Avalanches and the Wasatch Mountains
Loose Snow Avalanches
Loose snow avalanches involve individual snow crystals that can move separately from each other. This means the snow either does not stick together or only slightly sticky. There are two types of loose snow avalanches, dry and wet.
For either of these avalanches to form the snow must be on a surface that is steeper then the angle of repose. The angle of repose varies from as shallow as 15° for snow wet enough to classify as slush to as steep as 50° at which point snow will normally not accumulate. The most common angle at which avalanches occur is close to 30°. The shape of the snow crystals allow loose snow to accumulate on slopes steeper then the angle of repose would indicate. When snow is in this condition it is very unstable and once something but once one part moves it starts the snow below it moving this creates a teardrop shape in the overall avalanche.
Dry loose snow avalanches occur where there are low temperatures and little or no wind. These avalanches are most common after a storm ends in cold weather. Dry loose snow avalanches are generally not as dangerous as other types of avalanches. If there is persistent cold weather avalanches can occur several days after a storm has subsided, when they are not expected.
Wet loose snow avalanches are the second variety of loose snow avalanches. These avalanches are normally triggered by warming through either sunlight or rain water. These avalanches can be much more dangerous then dry loose avalanches due to the heavier weight of the wet snow.
Slab Avalanches
Slab avalanches are usually more dangerous then loose snow avalanches, and are caused by the failing of a layer of snow under the shear force caused by the snow pack trying to move down hill and the ground or a layer of snow staying stationary or moving at a slower rate. This failure in a layer of snow allows the overlying snow (which can be one or more layers and can be the entire snow pack) to slide down the slope as a block. Similar to loose snow avalanches the angle of repose is normally around 30° but can be as high as 55°.
The weight of a skier or other traveler can be enough to break a slab avalanche loose and take the skier and possibly the rest of the party down the mountain. This can happen because the weight of the skier increases the tension in the snow pack above them. The point where the slab separates from the snow pack above is called the crown of the avalanche.
As with loose snow avalanches there are both wet and dry slab avalanches.
Definitions
Angle of Repose: The maximum angle at which a pile of loose material can remain stable.
Shear: A stress resulting from applied forces that tends to cause two connected masses to slide relative to each other.
Tension: A stress resulting from applied forces that tends to pull two connected masses apart.
Crown: The top edge of the slab which separates from the snow pack further up the mountain.