structure with air pockets, but with more or less stable connections between them. Whether the piece of polystyrene is one or two or five meters thick has nothing to do with its stability against fractures within it. It only depends on how good the connections from one part of the scaffolding to another are. Ice climbers know: The colder it is, the more brittle the ice is - that is, it is more fragile - just as plastics become more brittle in the cold. As we find different crystal forms in the snowpack that together make up the entire scaffolding, the bonds are not equally good everywhere and are sometimes stable and sometimes quite fragile.

On the one hand, the fragility depends on the shape of the crystals: Platelets and cups have few points of contact with each other - the framework only hangs together in a few places - whereas small, round shapes are much more tightly packed together and touch each other on a much larger surface. In contrast, fused lumps are cemented together with water ice and also hold together well despite their size.

On the other hand, the fragility depends on the current temperature of the crystals: The colder they are, the more brittle the framework is, so it breaks more easily. Slab avalanches are caused by fractures in this structure. If a particularly thick snowpack is relatively homogeneous and ideally consists only of decomposed round grains with the same degree of hardness, there is practically no possibility of a fracture.

The less pronounced temperature gradient in a thick snowpack, the probably less frequent or long periods of good weather with blasting, and the fact that the snow is less brittle, make it easier for avalanches to occur. The less pronounced temperature gradient in a thick snowpack, the probably less frequent or long periods of good weather with sunny nights during the build-up of a thick snowpack and the probability that any weak layers are so deep inside that they can no longer be disturbed, make snowy winters safer overall than winters with little snow.

Excursus: Bound snow has nothing to do with hard or soft
Here is another video from October where you can see very nicely that bound snow for a snow slab is only conditionally related to the snow hardness:

Note: Snow is porous, hot and transformable. The transformation of snow is always related to the temperature within the snowpack. Snow slabs are formed by a fracture and fracture propagation in the snow structure.

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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