September 1, 2014
Winter 2013 - 2014


Kinds of Avalanches

Avalanches come in many shapes and sizes and even small ones can be dangerous. In general, there are three types of avalanches:

1) Slab avalanches: Most people that die in avalanches, die in slab avalanches. Slab avalanches occur when a more cohesive or harder layer of snow sets on top of a less cohesive or softer and weaker layer of snow.  Some times the weak layer can barely support the layers above it and when additional weight like a skier or boarder is added to the upper layers, the weak layer collapses and the snowpack fractures and a slab avalanche occurs.

Slab avalanches often involve large volumes of fast moving snow. Victims like the skiers in the video to the right typically trigger slabs at mid slope below the fracture line which often makes escape very difficult!

See Anatomy of a Slab Avalanche below...

 
2) Sluffs or loose snow avalanches: Sluffs are cold snow powdery surface slides that typically are the least dangerous type of slide; however, sluffs can and often do injure skiers and boarders by pushing them over cliffs and rock bands in steep terrain.

Click on the video on the right to see some sluff avalanches in steep gnarly terrain.  Sometimes you can ride next to them or through them but if they knock you over in the wrong spot they can definitely ruin your day.

 



3) Wet avalanches: Wet slides occur when warm temperatures melt the surface snow layers and saturate them with water (photo below and right).  The water weakens the bonds between layers and avalanches often occur.  Wet avalanches move more slowly than dry avalanches but they can still be very dangerous:

  • If temperatures have been above freezing for extended periods then wet avalanches will most likely occur
  • If you are sinking into wet snow up to your ankles while you are on skis or a board, the snow is wet and prone to avalanche
  • If you squeeze a hand full of snow and it makes your glove wet or if water literally drips out of the snow, the snow may be prone to avalanche



Anatomy of a Slab Avalanche
Mouse over the terms below and get the definition for each

The top fracture surface of a slab avalanche. Usually smooth, clean cut, and angled 90 degrees to the bed surface. The surface over which a fracture and subsequent avalanche release occurs. Can be either the ground or a snow surface. The portion of an avalanche path where an avalanche releases. The portion of an avalanche path between the starting zone and the runout zone. The portion of an avalanche path where the debris typically comes to rest.



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