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.

Additional Terms:
Anchors Hard Slab Avalanche Slide
Aspect High Danger Sluff
Avalanche High Marking Snowpit
Avalanche Path Isothermal Soft Slab Avalanche
Avalanche Transceiver Layer, Snow Stability
Bed Surface Leeward Stability Test
Collapse Loading Starting Zone
Concave Slope Loose Snow Avalanche Stepping Down
Considerable Danger Low Avalanche Hazard Sun Crust
Convex Slope Melt-Freeze Snow Surface Hoar
Cornice Metamorphism, Snow Sympathetic Trigger
Corn Snow Moderate Danger Temperature Gradient
Couloir Persistent Weak Layers Terrain Trap
Cross Loading Point-Release Track
Crown Face Probe Trigger
Danger Ratings Propagation Trigger Point
Deep Slab Avalanche Rain Crust Upside-Down Storm
Density, Snow Remote Trigger Weak Layer
Depth Hoar Rime Weak Interface
Dry Snow Avalanche Runout Zone Wet Snow Avalanche
Extreme Danger Sastrugi Windward
Faceted Snow Settlement Wind Loading
Fracture Ski or Slope Cut Wind Slab
Glide Skinning, Skin Track Whumpf
Graupel Slab