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What causes seemingly innocent snow slopes to collapse?

What causes seemingly innocent snow slopes to collapse?

On January 10, 2024, an avalanche swept away skiers at Lake Tahoe's largest ski resort: a 45.72-meter-wide cylinder of snow slid down the mountain into a pile 3,048 meters deep. One person died in the avalanche and three were rescued. The slide occurred on steep terrain near the KT-22 lift, which had just opened that morning for the season. The next day, a second unexpected avalanche hit the same ski resort, but no one was injured. Avalanche fatalities are rare at ski resorts like Palisades-Tahoe, but the risk is increased in the wilderness. There have been 30 wild avalanche deaths reported in the United States during the 2022-2023 season. University of Colorado skier and physicist Natalie Friend, who studies avalanches, explains what happens in avalanches and survival techniques in The Conversation.

What causes avalanches? What can we do if we get caught in an avalanche?

Avalanche behavior depends on the structure of the snowpack, but this is only one element. An avalanche requires all the wrong conditions at the wrong time.
The slope of the mountain is important. Slopes between 25 and 40 degrees are the most dangerous. It is also ideal for skiing. Slopes of less than 25 degrees may have a slight slide, but will not increase speed. If the temperature is above 40 degrees, snow usually does not accumulate and there is no risk of avalanches. Then you need a trigger. A snowpack may appear stable until a snowmobile or skier disturbs it enough to move the snow. Strong winds or falling rocks can also cause an avalanche. Blowing snow generates wind loads and accumulates in snow edges, creating a bump that eventually falls and triggers an avalanche below.

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What happens in a snow mass during an avalanche?

Mountain snow masses are not the same. Because it accumulates over time, it is a snapshot of recent weather conditions and has stable, weak layers. When snow falls, it has a flaky, crystalline structure. But as temperatures rise and the snow begins to melt and freeze again, it becomes more granular. The most granular glacial snow is a weak layer. When new snowfall accumulates on top, the grains of the weak layer are sheared off, creating a surface for the avalanche to slide over. The weight of a new mass of snow can cause the entire surface of a mountain to collapse almost instantly. As the avalanche accelerates, more snow and debris becomes incorporated into the avalanche and it becomes very large and wild.

An avalanche falls down a mountainside near Winter Park in 2021.Source:

At a University of Colorado laboratory, the physicist studied small-scale laboratory avalanches. Using photoelasticity, they created thin avalanches to see what happens inside an avalanche. They tracked photoelastic particles with a high-speed camera and observed that the particles bounce and collide very quickly, within 1/1000 of a second. In a real avalanche, these violent collisions produce a lot of heat through friction, causing more melting. Once the avalanche stops, this fluid quickly freezes and holds the snowpack in place like concrete. People say that we swim to the surface during an avalanche, but we don't know if the surface is above or below. If the collapse is still moving and the grains have not yet hardened, you may be able to move a little, but it is very difficult.

What can skiers do if they are in an avalanche?

Friend conducted field work with real, intentionally induced avalanches in Switzerland. He was in a hideout in a valley with his colleagues and explosives were dropped from a helicopter on the top of the mountain. They used radar to look inside the avalanche as it moved toward them. The avalanche easily moved at a speed of more than 50 m/s. If the avalanche is small, you can easily bypass it. The big danger is when the snow is deep, a person can be buried by several meters of snow. Basically, as the avalanche slows, new snow accumulates on top of the person. People have reported that it is like falling into concrete and not being able to move a limb. Very terrifying experience.

An avalanche rescue dog pulls a ski patroller during avalanche training on Rz Mountain in Colorado.Source:

Backcountry skiers carry equipment with them, which increases their chances of survival. However, the best chance is with a human companion, especially in backcountry skiing where it takes hours for a rescue team to arrive.
There are some things we can do.

First, carry a transceiver that sends out a signal that pinpoints your location. If we're in an avalanche, send a signal.

Our friends turn their transceivers into “receive” mode and try to locate our signal. It is also important that we have an avalanche sounder and shovel out in the wilderness in case we locate our friends. The snow is like concrete and it's going to be hard to get us out of it. Avalanche airbags can also help. James Bond explains this principle in his book The World Is Not Enough. With modern avalanche airbags, you have to get a tap on our back, and the airbag behind our head inflates, turning us into a larger particle. Larger molecules tend to stay closer to the surface, making it easier for them to locate us.

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How does avalanche danger change as winter temperatures rise?

This is an important question, and it's not as simple as higher temperatures meaning less snow, and therefore less avalanches. However, if mountain temperatures are more variable, there will be more stages of melting and refreezing during the winter, which means less snowpack compared to historical records. The historical circumstances in which societies emerged are changing. In 2017, there was a major avalanche in Italy that destroyed an entire hotel. This happened in an area where people did not expect an avalanche to occur based on historical data. There are computer models that calculate where avalanches are likely to occur. But when patterns of temperature, snowfall, and precipitation change, people may not truly understand the cause and effect of natural hazards like avalanches.

(Source: The Conversation:

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