Let's take a look at the difference between a snow profile at the avalanche crack with an autumn old snow base and a snow profile without an autumn old snow base.
General situation
While the probability of triggering is only slowly improving in the areas affected by the old snow problem, very good conditions can now be found where there was no continuous snow cover before the beginning of December. The snow surface in shady and flat, sunny terrain remains powdery due to the superficial, accumulating transformation. This is directly related to the fine weather and clear skies. As a result, the snow surface can cool far below the air temperature and the superficial fresh snow turns into small, angular crystals. On steeper, sunny slopes, the high temperatures mean that a brittle crust can usually be found. In combination with the temperature, this also occurs with the actually only weak sun - we are almost at the lowest sun position of the year.
Snow profile 1, 17.12.2020 - Pirchkogel/Hinterer Grieskogel avalanche
The snow profile was recorded two days after the avalanche was triggered in the middle of the outcrop. We are here in a 40° steep area exposed to the northeast and close to the ridge. The avalanche was probably triggered very close to this location. The snow cover here is about 70 cm thick. The wind has probably removed most of the fresh snow here, as the heavy snowfall from December 4 - 7 brought about 1m of fresh snow here.
The snow from the snowfall since December 4 is between 70cm and approx. 35cm high. Underneath we find the old snow from October and November and perhaps the September snow has also remained here. Deep rime crystals had formed on the former old snow surface, which can now be found in the snow profile between a height of around 31cm and 28cm. Underneath is a melting crust, which has also been massively eroded by the accumulating transformation. This is why the spectacle symbol of the melting crust also contains a deep frost symbol.
In the extended column test, a fracture in the weak layer of floating snow was produced across the entire block on the 10th blow. This fits very well with the avalanche triggering.
The weak temperature gradient in the lower half of the snowpack shows that the degrading transformation is currently taking place there. In the upper half, on the other hand, the temperature gradient becomes stronger and the curve becomes increasingly flatter, especially in the uppermost area. This is where the constructive transformation is currently taking place. This can also be seen in the small, angular crystals that have already formed in the fresh snow. After a brief phase of degradative transformation after the end of the snowfall, the constructive transformation has now taken over due to the clear sky and therefore low surface temperature.
