Given the right conditions, snow on any slope can become unstable and suddenly flow down the slope with devastating affects. This phenomenon is known as an avalanche. While there are a variety of factors that can lead to an avalanche, it is usually caused by one or more triggers. The trigger behind these powerful and sudden flows of snow can be divided into two categories; avalanches will either be triggered by natural or artificial means.
The factors that can cause or lead to an avalanche are quite numerous and may include: temperature, temperature change, weather, snowpack conditions, steepness of the slope, slope orientation, terrain, and vegetation. Individually, these factors may not lead to an avalanche, but when combined together, just about any snowy slope can become prone to an avalanche if it receives the right trigger.
Most snow on a slope has different layers to it that represent the different snow falls and the entire structure is usually called a snowpack. Each snowfall will be different because the conditions present when it was snowing where likely different than when the others layers were deposited on the slope. The differences can cause issues with the overall snowpack as one layer could be weaker than another and cause the entire slope or snowpack to become unstable.
The location and slope itself can also lead to an avalanche. Some areas are just naturally prone to avalanches because they have the right conditions needed. Areas with a steep slope of about 30 to 45 degrees are always more prone to avalanches than slopes of lesser inclines. Slopes that face the cardinal directions of north and east, as well as any direction in-between, are also more prone to avalanche than any other slopes. These slopes receive less sunlight and therefore are not subject to melting and freezing which helps the snow to become compacted and strengthen the bond between layers.
Many avalanches occur naturally and without any human or animal influence. The triggers responsible are the changing conditions and nature itself. Probably one of the biggest triggers for a natural avalanche is new snowfall. During a snow storm, or within a twenty-four hour period after a new snow fall, are the most common times for a natural avalanche to occur. The additional snow weight could cause enough stress to the snowpack for it to lose its stability on the slope. With too much weight and not enough support at lower levels of the snowpack it will begin to move and then gravity will do the rest.
While it may not seem like much, wind can deposit or move around a large amount of snow. Winds can cause more snow buildup, or deposit more snow in a given period, than an actual snow storm. In order to deposit that snow, the wind may also scour certain areas of a slope, removing the snow in one area only to deposit it in another. The actions of the wind are therefore able to cause slope instability without any other factors changing or occurring. With one area of the snowpack unstable because snow layers have been removed and one area heavier than it was, the result could be an avalanche. It is actually quite common for an avalanche to occur merely because of the wind.
The snow itself can also be a factor when it comes to avalanches. As stated previously, the snowpack on a slope has different layers. There is one type of snow, which is commonly referred to as depth hoar that can cause avalanches. This snow does not bond well with other snow and is usually the weakest layer in a snowpack. The depth hoar usually falls during early snow storms in the winter so it is also present in lower layers of the snowpack and can weaken an entire slope. Rain in-between snowfalls or drops in temperature can also cause problems with the snowpack as the water from the rain or snow melt will lubricate the rest of the snow, increase the weight on the slope, and promote avalanches.
In contrast to natural causes, artificial avalanches are caused by human or animal interactions with a slope. Activities such as skiing, hiking, walking, climbing, and others that humans or animals perform on a slope can weaken the structure of the snowpack. Around 150 people each year worldwide are killed by avalanches and a majority of avalanche victims were the cause for the avalanche being triggered. While some people may believe that sound is an avalanche trigger, it actually is not because the sound vibrations or pressure from human or animals are not enough to cause an avalanche. The amount of sound pressure needed to cause an avalanche is several orders of magnitude larger than what a person or animal can produce with their voice or most other means.
In some cases, people actually try and purposefully trigger an avalanche. In some areas or places where transportation lines (roadways or railway tracks) are close or on avalanche prone areas, explosives or artillery pieces are used to purposely cause an avalanche. The impact and concussive force on the slope is enough to trigger a small or medium sized avalanche and cause areas of a slope or snowpack, which are unstable, to run down the slope. This leaves the stronger areas intact and the remaining snowpack is safer than it was. The transportation lines can then be cleared and are safer for people to use until the next large snowfall. The same method may also be used to merely evaluate different slopes to determine how stable they are. Another method called ski checking may also be used to test slopes for avalanches. This method involves skiing across the higher areas of slopes, as opposed to down a slope, in order to try and trigger an avalanche. This method is done by experienced skiers with a partner.