The first step is to realize that a typical snowpack is actually a series of different layers stacked on top of each other. These layers are formed by precipitation, varying temperatures, and wind events that occur throughout the winter. The layers can deviate from very hard icy layers to very soft loose "sugary" layers and everything in between (check out the "Snow Pack Layering" video, top right).
The most treacherous type of unstable snowpack occurs when a slab sets on top of a weak layer. A slab is a layer of snow that has at least some cohesion. Slabs can be quite hard or they can be rather soft; the slab layer just has to be more cohesive or harder than the layer below it.
The most dangerous situation occurs when the upper layer is very cohesive and the lower layer is much less cohesive. In those situations, the upper or slab layer is literally a block that you can cut out and pick up and the lower or weak layer is like sugar.
Sometimes the weak layer can just barely hold up the block.
When that happens, it just takes a little additional stress or weight like a skier or snowboarder to collapse the weak layer and trigger an avalanche (see video at right).
Another type of unstable and potentially dangerous snowpack occurs when above freezing temperatures melt portions of the snow pack and saturate certain layers. This condition can cause wet avalanches.
To reduce your risk in avalanche terrain you must be able to recognize unstable snow packs. Mother Nature gives us lots of clues or red flags when the snowpack is unstable and there are also tests we can do to gage stability. But before we discuss Red Flags and Stability Tests, let's take a look at how weather affects snow stability.
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