The How Avalanches work.



How do avalanches work?

Avalanches seem to be something of a mystery to many people as they are things that are full of contrasts. Avalanches are a natural part of mountain life but they are also one of the deadliest aspects of it as well. They are beautiful and awesome to watch yet the destruction they leave behind is sometimes some of the ugliest destruction on earth. There are two different types of avalanches, loose snow avalanches and slab avalanches. Slab avalanches are when there is literally a slab of very strong and heavy snow rushing down the slope and it is this type that is most common to claim victims. Those who enjoy winter sports such as skiing, snowboarding, and snowmobiling through mountainous areas need to always be aware of the dangers of avalanches and truly understand how they work, what creates them, and how they take so many lives each year. It is only with this understanding that one can hope to survive an avalanche, should one occur while they are enjoying the fine mountain air. Of course, one would much rather prevent an avalanche from occurring rather than survive one and although they may seem to strike without warning, the truth is that many avalanches are started by the same people who the avalanches later bury. Knowing how they work is key in preventing this scenario from happening.

Understanding Snow

Avalanches are generally made up entirely of snow with the occasional rock, boulder, or ice pack getting in the mix of things as well. There are also other kinds of avalanches, such as rock avalanches which are made up almost entirely of rock but generally when people refer to avalanches, they are referring to snow avalanches. It is also snow avalanches that kill the most people so to understand them, one must first understand snow and its properties.

It’s true that no two snowflakes are identical but they all generally have a similar appearance. Snowflakes are generally six-pointed figures and depending on conditions such as humidity and temperature, they will have different shapes and sizes. When a lot of snow has fallen and begins to accumulate on the ground, it begins to form what is called a snowpack. Every snowpack is made up of different layers and the stability of the layers forms the stability of the snowpack. The common six-pointed snow crystals form the most stable layers but layers which contain substances such as rime and graupel create very unstable layers. Rime is formed when snow crystals in the air meet with air that is very, very cool. This rime collects and forms graupel deposits in the snowpack and layers within a snowpack that contain graupel are extremely unstable.

Once a snowpack has formed on the ground, it continues to change as the weather and temperature also change. If the surface of the snowpack thaws and refreezes repeatedly, a layer of ice will begin to form on top of the snowpack. This continual thawing and refreezing also causes ice particles to form on the surface of the snowpack but these ice particles often have spaces in between them, instead of forming one solid layer of ice. These small, multiple ice particles however, actually create an even more unstable layer within the snowpack and make it difficult for the next layer of snow to form a new, stable layer. The dew point in the air just above the surface of the snowpack also contributes to its changing properties. If the air does reach this point, a substance called hoar can develop around the edges of the snowpack. Hoar looks like feathery crystals and it will not attach itself to snow crystals, disallowing other snow crystals to collect and form as part of the snowpack.

Other changes within the snowpack are due to the temperature gradient, which is the difference in temperature between the top and bottom layers. The bottom layers will always be warmer than the top layers because they absorb heat from the ground. The temperature of the top layers will depend on the air that surrounds it, meaning that if it’s very cold outside, the top layers of snow will be very cold and if it’s warm, the snowpack surface will be as well. The individual snowflakes within the snowpack will all begin to change depending on the temperature. When there is a large difference in the temperature gradient of a snowpack, such as when the air is very cold therefore making the top layers very cold but the bottom layers are quite warm, this can cause the different snowflakes to form facets. Facets have flat surfaces, making it almost impossible for other surfaces to adhere well to it. Snowflakes can begin to develop many facets and once this happens, a depth hoar is created. A depth hoar is a very dangerous property of a snowpack as they are extremely unstable.

However, temperature gradients can also help snow packs form more stable layers. Of the difference of temperature is consistently low, or the temperatures are so cold that they fall to sub-freezing temperatures but remain consistently cold, this causes rounding. Rounding enable snowflakes to compress together more tightly and adhere to each other better. There is also water vapor among the snowflakes and as they are being squeezed together with water vapors constantly being exchanges, the vapor begins to form bridges and connect to other snowflakes. This creates a very stable and strong layer. It is these stable and unstable layers that both help to form avalanches.

How Avalanches are Formed

Avalanches need three components to form. These are snow, a slope, and something to cause them. Unstable layers within the snow packs consisting of ice, graupels, depth hoar, and faceted crystals also help with the formation of avalanches. If the unstable layer is close to the surface of the snowpack, it is called a sluff. A sluff is shaped in the form of a ‘V’ and consists of snow that is loose and powdery. These are considered to be avalanches but they are not considered to be of the dangerous sort and they can pass by people and objects leaving them undisturbed.

It is when the unstable layer is found deeper within the snowpack that the avalanche becomes far more dangerous. This is when slab avalanches are formed and the slabs consist of a layer that breaks entirely free from the snowpack and rushes down the mountain. It’s much like how the snow falls down a car’s windshield as it melts. The weak layer that causes the slab avalanche is sometimes called the failure layer and the strength of the avalanche will depend on the strength of the slab and the how deep within the snowpack the weak layer was. Slabs that are very stiff and have compressed tightly together will form large chunks of snow while looser snow will create smaller chunks. If the snow found within the slab is very wet, it will move much slower than snow that is dry but wet slabs are much more powerful.

Mountain slopes that are at a 25 to 60 degree angle from the ground have prime conditions for an avalanche but it is those with a slope between 35 and 45 degrees that have the most avalanches. Any slope that has a smaller slope than 25 degrees is generally not steep enough to create an avalanche and mountains with slope that exceed the 60 degree angle mark are so steep that they create continuous sluffs of snow rather than one large slab avalanche. There are also two different types of mountain slopes: leeward and windward. It is generally the leeward slopes that see the most avalanches occur on them.

And the third component to the avalanche is the trigger. Unlike many people believe, avalanches are not usually started by loud noises. There are some natural causes of avalanches such as falling trees, icy overhangs, or a sudden change in temperature but the avalanches that cause the most fatalities are those that people create. People cause the avalanche when they move over the slab with the weak layer, causing it to move and therefore moving the entire slab and the slabs around it. The slabs then begin to break apart and fall, following quickly behind the skier or other outdoor enthusiast. Once the avalanche has begun it has three parts to it. The starting zone is the first part and that is where the slab breaks away from the rest of the snow. The track is the second part of the avalanche and this is the path it makes as it rushes down the mountain. The runout is the third and final part of the avalanche and this is when the snow and debris it’s collected along the way slow down and come to a stop. This is the point that the snow sets and it becomes very hard. People have often compared it to concrete because it is so strong and when a victim is buried under it, this firmness is fatal as it’s nearly impossible to dig one’s way out and so, a rescue team is often the only hope.

Next: Avalanche Prevention and Control


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