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SnowFlurry

SnowFlurry 12 2019/20 | An old snow problem complicates tour planning

The much and little tracked slope.

by Stefanie Höpperger 02/08/2020
Tour planning proved to be rather difficult last week, as an old snow problem in the terrain is not easily recognizable. This makes it all the more important to use your head when planning at home and choose a suitable tour carefully.

The old snow problem is the most difficult of the five avalanche problems to deal with. Appearances are deceptive: you often can't see any danger spots in the terrain, which gives us a feeling of safety. The possibly existing weak layer is quickly forgotten in a seductive powder dream.

That's why you should explicitly avoid the exposures and altitude ranges specified as critical in the avalanche report on such days and avoid the main corridors of the much-traveled fashion tours. Even if the untracked slope just next to it looks fantastic!

As Lukas already mentioned in the previous Gestöber, the pimple powder and in some cases also the surface rime became a weak layer due to the overlay of new or drift snow. This was acute on W-N-E slopes, especially in areas with little or no traffic, before the snowfall. Why are corridors with a lot of traffic safer than those with little traffic - nap powder was also present there?

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We imagine an untracked slope with a great, loose snow surface made of nap powder - the surface is even and untouched, there are no tracks. In comparison, there is a slope on a fashion tour that has been heavily tracked: the snow surface is very irregular and the crystals have been partially destroyed by the ski tracks. These two slopes are now snowed in and covered with drift snow or bound fresh snow. These are exactly the right ingredients for an avalanche: a weak layer and a plank on top (bound snow).

In addition, the fracture propagation in the weak layer is also interesting. This is greater with a uniform, widely distributed weak layer. This brings us back to the two slopes: Tracked or untracked. On the tracked slope, the weak layer is no longer uniform or continuous due to the many tracks and the associated depressions, waves, hills, etc. and a fracture cannot spread or cannot spread far. On a slope with little to no tracks, the weak layer is continuous and uniform, the crystals have not been destroyed and are often more pronounced. The weak layer is therefore easier to disrupt and the fracture can spread over a large area.

Snow profile Sattelschröfen from 1.2.2020 2600m | NE | 22°

The profile in yellow clearly shows the snow-covered nap powder - the weak layer of angular crystals.

Above it in green is the fresh snow from 28.1.2020 - the board of bound snow.

Two partial fractures were created in this weak layer

Note: When a loose snow surface is snowed in, the degree of tracing before the snowfall is interesting.

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