Microstructural resistance of snow following first wetting

One of the main causes for the formation of wet snow avalanches is the decrease in strength following the introduction of liquid water. The influence of liquid water on initially dry snow was investigated in twelve field experiments in the Eastern Swiss Alps. For this, a variety of different snow types were artificially wetted and liquid water content (using the Snow Fork), microstructural penetration resistance (Snow Micro Penetrometer, SMP) and snow density were measured. Measurements were repeated up to eight times resulting in a total 1100 liquid water content, 355 penetration resistance, and 737 snow density measurements. Following wetting, microstructural penetration resistance decreased in layers consisting of faceted crystals or depth hoar at low liquid water content. Layers consisting of precipitation particles or small round grains, on the other hand, showed no significant trend at a similar water content. The results indicate that first wetting of faceted crystals and depth hoar layers has an impact on snow strength, and consequently contributes to a decrease in snow stability.

Research Highlights
► We measure micro-structural resistance in snow undergoing first wetting.
► Resistance decreases following first wetting in coarse facets and depth hoar.
► No changes were noted in layers consisting of precipitation particles and round grains.
► Resistance changes depend on grain shape and size, density and liquid water content.