Avalanche

[1] Avalanches can be triggered spontaneously, by factors such as increased precipitation or snowpack weakening, or by external means such as humans, other animals, and earthquakes.

Primarily composed of flowing snow and air, large avalanches have the capability to capture and move ice, rocks, and trees.

Though they appear to share similarities, avalanches are distinct from slush flows, mudslides, rock slides, and serac collapses.

One of the aims of avalanche research is to develop and validate computer models that can describe the evolution of the seasonal snowpack over time.

[5] A complicating factor is the complex interaction of terrain and weather, which causes significant spatial and temporal variability of the depths, crystal forms, and layering of the seasonal snowpack.

Despite the low speed of travel (≈10–40 km/h), wet snow avalanches are capable of generating powerful destructive forces, due to the large mass and density.

[10] These degrees are not consistently true due to the fact that each avalanche is unique depending on the stability of the snowpack that it was derived from as well as the environmental or human influences that triggered the mass movement.

Doug Fesler and Jill Fredston developed a conceptual model of the three primary elements of avalanches: terrain, weather, and snowpack.

In coastal mountains, such as the Cordillera del Paine region of Patagonia, deep snowpacks collect on vertical and even overhanging rock faces.

Avalanches are unlikely to form in very thick forests, but boulders and sparsely distributed vegetation can create weak areas deep within the snowpack through the formation of strong temperature gradients.

[20] The rule of thumb is: A slope that is flat enough to hold snow but steep enough to ski has the potential to generate an avalanche, regardless of the angle.

[22] At temperatures close to the freezing point of water, or during times of moderate solar radiation, a gentle freeze-thaw cycle will take place.

[citation needed] Any wind stronger than a light breeze can contribute to a rapid accumulation of snow on sheltered slopes downwind.

During clear nights, the snowpack can re-freeze when ambient air temperatures fall below freezing, through the process of long-wave radiative cooling, or both.

[27] Attempts to model avalanche behaviour date from the early 20th century, notably the work of Professor Lagotala in preparation for the 1924 Winter Olympics in Chamonix.

[28] His method was developed by A. Voellmy and popularised following the publication in 1955 of his Ueber die Zerstoerungskraft von Lawinen (On the Destructive Force of Avalanches).

[29] Voellmy used a simple empirical formula, treating an avalanche as a sliding block of snow moving with a drag force that was proportional to the square of the speed of its flow:[30] He and others subsequently derived other formulae that take other factors into account, with the Voellmy-Salm-Gubler and the Perla-Cheng-McClung models becoming most widely used as simple tools to model flowing (as opposed to powder snow) avalanches.

[33] Preventative measures are employed in areas where avalanches pose a significant threat to people, such as ski resorts, mountain towns, roads, and railways.

The simplest active measure is repeatedly traveling on a snowpack as snow accumulates; this can be by means of boot-packing, ski-cutting, or machine grooming.

Interferometric radars, high-resolution cameras, or motion sensors can monitor instable areas over a long term, lasting from days to years.

Modern radar technology enables the monitoring of large areas and the localization of avalanches at any weather condition, by day and by night.

Complex alarm systems are able to detect avalanches within a short time in order to close (e.g. roads and rails) or evacuate (e.g. construction sites) endangered areas.

[40] During World War I, an estimated 40,000 to 80,000 soldiers died as a result of avalanches during the mountain campaign in the Alps at the Austrian-Italian front, many of which were caused by artillery fire.

[43] In the northern hemisphere winter of 1950–1951 approximately 649 avalanches were recorded in a three-month period throughout the Alps in Austria, France, Switzerland, Italy and Germany.

[44] A large avalanche in Montroc, France, in 1999, 300,000 cubic metres of snow slid on a 30° slope, achieving a speed in the region of 100 km/h (62 mph).

[50] In Europe, the avalanche risk is widely rated on the following scale, which was adopted in April 1993 to replace the earlier non-standard national schemes.

[60] Predictions also show an increase in the number of rain on snow events,[61] and wet avalanche cycles occurring earlier in the spring during the remainder of this century.

[65] The warm, wet snowpacks that are likely to increase in frequency due to climate change may also make avalanche burials more deadly.

Denser avalanche debris decreases the ability for a buried person to breath and the amount of time they have before they run out of oxygen.

[66] Additionally, the predicted thinner snowpacks may increase the frequency of injuries due to trauma, such as a buried skier striking a rock or tree.