A disturbance that initiates fracture within the weak layer causing an avalanche. In 90 percent of avalanche accidents, the victim or someone in the victims party triggers the avalanche.

Most avalanches are “naturally” triggered, meaning that weather (wind, snow, rain or sun) stress the snowpack to its breaking point. Like a tree falling in the woods, for the most part, we only care about the ones that affect people. Luckily, in 92 percent of avalanche accidents, the avalanche is triggered by the victim or someone in the victim’s party. In other words, most avalanche accidents happen by choice, not chance. These are “human triggered” avalanches. In other words, weather adds stress to the snowpack until it nearly equals the strength of the snowpack. Then, the added weight of a person provides the final thump to initiate a fracture within the buried weak layer. (No, noise does NOT trigger avalanches. It’s a cliché plot device in the movies, but noise is simply not enough force to trigger an avalanche.)

Trigger Size Required to Initiate an Avalanche
Stability Large Cornice Explosive Human Natural
Very Poor X X X X
Poor X X X
Fair X X
Good X
Very Good

We can also think of snow stability in terms of the size of trigger required to trigger an avalanche. Notice that when natural avalanches are occurring, the stability meter is pegged out at the top of the scale. That’s why the best sign of avalanche danger is another avalanche on a similar slope.