Avalanche Basics

Types of Avalanches

Loose Snow Avalanches or Sluffs
Loose snow avalanches occur in fresh, unconsolidated snow that have little cohesion. They are also called sluffs or powder avalanches. Usually, this type of avalanche starts out at a single point and fans out, entraining more snow as it travels downhill. They are generally small and manageable, but can be dangerous depending on the situation. Sluffs of new snow can knock you off your feet, which can be deadly in steep terrain over cliffs or crevasses. Depending on the conditions and runout, they can also entrain a lot of snow as they flow downhill, potentially burying or injuring a victim.
Loose snow avalanches generally start on steep terrain during or immediately after a storm.

Wet Snow Avalanches
Wet snow avalanches are similar in some ways to loose snow avalanches that occur during or after a storm. Like powder avalanches, they start out at a single point and fan out as they flow downhill, but unlike powder avalanches they tend to move much more slowly.
Large wet snow avalanches commonly bury victims, and can knock down trees or buildings. Look out for wet snow avalanches after rain, during rapid warming, or in spring when the snowpack does not freeze overnight. If you’re sinking into wet slush up to your calves, get off the slope!

Cornice Fall Avalanches
Cornices are big overhanging blocks of snow at ridge tops formed by the wind. If you’re traveling along a ridge top you can accidentally step on a cornice and cause it to collapse under the extra weight supplied by your body. If this happens, you’re going to have the snow beneath you fall away and be hurdled down the slope below. The sudden weight added by the cornice dropping onto the slope is like a bomb, and will often trigger a dangerous avalanche.
Cornices will often break off further back than you expect. Treat them like backcountry land mines; stay away!

$Slab Avalanches A slab avalanche occurs when a cohesive mass of snow breaks off from the slope and begins to move downhill. These are the most fatal, scary, and destructive avalanches because such a large amount of snow fractures all at once. There are two types of slabs that we will talk about in this course; hard slabs and soft slabs. Soft slabs occur within new snow, during or shortly after a storm. The tend to break off right at your skis, board, or snowmobile. The snow disintegrates into a cloud of powder as it flows downhill. Hard slabs are made of stiffer, denser snow. The typically break off uphill of the trigger point, and are usually larger than soft slabs. The most common type of hard slab is a wind slab, which will be discussed later. The next few pages discuss the ingredients and anatomy of a slab avalanche.$ shapeimage_6_link_0

Image courtesy Logan Avalanche Forecast Center

Photo: Mike Jenkins

Hard slabs can be recognized by blocky, angular piles of deposition, while soft slab debris (right) have a much smoother appearance.

Photos: Mike Jenkins

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Citation: factcouraud. (2007, May 08). Avalanche Basics. Retrieved November 24, 2009, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/avalanche-and-snow-dynamics/AvClass/AvalancheBasics/Avalanche%20Basics.html.