Avalanche Basics
An avalanche is basically a mass of snow moving downhill. This can occur almost anywhere there is snow on a slope if the conditions are right. Avalanche conditions include weather, temperature, snowfall depth, snowpack depth, windpack, moisture content, slope steepness or angle, slope aspect or orientation, slope shape, vegetation, elevation, and layering. The key in evaluating snowpack for stability is in the correct mixture of these elements.
There are two main types of snow avalanches:
Slab Avalanches- slide starts as a giant slab, where the entire surface of the snowpack slides as one, though often breaking apart as the terrain changes. Slab avalanches can occur on slopes anywhere between 20 and 60 degrees
Image from "Snow Sense" By Fesler and Fredston
Point Avalanches- starts from a single point, common in triggering avalanches with detonators, also often called-
Loose Snow Avalanches (Sluffs)- Loose snow triggers more loose snow to tumble downslope: Relatively Harmless.
Wet Snow Avalanches- Wet snow slides like a mudslide, pouring down shallower slopes. These slides can occur on slopes less than 15 degrees, and are mostly driven by high water content in the snowpack. Generally, wet snow avalanches are slab avalanches, due to the cohesion of layers, often with a layer of frozen snow beneath heavy, wet, sticky snow that slides as a slab.
Powder Avalanches- this is the quintessential slide type, the type most people think of where a mass of dry snow flies down a steep slope through a slide path. These avalanches occur most commonly between slope angles of 30 and 50. The dry snow is mixed with air, which is why these avalanches are accompanied by a large cloud of snow, and often followed by a powderblast, a large blast of high velocity air, which can sometimes do more damage than the snow itself. The powderblast often travels further than the snow in an avalanche, and can flatten trees (and anything else) on the opposite sides of valleys as well as in the runout zone.
This is a controlled powder avalanche at Telluride from http://www.avalanche.org/pictures/avalanche-.jpg
