An avalanche is the sudden flow of a large amount of snow or ice down a slope or cliff. There are several factors that can effect the creation of avalanches including: weather, temperature, steepness of the slope, the direction of the slope, the wind direction, the terrain, the vegetation, and the snowpack conditions. If the weight of new snow being deposited by wind, or snowfall on a slope is being added faster than the weakest layer of snow on the slope can adjust to, then the snowpack fractures and creates an avalanche. Contrary to common rumors, noise can not trigger an avalanche. A simple shout will not make the snow come down over your head. In fact, ninety percent of all avalanches that cause a person to die are set off by the weight of the victim, or someone in the victims party. When an avalanche happens naturally, new, or windblown snow strains the weak layers, or it happens as the result of a rapid rise in temperature. Although wind, snow, and rapid warming do cause avalanches, an avalanche doesn't happen every time the wind blows, snow falls, or the temperature rises rapidly. Whether or not an avalanche happens depends on the condition of the snow that was already there. If every layer in the snow pack was pretty stable, chances are, and avalanche won't happen.
Even if a weak layer is at the bottom of the snowpack, it can still cause an avalanche and usually the avalanche will be even more dangerous that if the weak layer is near the top of the snowpack because more snow will slide down the slope. A particular kind of snow called depth hoar (which is coarse and grainy, like dry sand) is a snow crystal that doesn't bond very well with other snow particles. The weather conditions that are necessary to create depth hoar unfortunately usually occur pretty early in the winter, which means that the weak layers are buried under tons of other snow.
"Sluffs," sometimes called "loose snow avalanches" happen when loose snow slides down a slope. The type of avalanche that most people think of when they're talking about avalanches, are called "slabs." In a slab avalanche, a sheet of snow, sort of like a pane of glass, breaks off with a loud noise, and then slides down the mountainside. There are two kinds of slab avalanches: wet and dry. Most people are killed by dry slab avalanches instead of wet ones.
Dry slab avalanches occur when the weak layer of snow breaks, usually because too much weight has been added to the snowpack too fast which overwhelms the weak layer. Snow is extremely sensitive to how fast more weight is piled on. If a slope accumulates two feet of snow in two weeks, it is very unusual for an avalanche to start, unless something else, like the weight of a human, triggers it. Two feet of snow in two days is a little bit more of a problem and an avalanche might happen on that slope. If two feet of snow accumulates in two hours, there will almost definitely be an avalanche. Wet slab avalanches occur when unfrozen water dissolves the bonds between the individual snow grains, and decreases the strength of the weak layer until it finally breaks. If the sun is out for a long time, or the temperature warms up and stays up, a wet slab avalanche can easily be caused. There is a very delicate balance between the snow melting and refreezing, which helps create strong bonds, and the snow melting and not refreezing fast enough so it creates a wet slab avalanche. The snowpack may remain pretty unstable if the temperature stays below freezing, especially below zero degrees farenheit, because the snow will not bond together as well.
Although avalanches are sometimes caused by snowfall from storms, more often they're caused by wind, or wind-loading. Wind-loading happens when the wind erodes snow from the side of an obstacle (like a large rock) that is upwind, and placed on the lee (downwind)side. Almost all avalanches happen on a slope that is between thirty five and forty five degrees (a black diamond run at a ski resort is about thirty five degrees). Once the angle is below thirty degrees, gravity doesn't have as much pull, which helps keep avalanches from occurring. On slopes that are steeper than fifty degrees, sluffs happen so often, that the snow isn't usually able to build up enough to cause and avalanche. Trees and rocks that stick up though the snow, can act as anchors and help stabilize the snowpack. But there needs to be many trees or rocks in order to do any good. If there is a lone tree or rock it can instead help create a fracture point for the snowpack.
There are three main parts to an avalanche: the starting zone, the avalanche track, and the run out zone. The starting zone of an avalanche is where the unstable sheet of snow breaks loose from the rest of the snowpack and starts to slide down the slope. most starting zones are at pretty high elevations on the slope, but given the right condition, the snow can break at any point. The avalanche track is the path the avalanche takes. If avalanches occur frequently at a certain place, large sections of trees may be missing, the snow might be piled up in two places forming a sort of chute. If avalanches break off from the same point multiple times during a winter, they will tend to take the same path. The run out zone is where the snow slows down and stops moving, this is where the snow and debris that were picked up usually pile the highest. If a person was caught in an avalanche, most of the time they end up in the run out zone.
In the northern hemisphere, at latitudes between twenty and sixty degrees, the direction a slope faces has a lot to do with when avalanches happen. In the winter, north facing, shady slopes usually have more avalanches because the snow doesn't melt and freeze, so it doesn't bond as much, and the snowpack isn't as stable. In the spring, slopes that face south and are sunny have more avalanches because the strong sun melts the snow and the resulting water creates a wet slab avalanche. At latitudes near the equator, or in the Arctic, the direction a slope is facing doesn't really matter, with respect to the sun. There could be other reasons in that area that a slope facing one direction is more stable than a slope facing another.
Avalanches can move at speeds over one hundred miles per hour. Most dry slab avalanches travel at speeds between sixty and eighty miles per hour. They usually reach these speeds within five seconds of the time they fractured. Wet slab avalanches usually travel around twenty miles per hour. A large avalanche can release three hundred thousand cubic yards of snow (that would be about twenty football fields each piled ten feet high with snow), so if one is coming down at you, there's really no way to move out of it's path.
Most avalanche victims are between the ages of twenty, and twenty nine (the average age is 31). Only one tenth of one percent of avalanche deaths happen at ski resorts, or on highways, because avalanche professionals continually use explosive to knock down the loose snow and keep avalanches from forming in those areas. Once you leave a maintained area though, it's a whole different story. Three fourths of all avalanche victims are back-country recreationists, including, snow mobiles, climbers, snow boarders, snowshoers, skiers, and hikers. Snowmobilers are at the top of the list, having almost two times the number of fatalities as any other activity. Most avalanches happen between the months of December and April because of the high amounts of snow on the ground during the winter. The highest number of avalanche deaths occur in January, February, and March, but in the U.S., there are recorded avalanche fatalities for every month of the year. In the summer, the main people caught in avalanches are climbers who go high up into the mountains where there's still snow.
Even extremely dense snow is about sixty to seventy percent air, so when people who are buried in avalanches die, it isn't usually lack of oxygen that kills them. Instead, the die because carbon dioxide from the air they breathe out builds up n the snow that's near their mouth, and they die of carbon dioxide poisoning. When a person has been buried in a avalanche,the time that it takes to get them out plays a big part in whether or not they survive. If a victim is dug out within the first five minutes of when they were buried, they have a ninety three percent chance of still being alive, but after forty-five minutes, only about twenty to thirty percent of victims are still alive. After two hours, almost no one is still alive. Avalanche beacons (transceivers) are the rescue tool that is used most often. If a victim was wearing a transceiver, it will emit a frequency that the transceivers of the searchers pick up, directing them to the victim.
In the United States between the years of 1950 and 1997, five hundred fourteen people have been killed in avalanches in fifteen states. Colorado had recorded about a third of those fatalities.
The first image is a picture of an avalanche sliding down a slope, courtesy of http://www.cs.umd.edu/class/spring2001/cmsc838b/Project/Parija_Spacco/images/avalanche.jpg The second, third, and forth images are graphs showing different avalanche statistics, all of them courtesy of http://www.avalanche.org/%7Euac?Common-questions.html