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: – A snow-covered slope – A slab of snow resting on top of a weak layer of snow – A triggering mechanism

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.