SnowFlurry 6 2017/18 | The compact snow cover

Yay for that! Finally a winter with a thick blanket of snow and, above all, a large area of compact snow cover in the Alps. The last time we had this honor was in the winter of 2011/12 and 2005/06. However, there were often enough stable conditions in the winters in between. Because "stable" does not mean "compact", but conversely "compact" usually means "stable". Below, we explain why.

Definition of compact & stable

When we talk about a "compact snow cover", we always mean a snow cover that is relatively hard throughout (except for the layers on the surface). In this case, "hard" always means harder than hardness grade 2-3, which means that no layer can be penetrated with four fingers of one hand pressing simultaneously. Only when you have to use one finger or even a pencil or knife to get through all the individual layers of snow can you speak of a "compact" snow cover.

This does not mean that there cannot be any old weak layers in a compact snow cover. These can be present, but are sintered again by pressure from the upper snow masses and decomposing transformation to such an extent that they become harder and no more fractures can propagate in them. It is indeed possible to generate partial fractures during stability tests in a compact snowpack, but only rarely and only under high loads. A compact snowpack always consists mainly of round-grained crystals, possibly in combination with fusion crusts or angular-rounded shapes. Other, softer crystal forms can only be found directly on the surface.

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A compact snowpack usually also has a positive effect on the avalanche risk - apart from short-term problems near the surface. — A compact snow cover usually also has a positive effect on the avalanche risk - apart from short-term problems near the surface.

LWD Bayern

Weak layers, in which fractures can propagate and thus form slab avalanches, are always equally soft or softer than hardness grade 2-3 when dry. A snowpack that is described as "compact" is therefore always relatively hard throughout (except for the surface) and has no significant weak layers.

There are also largely stable conditions in a non-compact snowpack: when the weak layers are no longer relevant for the skier for a variety of reasons. Stable conditions in a snowpack rich in weak layers are admittedly relatively rare, but usually occur in spring, at least for a few days - although the snowpack could never be described as "compact" throughout the winter:

First due to the weak layers being built up and transformed
Then due to the initial soaking of these layers and the associated loss of strength.
Subsequently due to the soft and low-binding melt forms later in the spring.

So always soft stuff with the possibility of fracture propagation, but in some cases so deep in the snowpack,

that not relevant,
or fracture propagation tendency so low that hardly relevant,
or overlaid by such hard layers that hardly relevant because not triggerable.

How does a compact snowpack develop?

As described in the SchneeGuide.com 5, heavy snowfalls such as those of the current season form homogeneous layers within the snowpack and favor the further development of the snowpack with regard to the degradation of weak layers or the weakened tendency to form new weak layers. A compact snow cover only forms with regular and, above all, intensive snowfall in early winter without long periods of good weather.

Why a lot of snow is positive - a comparison

Fractures in a rock (also a solid material like snow or ice) can be created most easily in porous, less solidified areas. It therefore depends on the nature of the material, not its thickness. The same applies to snow: whether it is a 50 cm thick layer of snow or a 5 m thick layer - if it is compact, i.e. hard throughout and not porous, you can trample on it as much as you like, nothing will break. If there are porous areas somewhere, you can cause it to break. Due to the temperature gradient, porous areas tend to form in a thin layer of snow, not in a thick one...

On the tricky search for relevant weak layers in winter 2017/18 — In the tricky search for relevant weak links in winter 2017/18

Lukas Ruetz

Example 2011/12

In the winter of 2011/12, the snow cover was also compact and the warming in spring was so subtle - i.e. slow and usually associated with low humidity - that the well-known, annually recurring base avalanches in the home of the snow rummager never occurred throughout the winter. This has never been the case since the memory of the village elders. There were practically no old or weak layers close to the ground that could be weakened by moisture penetration in spring.

This brings us to another point: intense periods of precipitation at the beginning of winter. In the fall of 2011, there was no precipitation and warm temperatures throughout November. It was not until the beginning of December that the first significant snow of the season fell. This was followed by regular and sometimes intense snowfall. By mid-January, a relatively thick and de facto old snow problem-free snow cover was able to build up - as there were only a few radiation days with the still thin snow cover, i.e. days with fine weather and massive, superficial cooling.

The striking cold phase at the beginning of February 2012 (with the last days below -20°C in the populated areas of Tyrol) was then only able to form significant weak layers in the snow cover superficially and briefly. The snowpack was now too thick to build up in deeper areas. In terms of snowpack build-up, late snowfall followed by sustained heavy snowfall is therefore desirable. This is because as soon as conditions for weak layer formation occur, only layers close to the surface usually transform and cause problems in the short term. This is directly related to the thickness of the snowpack: Lots of snow - less pronounced temperature gradient - weaker accumulating transformation.

With this in mind, here's to 2017/18!

Note: Stable conditions occur every winter, at the latest in spring - but a consistently compact snowpack only occurs in a fraction of all seasons.