Avalanches: Their Dangers and How to Reduce Your Risks

February 11, 2020, 10:59 PM EST

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Above: Security officers and villagers carry a victim of an avalanche near the town of Bahcesehir, in the eastern Turkey province of Van, on February 5, 2020. Several dozen rescue workers were hit by a second avalanche while on a mission to find two people missing in a previous snowslide that struck late on February 4. (DHA/DHA/AFP via Getty Images)

We all have heard the term. It may be foreign to those who do not work and play in mountainous winter climates, but for those who enjoy winter activities outside, especially in the mountains of the West, and even more important, for those who venture off groomed trails into the backcountry, it is a danger to be reckoned with.

An avalanche is a mass of snow, sometimes mixed with ice and debris which travels down mountain sides, destroying everything in its path. One recent example of the deadly nature of avalanches happened in Turkey in early February, when a pair of avalanches led to at least 41 fatalities and 84 injuries. The second avalanche killed rescuers who were trying to extricate people from the first one.

On average, in the U.S., avalanches kill 25 to 30 people each winter season and injure many more. Many of those fatalities occur in the backcountry on unmarked or unpatrolled areas either inside or outside of a ski resort’s boundaries.

Sobering statistics

Because the highest elevations of the mountains in the western U.S. (including Alaska) maintain significant snowpack through the entire year, avalanches can occur at any time. Avalanche fatalities are most common during a core of five months that span a time frame from late fall through early spring, or December through April. However, in records kept since 1950-51, fatalities have been recorded in every month of the year, as shown below.

Over the decades, avalanches have killed people who were taking part in a wide variety of activities. Prior to World War II, most Americans who died in avalanches were miners or railroad operators who worked in the mountains and were at peril when avalanches struck. After World War II, those ways of life decreased and recreation took over. The term “back-country enthusiast” takes into account the significant number of ways people enjoy the snow (and ice). Below is a breakdown of the types of activities people were engaged in when they suffered a fatality.

When researchers have analyzed avalanche fatalities, some interesting trends were revealed. I spoke with Dr. Ethan M. Greene, director of the Colorado Avalanche Information Center, about some recent research into avalanche fatality trends. Rather than look at a year-to-year basis, which can have significant swings due to seasonal weather differences, Greene and colleagues looked at the five-year running average.

Across the period from 1950 to 2017, the first big jump in fatalities occurred in the early 1970s, likely due to a significant increase in backcountry recreation. From the mid-1970s through the 1980s, the running average leveled off somewhat.

Then came the 1990s, when the next big jump in fatalities occurred. That period was accompanied by an explosive jump in backcountry recreation by both skiers and snowmobilers. Big improvements in equipment technology allowed skiers and snowboarders to travel deeper into the backcountry. In addition, there were significant improvements in snowmobile technology with the development of the first true mountain snowmobiles, allowing riders to access avalanche terrain even in periods of more elevated avalanche danger. In the 40-year period from 1951 to 1990, there were only 14 snowmobiler fatalities. But starting in the late 1990s, there have been on average about 10 snowmobile deaths per season!

Around 1995, the five-year running mean shifts again, leveling off to about 25 to 30 deaths per year despite rapidly rising backcountry use. Nobody can accurately gauge the exact number of backcountry recreationists, given that they are exploring off the grid. So, based on the rapid increase in people accessing back-country forecasts from avalanche centers over the past couple decades, Greene and his colleagues used that information as an “imperfect proxy” for increased backcountry use. They conservatively estimated an eight-fold increase in backcountry use from 1995 to present. With a fatality rate that has remained nearly constant at 25 to 30 deaths per season, the overall data suggests that, based purely on such a huge increase in backcountry use, one would expect the fatality rate to be much greater. The combined progress in the establishment of avalanche centers, avalanche forecasts, improved gear, and education appear to have all contributed to an increase in backcountry safety.

Where U.S. avalanches occur

It is no surprise most U.S. avalanche fatalities occur in a core of eight states across the Rockies and Cascades. However, fatalities have occurred in the northeastern U.S. as well, in parts of the Appalachian Range. The breakdown below shows that 17 states have recorded avalanche deaths since 1950, with Colorado by far standing out as the place where most deaths have occurred.

Why does Colorado have such a high number of fatalities? Brian Lazar, an avalanche forecast from the Colorado Avalanche Center, shed some light on the reasons. First and foremost, there are just so many people who either live in the Colorado mountains or travel there to get that backcountry experience. Much of the area is close to major cities, and there are also major highways that cut right through prime recreational land in the heart of the Colorado Rockies.

