TYPES OF AVALANCHES



Exist different types of avalanches, although some could look like. Some avalanches are very dangerous and can cost a personís life while others only have mild effects on people. There are many types of avalanche; each varies according to cause of occurrence.

Loose snow avalanches

Loose snow avalanches usually happen when weak surface snow is on a slope that is steeper than its critical angle of repose. Typically the cohesionless snow is either dry unsintered fresh snow or wet snow formed from melting. These are called dry loose-snow and wet loose-snow avalanches respectively.

When the snow is perturbed, the loose snow undergoes a localized rotational slip and then moves downslope in an inverted V-shape pattern. The initial slip involves very small masses of snow that range in size from one grain of snow to a the size of a large snowball. Typically they contain less than 1 cubic meter of snow. As the avalanche moves downslope, it can set other cohesiveless snow in motion. The avalanche finally comes to rest once the snow reaches its kinetic angle of repose.

This is a gentle and usually safe type of avalanche. It does not gather enough snow on the way to be able to kill or bury a person. Since the speed of a loose snow avalanche is relatively slow and the initial mass of snow is small, they typically do not cause much destruction. Nonetheless hazards are still involved.

Ice fall avalanches

Ice fall avalanches happen when a glacier finds a steep drop. Pieces of ice "calf" off as the glacier slowly flows downhill under the force of gravity. Ice fall avalanches are unrelated to temperature, time of day or any other factors, despite conventional wisdom to the contrary. Many believe that temperature and time of the day can affect the occurrence of this type of avalanche, but the truth is that this avalanche happens when glaciers drop steeply from the mountain. The best way to avoid these avalanches is to not travel beneath ice cliffs and through ice falls, or if necessary, roll the dice and travel through quickly.

Cornice fall avalanches

Cornice fall avalanches arise when cornices break loose from the lee side of ridges. Cornices look like frozen ocean waves stretched along mountain ridges. They form their characteristic "eaves" of cantilevered snow when prevailing winds remove snow from slopes on one side of a ridge and deposit it on the other side of the ridge. The snow that forms cornices is very dense and hard, yet can be extremely fragile. It is often difficult to determine from the ridge top where the ground ends and the overhanging cornice is not supported. This type of avalanche is easily avoided by staying back from the peak of ridges, but can be deadly as the victim tumbles downhill amid massive, hard and heavy chunks of snow which often trigger secondary slab avalanches as they pass.

Slab avalanches

The most dangerous type of avalanche is the slab avalanche. Slab avalanches arise when cohesive snow begins to slide on a weak layer. The rupture line where the moving snow breaks away from the snowpack makes this type of avalanche easy to identify. Slab release is rapid. This type of avalanche is the primary cause of the most number of casualties among travelers.