Anyone seeking to understand how the properties of snow affect avalanche formation has to take an interest in the microstructure of snow as well. Even minor changes in the snow microstructure can exert a massive influence on the behaviour of the snowpack and avalanche formation. Since snow is usually very close to its melting point, snow grains are in a state of constant flux. This change in the microstructure is known as snow metamorphism. The primary influencing factor is the temperature conditions in the snowpack, which determinehow the snow changes as it undergoes metamorphism. The existence of a similar temperature throughout the snowpack results in a structure consisting of rounded compact grains. A much more frequent occurrence, however, is a vertical temperature difference in the snowpack. These conditions give rise to large faceted crystals, an extreme form of which is called depth hoar (Fig. 1). There is little bonding between these faceted grains of snow, so that the overall structure is weaker. Layers consisting of faceted crystals are often weak and ultimately responsible for avalanches. If the snowpack is suddenly exposed to a load, typically by a skier, the bonds that exist can break down.

While metamorphism is taking place, the snow is also settling. These simultaneous processes influence one another. Observations in nature have shown that different types of snow affect settling in different ways. Depth hoar, for example, settles more slowly than round snow grains. Mareike Wiese, a PhD candidate belonging to the snow physics research group, recently set herself the goal of examining why settling rates differ in this way.

Lab experiments illuminate structural changes in the snow

The researcher used a new device, the Snowbreeder 5 (Fig. 2) developed at the SLF, to expose snow samples to different temperature conditions in the lab, while a weight placed on the samples brought about settling at the same time. A laser measured the extent to which the snow settled. Mareike also scanned the samples at regular intervals using an X-ray CT scanner. The 3D images produced in this way depicted the snow structure as it was changing.

Compared to the effect of a consistent temperature, the results show that vertical temperature differences within the snow sample reduce the rate of settling by 50%. The images also illustrated the formation of vertical structures in the snow as metamorphism took place (Fig. 3). The structures are comparable to the columns or pillars that support buildings. The effect they have in snow is to reduce settling and compaction. Although the vertical structures are capable of supporting the weight of the snow, the bonds can easily break when exposed to lateral shear forces. If the temperature is uniform throughout the snowpack, in contrast, the absence of distinct vertical layers allows the snow to settle quicker. Given the greater prevalence of bonds in dense snow, its structure is stronger.