An avalanche is a mass of snow that slides rapidly down an inclined slope, such as a mountainside or the roof of a building. Avalanches are triggered by either natural forces (e.g. precipitation, wind drifting snow, rapid temperature changes) or human activity. In mountainous terrain, they are among the most serious hazards to human life and property. Avalanches are sometimes called snowslides.

An avalanche occurs when stress from the pull of gravity and/or applied load (such as a skier) exceeds the strength of the snow cover.  Strength is derived from bonds between snow grains. A slab (a cohesive layer within the snowpack) avalanche can occur when the following three conditions are present:

1.      A snow-covered slope

2.      A slab of snow resting on top of a weak layer of snow

3.      A triggering mechanism

The level of avalanche danger increases during and after major snowstorms and periods of thaw. More than 80% of the avalanches reported to the CAIC strike during or just after large snowstorms. The most avalanche-prone months are (in order): February, March, and January. Avalanches caused by thaw occur most often in April. An avalanche-caused fatality has occurred in Colorado in every month of the year except September.

The graph above shows the number of U.S. avalanche fatalities by month between 1950-51 and 2009-10.

Most avalanches occur in the backcountry, outside the boundaries of developed ski areas. About 90% of all avalanches begin on slopes of 30-45 degrees, and about 98% occur on slopes of 25-50 degrees. Avalanches strike most often on slopes above timberline that face away from prevailing winds (leeward slopes tend to collect snow blowing from the windward sides of ridges). However, it is possible for avalanches to run on small slopes well below timberline, such as in gullies, road cuts, and small openings in the trees. Very dense trees can help anchor the snow to steep slopes and prevent avalanches from starting; however, avalanches can release and travel through a moderately dense forest.

There are two basic types of avalanches: loose snow and slab. When grains of snow have little cohesion between them, the snow will slide. This is known as a loose snow avalanche, point-release avalanche, or sluff avalanche. Because they typically only involve snow layers close to the surface, loose snow avalanches rarely result in fatalities or serious injuries. However, they can be dangerous to backcountry users if they occur above a cliff, crevasse, or gully.

A slab avalanche, on the other hand, is the most dangerous kind of avalanche. Slab avalanches occur when a large block (slab) of snow breaks free from a mountain side and slides downhill. The weight of a single skier is enough to set off a slab avalanche. From 1950 to 2010, 99% of avalanche fatalities in Colorado were caused by slab avalanches. (In the United States, slab avalanches account for 96% of fatalities.)  There are three types of slab avalanches:

• Soft slab: slab made of new or low-density snow that is “light and fluffy” and was deposited  with little or no wind conditions
• Hard slab: slab made of high-density, hard, wind-compacted snow
• Wet slab: slab consisting of snow that has a significant amount of free water running through it.