Avalanche problem old snow
Little snow and still high avalanche danger...
Learn more about avalanches with us!
How does an old snow problem arise?
An old snow problem arises when there is little snow and there is a build-up transformation due to a strong temperature gradient in the snow cover. The temperature gradient is there because the ground is at 0°C and the surface temperature drops sharply on clear nights. This causes vapour diffusion and recrystallisation, a build-up transformation. The built-up snow is now a weak layer and with the new snowfall there is a snow slab above the weak layer.
Weather conditions with little precipitation
The trend of global warming is causing the jet stream (strong, channelled high-altitude winds) to move further and further south. The jet stream meanders in waves from west to east. Surrounding the jet stream are areas of high and low pressure, which are often arranged in an omega pattern. An omega high is a particularly stable weather pattern that lasts for weeks and has been observed more and more frequently in recent years. Cold temperatures and fine weather are the result. This is exactly what we experienced at the end of January and beginning of February. This is just one explanation; the following factors also play a role:
- Warmer temperatures mean it rains instead of snowing (around Christmas it rained at an altitude of around 2200 metres).
- At the beginning of winter, the weather came from the south and the snowfall in Lower Valais and Chamonix was quite considerable.
- The angle of incidence of the fronts determines where it snows the most. The last heavy precipitation fell mainly in Austria, and the Prättigau got a lot of it.
Further material on the Omega High:
Why is little snow also dangerous for avalanches?
Little snow is more dangerous for avalanches than a lot of snow. This is not intuitive, but can be explained as follows: 5 factors are needed for an avalanche:
- Snowboard
- Over a weak layer
- Over a sufficiently large area
- Slope inclination over 30° (wet snow from 25°)
- Additional load (often ourselves when skiing)
The crux of the matter with little snow is the combination of weak layer and snow slab. If only a little snow falls at a time, very different layers form and an unfavourable combination occurs. If a lot of snow falls, it is dangerous for a short time (up to 3 days), but then a thick, homogeneous and therefore stable layer forms. Little snow often leads to old snow problems, which are preserved by the cold and last for weeks. As we experienced at the beginning of February. The following factors lead to old snow problems:
- Little snow and fine weather conditions (omega high): Cold temperatures lead to the transformation of the snow cover and the formation of frost (formation of a thin layer).
- Little fresh snow and wind lead to the formation of a snow slab.
The warm temperatures of the last few days have cleared the whole thing up again. The snow cover has solidified and the avalanche situation is currently very favourable. You will learn this and much more in our Avalanche courses.
Further material:
What is the best case scenario?
Heat and strong winds work the snow surface hard. It snows a lot, the wind decreases and the temperature drops from warm to cold during the snowfall. This causes the new snow to bond with the layer below. Due to the decreasing temperature, the layering in the new snow is also good: loose on bound, so no snow slab on loose snow, but exactly the opposite. For us skiers, this is also great for freeriding! You can experience and learn this with us: Freeriding, Ski tours and Avalanche courses.
