Avalanches and microstructure

At the most basic level, the ingredients for a snow avalanche are, quite simply, snow on a slope. However, there is clearly more involved, since these are also the ingredients essential for downhill skiing. Two events that are best kept separate! Snowflakes accumulate on the ground to form a granular ice structure, which is in constant transformation. Environmental conditions may dictate that the grains develop into generally rounded shape with ample bonding; or under different circumstances new faceted crystals may develop that are indicative of snow with a low structural integrity. Snow on the ground may strengthen or weaken; in fact both processes may occur simultaneously within the snowpack. It is the microstructural arrangement that ultimately determines instability and the scale of an avalanche. The sensitivity of the snow to environmental conditions causes a seasonal snowpack to develop a layered stratigraphy, where both the intergranular and interlayer strength is essential to determining the avalanche potential. It is also important to consider that the transfer of a trigger force, such as a skier or an explosive, to a weak layer is influenced by the mechanical properties of the overlaying snow. Weak layers that develop while at the snow surface may become problematic when subsequently buried, providing a fragile layer that cannot support the overburden of subsequent snowfalls or triggers. Alternately, layers near the ground may weaken over time, resulting in full depth avalanches. An understanding of the energy interaction between the snow and the environment is necessary to determine the likely metamorphism. Despite the dramatic, sometimes large scale destructive nature of avalanches, it is an assessment of the microstructure of the snowpack which is a key component to forecasting.