By: Marlene Affeld
Have you ever witnessed an avalanche? They are awesome, terrifying and among the greatest dangers to human life that one may encounter in the back country. In the United States, over the past ten years there has been an average of 25 avalanche fatalities per year. The majority of avalanches affecting people are triggered by people. Skiers, snowmobilers and everyone that travels in avalanche-prone areas should remain vigilant and aware.
An avalanche is a rapid and abrupt mass flow of snow, generally mixed with water and air that thunders down the mountainside, burying anything in its pathway. Typically avalanches are caused by an over-burden of snowpack that is too massive and unstable for the slope that supports it. As an avalanche crashes down the mountain it will also tear loose trees, rocks and boulders that become part of the devastating barrage.
It is extremely dangerous in the mountains when a period of very cold weather is followed by a chinook or sudden warming trend or when rain is falling on snow pack. Free flowing water in the snow pack lubricates weak or fragile layers of the snow and often massive avalanches result. Like intense snowstorms, this is a very dangerous time to be traveling in the mountains.
There are several different types of avalanches that occur. Loose snow avalanches happen when the massive weight of the snowpack exceeds the shear strength within it. This type of avalanche is most common on steep terrain and will normally release at a narrow point and then widen as it crashes down the mountain, forming a teardrop pattern.
Slab avalanches are the biggest danger and account for over ninety percent of avalanche-related fatalities. Slab avalanches form when heavy accumulations of snow are deposited on the leeward side of a mountain. These types of snow deposits form stiff, strong layers of snow. When the slab layer fails, the fracture or break in the weak layer allows massive volumes of snow to instantly fall away. Slab avalanches are often many feet thick and hundreds of yards wide.
An isothermal avalanche, a third type of avalanche, occurs when the built-up snowpack becomes heavily saturated by water. This type also normally starts at a narrow point and spreads out as it proceeds downward.
Avalanches that form from fresh or loose snow are known as powder snow avalanches or a powder cloud. Powder clouds are a turbulent suspension of snow and air that flow as a gravity current. Powder snow avalanches are the largest of all avalanches and can flow for extremely long distances at speeds in excess of 300 miles per hour and have been known to contain over 10,000,000 tons of snow. The massive flow of snow from powder snow avalanches can flow along flat valley bottoms and even short distances uphill.
Avalanches are one of the strongest forces of nature and unpredictable by nature. Winter travel or recreation in avalanche-prone areas is never truly safe. Check the weather, don’t get caught out when big events are imminent. Don’t get caught out when you may lose visibility and not be aware that you’re traveling under or near avalanche terrain.
Pay attention to avalanche warnings. Only travel in the back country with partners you’d trust with your life. Carry safety and winter survival equipment and know how to use it.
If you are going to venture into areas where avalanches may occur, carefully consider your route. When you ski or snowmobile, never follow in the tracks of others without your own evaluation of the risk. Snow pack can change from small and benign to deadly overnight; conditions change constantly and a area that was previously stable may change quickly. A mountain slope should never be considered safe just because others have crossed it.
Observe the terrain and and take note of obvious avalanche paths where vegetation is damaged or missing, where there are few surface anchors and avoid areas of overhanging ice formations or cornices. Do not travel below others who might trigger an avalanche.
The majority of avalanches start on mountain slopes that are 30 degrees or greater. If you stay off of slopes that are 30 degrees or greater and avoid traveling beneath them, your risk is substantially minimized. Pay attention and avoid such obvious risks.
Even small avalanches are life threatening; over fifty percent of victims buried do not survive. If you remember anything about avalanche safely, remember this; most avalanches happen during or shortly after periods of intense snowfall. If snow is accumulating at a rate of an inch or more an hour avalanche danger is extreme.
Avalanches can and do happen anywhere. If you are skiing or snowmobiling, please learn about the inherent risk of avalanches and the areas you will be traversing. Accessing the most recent information for your area is crucial to enjoying your winter recreation safely and will make you a more alert and an all-around better outdoors person.