Not all snow is the same
Last week, our trainee Michael dug some snow samples as part of the Alpine course at the Tyrolean State Ski Instructors' Association. He also came up with our special topic for this week: "Not all snow is the same". When analysing snow samples and stability tests, we repeatedly noticed a thick layer of floating snow. That's why this week's topic is snow transformation processes. When snow falls fresh and powdery from the sky, its transformation begins immediately after landing. But what does this mean for freeriders and ski tourers? Why can these processes be so decisive for snow conditions and the risk of avalanches?
Decomposing conversion
During the degradative transformation, the originally sharp-edged snow crystals gradually disintegrate, round off and become increasingly densely packed. Two processes are primarily responsible for this - one mechanical and one thermal:
Mechanical conversion:
When the wind transports snow, the crystals break up, compacting the snow cover.
Thermal conversion:
If the temperature remains relatively constant over a longer period of time, the crystals change towards a rounder shape. This happens because a spherical structure is energetically more stable than an angular one. The result: the snow becomes firmer and more compact. This type of transformation often helps to create a stable base for the snow cover.
Constructive conversion
Accumulating snow transformation occurs when there are large temperature differences within the snow cover. This is particularly common with a thin snow cover when the ground is warmer than the snow surface. This temperature gradient also creates a water vapour pressure gradient - water vapour rises upwards in the snow cover and accumulates again on the crystals. This causes angular, loose crystals to grow, which are no longer connected to each other. If this process occurs within the snowpack, it is referred to as floating snow or "dead snow" - an extremely unstable layer. If it occurs on the surface of the snow, the result is "surface rime" - a thin but extremely fragile weak layer that can lead to problems with fresh snow or wind drifting. These loose, sugary layers are often treacherous as they form the perfect basis for avalanches.
Why is this important?
The most common type of avalanche that affects winter sports enthusiasts is the slab avalanche. Three main ingredients are needed to trigger it:
1. a weak layer - often a loose layer created by a build-up transformation.
2. a bound snow cover - i.e. an overlying layer compacted by decomposition.
3. an additional load - such as a skier or a snowfall that exceeds the critical limit.
When these factors come together, the snowpack on the weak layer can slide off. It is therefore highly recommended for anyone travelling in the backcountry to look at the snowpack, recognise possible weak layers and interpret their avalanche risk.
