how does Avalanches Work..

Avalanches can be surprising, sublimely beautiful and deadly. They can sweep trains off their tracks, crush buildings, uproot trees and bury people. Some avalanches have even covered entire houses with people still inside them. Even though movies and news reports say that they "strike without warning," most deadly avalanches start when victims trigger them. So what do you do if you're caught in an avalanche? How can you stay alive, and what does it take to rescue people who've been buried in the snow? In this article, you'll learn how avalanches form and what triggers them. You'll also learn how to survive and how to rescue others.

Avalanches can be surprising, sublimely beautiful and deadly. They can sweep trains off their tracks, crush buildings, uproot trees and bury people. Some avalanches have even covered entire houses with people still inside them. Even though movies and news reports say that they "strike without warning," most deadly avalanches start when victims trigger them. So what do you do if you're caught in an avalanche? How can you stay alive, and what does it take to rescue people who've been buried in the snow? In this article, you'll learn how avalanches form and what triggers them. You'll also learn how to survive and how to rescue others. The Properties of Snow To understand how avalanches form, you need to understand the properties of snow crystals. Depending on the temperature, humidity and other atmospheric conditions, snow crystals can have a variety of shapes, but all are generally hexagonal or six-pointed. In areas that get a lot of snow, the snow on the ground forms a snowpack. The layers within the snowpack have different qualities due to the shapes of the crystals in the layer. For example, six-pointed crystals can interlock more easily than needle-shaped crystals, so they create a steadier layer. On the other hand, when super-cooled water comes into contact with snow crystals in the air, it creates rime. Heavy rime deposits can cause pellet-like snow called graupel, which creates a very unstable layer. Snowpack layers also have different qualities because of changes that take place once the snow is on the ground. Changes in the weather lead to changes on the snowpack's surface. * If the top of the snowpack melts and re-freezes, it can form a layer of slick ice. * If air just above the snowpack reaches the dew point, the snowpack can develop hoar, which is a light, feathery crystal that does not bond well to snow. * If the top of the snowpack freezes and thaws repeatedly, it can develop clusters of frozen particles with space in between, which creates an unstable surface for the next layer of snow. Changes within the snow pack take place due to the temperature gradient -- the difference in temperature between the upper and lower layers. The snow near the bottom is relatively warm (close to 0° Celsius/32° Fahrenheit) because of residual heat from the ground. The temperature in the upper layers depends on the temperature of the air. Snowflakes within the snowpack undergo different types of metamorphosis depending on the size of the temperature gradient. In snowpacks with a high temperature gradient -- a large difference in temperature -- crystals tend to develop facets. The flat surface of a facet cannot bond well to other surfaces. Heavily faceted crystals located deep in the snowpack are called depth hoar and create dangerous instability. On the other hand, low temperature gradients and consistent sub-freezing temperatures cause rounding, which allows crystals to compress more tightly. The exchange of water vapor during rounding also creates bridges between crystals and parts of crystals, creating a firm, stable snowpack. Regardless of whether they are the result of temperature gradients, atmospheric conditions during snowfall or melting and refreezing, strong and weak layers of snow make avalanches possible. Next, we'll look at how avalanches form and what can trigger them. Avalanches have three ingredients -- snow, a sloped surface and a trigger. A weak layer within the snowpack, caused by ice, surface or depth hoar, faceted crystals or graupel also contributes to the process. If the weak layer is near the surface, it causes a sluff -- a cascade of loose, powdery snow in an inverted "V" shape down the slide of the mountain. Sluffs are like sand rolling down a dune, and they usually cause minimal damage to people and property. If the weak layer is deeper in the snowpack, it can cause a slab avalanche, which is far more dangerous. In a slab avalanche, a strong, cohesive layer of the snowpack slides down over a bed layer of snow, like thawing snow sliding down a car's windshield. Sometimes, the entire snowpack breaks free from the mountain and slides over the ground. The strength of a slab avalanche depends on the properties of the slab and the depth of the weak layer, also called the failure layer. Hard, cohesive slabs create very large chunks of solid slow, while softer slabs create smaller blocks. Slabs of wet snow cause generally slower avalanches than dry slabs, but they typically hit obstacles with more force. Avalanches usually start on mountain slopes that are at a 25 to 60 degree angle to the ground. Slopes less than 25° generally aren't steep enough to produce avalanches, and slopes steeper than 60 degrees usually sluff their snow constantly, giving slabs little chance to develop. Most avalanches begin on 35 to 45 degree slopes Most slab avalanches take place on leeward, rather than windward, slopes. They can have a natural trigger, like a sudden change in the weather, a falling tree or a collapsing cornice -- an icy overhang of wind-driven snow near the ridge. In spite of what movies and cartoons depict, the trigger is almost never a loud noise. In most fatal avalanches, people create the trigger. Once they begin, they have three segments: * A starting zone, often above the tree line and near the ridge, where the slab breaks away from the rest of the snow. * A track, or the course the avalanche follows down the mountain. You can often see avalanche tracks even in the summer because of missing trees. * A runout, where the sliding snow and debris eventually comes to a stop. When the snow stops, it compacts and sets up like concrete. This is what makes avalanches so dangerous to skiers, hikers and snowmobilers -- they generally cannot dig themselves out and must wait for rescue. Avalanche fatalities are most common in the winter months, but since early-season snowfalls and spring thaws are also dangerous, they can occur in every month of the year. In addition to the threat to human life, avalanches can cause tremendous damage to buildings and property. They can also close roads, cover train tracks and disrupt local economies. So, ski patrols and other organizations usually take steps to prevent major avalanches. One technique is to deliberately trigger small, controlled avalanches when no one is on the slope. Staff and researchers first study the snowpack either by digging pits and analyzing each layer or by using radar technology. They then start an avalanche with explosives or artillery fire. On small test slopes, they may also perform ski checking by deliberately skiing along fracture lines high on the slope. People performing ski checking always work with at least one partner, who remains in a safe location in the event that the skier gets caught in the avalanche. Other techniques involve preventing the conditions that lead to avalanches or interrupting the flow of snow. In some locations, fences, posts, nets, anchors and windbreaks change the way snow collects, reduce the size of the slab or provide physical obstacles in the event of an avalanche. Authorities in parts of the United States and Canada have also reforested areas that underwent heavy logging (clear-cutting in avalanche-prone areas is illegal in most of Europe). But avalanches can happen in spite of all preventive measures, especially in the first 24 hours after fast, heavy snowfall. Next, we'll look at how people can avoid triggering avalanches.