3. Why do avalanches occur?

Avalanches only occur when the stress on the snow exceeds the shear, ductile, and tensile strength either within the snow pack or at the contact of the base of the snow pack with the ground or rock surface. A number of the forces acting on a snow pack can be readily determined; for example, the weight of the snow is straightforward to calculate, however it is very difficult to know the shear, ductile, and tensile strength within the snow pack or the ground. These strengths vary with the type of snow crystal and the bonding between them. The thermo-mechanical properties of the snow crystals in turn depend on the local conditions they have experienced such as temperature and humidity.

Figure 5 - The so called "rutsch block test", used to verify on site the degree of stability of the snow pack. Once a snow block has been created, a skier jumps on it: the more jumps needed to make the block slide, the more stable is the snow pack. Source: www.swissmountainleader.com
Figure 6 - The analysis of the snow pack stratigraphy is a fundamental step in detecting its possible instability. Weak layers within the snow pack can be detected, which may favour future avalanche occurrence. Photo: A. Ghinoi.