The most dangerous conditions for an avalanche occur when a lot of snow has fallen and/or blowing wind has caused snow to accumulate within a short period of time (hours or a few days, versus weeks). A “dry slab” avalanche is the most hazardous. This is when a heavy slab of snow that has formed quickly is resting above another layer of snow that is weaker but formed over a longer period of time. Dry slab avalanches are usually set off by a person walking over the unstable layer. There are also “wet slab” avalanches, which, as one might guess, involve a layer of wet snow over a harder layer of snow.

Avalanches are most likely to occur on hills with inclines of 30 to 45 degrees, though wet snow can tumble down a hill with a grade of as little as 10 degrees, and dry snow regularly causes avalanches on hills with about 20 to 22 degree slopes. Avalanches happen abruptly, and once a slab has broken off, there is usually no escape for someone downhill. Traveling at 60 to 80 miles (95 to 130 kilometers) per hour, an avalanche will quickly bury everything in its path.