But there is a second factor. It has to do with the character of the snowfall that occurs in Colorado. The area has a continental climate (one far from oceanic influence), which results in the depositing of successive layers of thin and relatively dry snowpack in cold air. Contrast that with the Sierra or Cascade ranges on the West Coast, which experience a much wetter marine climate and water-laden snow, being in close proximity to the ocean. One important characteristic of the continental snowpack of the Colorado Rockies is that it results in weak layers that can produce a variety of slab avalanches.

What causes an avalanche?

There are many factors that can trigger avalanches, and conditions vary greatly from place to place. Rain and wind can play prominent roles in the development of avalanches, but one of the leading causes is the development of weak layers.

Snow can fall in one big dump to create a cohesive slab on a mountainside. The surface of that snow cover can undergo lots of minute changes, including melting of the snow crystals from the sunshine and refreezing at night. Those beautiful little snowflakes with all of their interlocking legs that stabilize a snowpack can morph into structures with different properties, including facets or sharp edges that after a few days weaken the cohesion of the snowpack. In fact, the snow often begins to feel “sugary”. If another snowstorm comes through and deposits several more inches or even feet atop that first slab of snow, after it has undergone these kinds of structural changes, you end up with a weak layer between the old and new slabs.

Slab avalanches are probably the most destructive and dangerous type of avalanche. Once a slab of unstable snow is in place atop a weak layer, all that’s needed for a slab avalanche is a trigger to break it loose and get it moving down the mountainside. A slab avalanche may be only a few inches deep and 50 feet across, coasting downhill at 30 mph. Or it can be a ten-foot-deep monster, wider than a few city blocks and churning at more than 100 mph. A “powder cloud” often builds atop the biggest avalanches, as prominently featured in the 2014 French film “Force Majeure” and the English-language remake, “Downhill”, which debuts this month. Despite its soft, feathery appearance, a fast-moving, snow-laden powder cloud may pack enough heft to knock down trees and even buildings.

With all that snow on the slope of a mountain, there are several triggers that can result in an avalanche. They include overloading, temperature, slope angle, snowpack conditions, and vibration.

Overloading is simply the increase in the weight of a snow layer to a point that can overcome the layer’s cohesion to the snowpack underneath.

Temperature also has an effect on the cohesion of snow. A temperature rise will cause melting which weakens those bonds between snow crystals; a drop in temperature can increase the brittleness of the snow crystals and weakens the slab.

The angle of the slope is another very important factor to consider. I call it the “Goldilocks Effect”. About 90% of all avalanches occur when there is an angle to a slope between 30 and 45 degrees. If the angle is less than 30 degrees, it’s typically not steep enough to trigger an avalanche. If it’s greater than 45 degrees, it’s much more difficult for a slab to build to any great depth—it just slides down the mountain before it gets deep enough to trigger a significant avalanche. What’s interesting about slope angle, is that for the most part, the angle doesn’t change on a mountainside as snow builds up. This means there are typical areas where avalanches are much more common and therefore, easier to avoid.

Any type of vibration can trigger an avalanche to occur. This includes naturally occurring shakes from wind and lightning or manmade sounds like that of a loud engine. In fact, avalanche forecasters use explosives to produce the concussions necessary to release a layer of snow and remove the threat in avalanche-prone areas where there are lots of people including within the boundaries of a ski area or along certain mountain highways like Little Cottonwood Canyon above Salt Lake City, Utah.

By the same token, any vibration caused by a skier/snowboarder or snowmobiler moving across dangerous terrain could trigger an avalanche. A snowboarder jumping off a cliff, a skier swooshing below the precipice of a ridge, and certainly a snowmobiler—who sits atop a loud, fast-moving, several hundred-pound machine that grinds across the snow—all can produce enough vibration to trigger that release or crack on a slab of snow and the ensuing avalanche.

Someone in the backcountry may be able to assess the slope of the landscape and may be able to even assess other visible threats, like overhanging cornices of snow on the lee side of a ridge. However, it is often impossible to determine the state of the layer of snow you are about to cross. Are there weak layers below? How long has this snow been there? How strong is the cohesion of the pack? Without knowing these things, you are playing a high-stakes game that could release the snow into an avalanche and put you and potentially others into danger.

Fortunately, there is a way to get updated on those questions with a simple click of a button, as we will see in Part II of this post.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Tom Niziol

Tom Niziol recently retired as winter weather expert for the Weather Channel after a 32-year career as a forecaster, science and operations officer, and meteorologist-in-charge at the National Weather Service office in Buffalo, NY. Tom has published several papers and taught forecasters around the world through the COMET Program. His keenest winter weather interest is lake-effect snow.


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