Welcome to Avalanches 101. What is an avalanche? Avalanches come in all shapes, sizes and forms and similar to the snow crystals they are composed of, no two slides are exactly alike. They may contain dry snow, wet snow, ice, or sometimes a combination of these elements. They may contain loose snow, travel a short distance, and have little mass, or they may contain consolidated heavy snow, travel down an entire mountain, and have a considerable mass.
Regardless of their size or shape, avalanches have one thing in common: when the stress applied on the snow exceeds its strength, a failure is initiated and an avalanche is formed. A pretty simple concept if you think about it, but with possible devastating consequences. Under a given applied stress, whether or not snow failsdepends on a number of physical properties that include, among others, density, hardness, temperature, rate of deformation and quality of bonding to adjacent layers.
In the process of making an assessment of the likelihood of avalanche formation it is always very useful to consider how the following can influence the snowpack. Layering. A snowpack is made up of layers, created snowfall after snowfall, that might differ in strength, structure and their adhesion to each other. Changes in the snow crystals. Continuous changes occur in their density, in the way in which they bond to each other, in their tensile strength and visible appearance. Variation of terrain. The depth of the snowpack and layering varies with elevation, aspect and slope shape.
Generally snow is strong when the density or specific gravity is high, the grains are rounded and small and the snow temperature is low. Inversely, snow is usually weak when the density is low, the grains are large, the temperature is at or close to freezing and the snow is wet.
As far as changes in the snowpack are concerned, I must mention the various effects the wind has on it. It removes snow from windward slopes and deposits it on the lee slopes. As a result the snowpack will be shallower in some areas and deeper in others. Wind also breaks up snow crystals. One of the consequences is that wind transported snow is made up of smaller grains which once tightly packed together form very dense layers that rapidly sinter and form solid and rigid layers.
These are the reasons why having a mental image of the snowpack and of how it has been transformed throughout the season as a result of the above mentioned changes, is an extremely valuable piece of information. It will have a key role in forecasting the avalanche hazard for an entire area and for localized slopes.
Avalanches release from slopes in two different ways. A loose snow avalanche starts when a small amount of loose snow slips and begins to slide down setting additional snow in motion. A slab avalanche occurs when a plate or slab of cohesive snow begins to slide as a unit before breaking up. Following a slab avalanche, a distinctive fracture line, also known as crown fracture, is visible at the top of the slide. Due to the bigger amount of snow involved comparatively to a loose snow avalanche, this type represents a greater hazard for sliders (this is how I call skiers and boarders) and climbers. It requires that one or more layers of cohesive snow lie on top of a weak layer. So the fractures that release the slab avalanche start in the weak layer.
Most avalanches are triggered when are loaded with additional snow. However, the majority of avalanches that injure or bury people are usually triggered by the victim or by a member of the same group. Whilst slabs are generally triggered when the stress caused by weight of the slider and the slab itself exceeds the strength of the weak layer in the snowpack, it’s not uncommon that natural triggers play a role too. Those include loading of slope by snowfall or wind deposited snow, warming temperatures, rain, cornice fall, etc.
I am sure you will find the next post more interesting. I will write about recognizing and traveling in avalanche terrain, by far my favorite kind of homework!