Outdoor recreation in any season can pose risks such as getting lost or becoming injured due to trips, slips, falls, and collisions. Winter recreation can pose additional risks due to cold, such as hypothermia (also common in other seasons) and frostbite. Snow itself can create some additional hazards. Even something as simple as falling snow covering up your tracks can present a danger if you aren’t familiar with the area and end up losing your way home. (Always pay attention to landmarks and trail markers, and bring a map and know how to use it!) Learning about how to stay safe in the snow and making responsible choices can make the difference between life, injury, and death in some situations.
On a sunny day, ultraviolet light reflecting off the snow’s surface can burn the corneas of the eye. In addition to being painful, this can result in temporary blindness (“snow blindness”) if one’s is not wearing sunglasses or goggles, which have been used by indigenous peoples of the North for hundreds and even thousands of years. On the other hand, low light conditions can cause an all-white landscape look “flat,” making it difficult for a snow traveler to identify differences in terrain, such as drop-offs and the boundary between a packed trail and surrounding deep snow.
An avalanche is a mass of snow moving downhill. Avalanches can be small or large, wet or dry, powdery snow or hard “slabs.” Three main factors determine the likelihood of an avalanche occurring in any point of time in a particular location: Snowpack characteristics, weather conditions, and terrain. These three factors are sometimes depicted as an Avalanche Triangle, and the sides of the triangle remind us that the three factors interact to determine avalanche risk.
Avalanche Hazard Triangle
Understanding Avalanche Conditions
Avalanche risk increases when the snowpack is unstable, meaning that it is more likely to slide. Snowpack instability is related to the differences between different layers in the snow on the ground and how they are arranged. Due to changes in the snowpack after snow accumulates on the ground, snow crystals can take on a variety of different characteristics. When a weak layer of crumbly, fragile, poorly bonded snow crystals lies underneath a layer of heavier, denser snow, the snowpack is likely to be unstable. When a weak layer is deposited on top of a heavier, denser layer, the snowpack tends to be more stable. Icy layers in the snowpack provide a smooth surface that snow layers above it can slide on easily. In some avalanches, only the top layers of snow slide, and in others, the entire snowpack slides on the ground underneath.
Weather influences avalanche risk because it influences the amount of layering in the snowpack and the characteristics of those layers. When a snowpack is isothermic, meaning that the temperature of the snowpack is the same at all depths, it is generally more stable. When there is a large temperature gradient, meaning a difference between the temperatures at the bottom of the snowpack and the top of the snowpack, the snowpack is generally more unstable. Large temperature gradients occur in colder places with shallower snowpacks, because snow at the bottom of the snowpack is always approximately 0 degrees Celsius (32 degrees Fahrenheit), and the top of the snowpack is the temperature of the air.
In addition to temperature, precipitation influences snowpack stability, because additional snow or ice to the top of the snowpack adds weight. If a lot of snow falls in a short amount of time, the underlying snowpack doesn’t have enough time to adjust to the heavy load, which creates instability. Often, strong winds have the same effect even if it isn’t currently snowing, because they can blow large amounts of snow and deposit it quickly, often on the other side of a ridge. Leeward slopes (sheltered from the wind) can be more unstable than windward slopes (facing toward the wind) for this reason.
The steepness of a slope, the compass direction toward which it faces, and the underlying surface can all influence the likelihood of an avalanche occurring. Avalanches occur most commonly on slopes between about 25 and 60 degrees from horizontal. On slopes shallower than 25 degrees, the force of gravity is not great enough to overcome the strength of the forces keeping the snowpack together. On slopes steeper than 60 degrees, snow constantly sloughs off and doesn’t accumulate. Smooth ground surfaces and ice layers generally slide more readily than areas with lots of trees or big rocks, which can act like “anchors” to keep the snow in place under some conditions. (In cases where the snow is shallower around trees or big rocks, however, they can actually increase rather than decrease avalanche hazard.)
In addition to the conditions created by the interacting sides of the avalanche triangle, there must also be a trigger for an avalanche to occur. Most avalanches are started by natural triggers, such as a rapid deposit of heavy snow during a storm, but the avalanches that we usually hear about are human-caused. Human-caused triggers can be purposeful, as when safety professionals at established ski areas use artillery to set off avalanches before opening the area to people, or accidental, as when the weight of a skier, snowmobiler, or other recreationist causes the avalanche to start.
Experienced winter backcountry travelers learn about how to observe snow conditions and perform tests to assess the stability of the snowpack. The most important thing people can do to stay safe in the backcountry is to pay close attention to your surroundings, including looking for evidence of past avalanches, since avalanches often occur in the same locations under similar conditions. Sounds that the snow makes as you travel over it also provide clues to snowpack conditions. A “whumpf” sound indicates that your weight is causing weak snow layers to collapse underneath the surface.
Another step in assessing avalanche conditions is to dig a snow pit in order to observe the layering in the snowpack. This is also one of the primary ways that scientists study snow and a good example of how scientific understanding can be applied to practical situations, and one that can determine the difference between life or death. Training is required to be able to interpret snowpack observations, however. Most importantly of all, people traveling in potential avalanche areas need to make responsible and safe decisions about when and where to go.
Because avalanche dynamics are complex and depend on a variety of factors and how they interact in time and space, learning more about avalanche safety is critical for anyone planning to recreate outside of established areas maintained by avalanche professionals. This site does not provide enough information for you to make decisions about avalanche safety.
If you want to learn more about avalanches and avalanche safety, check out the links on this page.
By following safety precautions such as finding out what the local snow and weather conditions are ahead of time, traveling with other people, bringing the necessary gear, and not going when and when avalanche risk is high, people can safely enjoy the beauty and magic that snow provides.
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