What is an Avalanche?


Technically, an avalanche is any amount of snow sliding down a mountainside.  It can be compared to a landslide, only with snow instead of earth.  Another common term for avalanche is “snowslide”.  As an avalanche becomes nearer to the bottom of the slope, it gains speed and power, this can cause even the smallest of snowslides to be a major disaster.


There are two common types of avalanches, a Surface Avalanche that occurs when a layer of snow with different properties slides over another layer of snow.  For example, when a layer of dry loosely packed snow slides over a dense layer of wet snow.  The other common avalanche is known as a Full-Depth Avalanche which, as it’s name would lead you to believe, occurs when an entire snow cover, from the earth to the surface, slides over the ground.


An avalanche can be composed of many different kinds of snow depending on the region, temperature and weather.  It could be compiled of loosely packed light fluffy snow, which can still be very dangerous even though it may not appear threatening.  It could also consist of a thick “slab” which is an area of tightly packed together snow that separates itself from the surroundings.


Why Do Avalanches Happen?


The snow packed down on the surface cannot support itself with all the weight. When another factor is introduced, such as a person’s step, this helps to loosen the snow and an avalanche occurs. Major temperature changes, rapid wind speed and man-made influences are the main causers of why avalanches occur.

Most avalanches begin within weak layers of snow, which evolve within the snowpack or form on top of the snow and become buried.  Eventually these weak layers can no longer hold up the weight on the overlying snow, and will give way causing the snow above them to break free and slide downhill.


Snow Pack Test


The snow pack test is checking for recent avalanche activity, fresh snow drifts and large snow masses. Depth of each footstep, cornice build up and wind patterns show how well the snow is packed together. If snow breaks up when you step on it avoid steep areas for an avalanche danger is very high. To do a snow pack test is very simple. Cut out a square slab of snow. Check the pack on the layers using a shovel, fist or pick depending on the hardness you are dealing with. By figuring out the pack of the snow can make silent avalanches become more exposed to your senses.




Where and When Avalanches Can Happen


Avalanches happen on mountains with extreme amounts of slow fall and build-up. Wherever snow is lying on ground on an extreme and sufficient angle there is potential for a sleeping avalanche. The steadily increasing numbers in popularity of winter activities along with the growth of interest in skiing has resulted in a much greater hazard. There are many sites around the world that are potential or have already experienced avalanches. Europe, France, Swiss mountains, Western Canada, Utah, Alaska, and Colorado are just a few places that have high probability of avalanches. All of these locations go though a thaw and freeze during the year at the bases of though mountains. This is very stressful on the snow built up above and packs it tighter together.




Three main factors effect whether or not avalanches are probable to occur. These three factors are the weather, the snow pack and the terrain. The weather is the most important when deciding whether avalanches are likely to happen. The height of the snow pack is dependent on the weather also. From the weather the temperature, wind speed and direction are the factors to watch. With a quick change in any of the weather factors an avalanche could is expected. For example, if the temperature were to have a rapid increase then a wet slab avalanche is likely to occur.




Many avalanches that occur are cornice (an over-hanging mass of snow above a cliff, pictured above) triggered. These happen during snowstorms, strong winds (where snow drifts form quickly), and usually occur one to two days after or shortly after a quick thaw or temperature rise. Finally the terrain factor of avalanches depends on the slope angle, ground surface and slope profile. Any slope that is between 25 to 45 degrees is susceptible to snow movement. The smooth or roughness of the ground and rocks located under the snow will determine how easily the snow will move. The larger the rocks the slower and less chance the snow have to move. But a warning, that once enough snow has fallen and the rough terrain has been covered these rocks will not yield the avalanche. While the slope profile should also be considered. If the slope is a convex slope more tension occurs in the snow causing the chances of a slab avalanche to increase. All slopes should be avoided after storms or high wind speeds. Avalanche locations will move according to the direction of the wind.




Obviously after Mother Nature has done a chore (such as a storm or high winds) the chances of an avalanche increase. Rapid snow build-up (usually more that 2cm/hr) will produce very unstable conditions. More than 30cm of build up within 24 hours is very hazardous. Lose snow and air packets caught under the snow will make danger levels increase. Sudden temperature change, wind speed and direction also alter snow stability. Most importantly above all is human interference. 90% of all avalanches happen during snowstorms, with 90% percent of all avalanches being people trigger accidents.




Danger of Avalanche=Danger of Death


After one hour, only one in three victims who are buried in an avalanche are found alive.  The most common cause of death of these victims is asphyxiation (suffocation), wounds and hypothermia. 



How do you avoid getting caught in an avalanche?


The most simplistic way of avoiding getting caught in an avalanche is to recognize where one is most likely to occur.  If you understand that there is a high possibility of an avalanche, you are less likely to fall victim to the surprise occurrence of one.  This can be accomplished by understanding how the path of an avalanche appears.  Most avalanche paths are obvious, appearing as an open slope, bowl or gully shape.  Other common give-aways are bent or damaged trees. 



Although it is a very popular myth, loud noises do not usually cause avalanches.  The pressure of someone or something that is making the noise usually causes them.





http://www.epicscope.com/gallery/video.htm (Swiss Avalanche)

http://juneaualaska.com/media/video/avalanche/avalanche.mov (be patient)























Atkins, Dale  “Colorado Avalance Information Centre”

            Avalanche facts http://geosurvey.state.co.us/avalanche/facts.html

            (17 Apr. 2003)


Avalanche Center  “Avalanche Organization”

            Avalanches http://www.avalanche.org

            (30 Apr. 2003)


Corney, Peter “Epicscope Video Gallery”

            Swiss Avalanche http://www.epicscope.com/gallery/video.htm

            (25 Apr, 2003)


National Geographic “Avalanches for Kids”

            All about Avalanches http://www.nationalgeographic.com/ngkids/0301/

            (17 Apr. 2003)


National Snow and Ice Data Center, The  “NSIDC Information”

            Avalanche Awareness http://nsidc.org/snow/avalanche/

            (26 Apr. 2003)


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            Avalanche Video http://juneaualaska.com/media/video/avalanche/

            (26 Apr. 2003)


The Scottish Sports Council. “About Avalanches”

Avalanches http://facstaff.uww.edu/rambadtd/Eurogeog/Kntowski/paper.htm

(17 Apr. 2003)


University of Glascow “Sport Scotland”

            About Avalanches http://sais.dcs.gla.ac.uk/about_avalanches/

            (26 Apr. 2003)