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Have you ever noticed the snowpack on a car windshield after a snowfall? While the temperature is cold, the snow sticks to the surface and doesn’t slide off. After temperatures warm up a little, however, the snow will slide down the front of the windshield, often in small slabs. This is an avalanche on a miniature scale. On the other hand, a mountain avalanche in North America might release 300,000 cubic yards of snow. That’s the equivalent of 20 football fields filled 10 feet deep with snow. However, such large avalanches are often naturally released. They are primarily composed of flowing snow but given their power, they are also capable of carrying rocks, trees, and other forms of debris with them. In mountainous terrain avalanches are among the most serious objective hazards to life and property, with their destructive capability resulting from their potential to carry an enormous mass of snow rapidly over large distances.
Avalanches are classified based on their form and structure, also known as morphological characteristics. Some of the characteristics include the type of snow involved, the nature of what caused the structural failure, the sliding surface, the propagation mechanism of the failure, the trigger of the avalanche, the slope angle, direction, and elevation. All avalanches are rated by either their destructive potential or the mass they carry. Avalanches only occur when the stress on the snow exceeds the strength either within the snow itself or at the contact point where the snow pack meets the ground or the rock surface. An avalanche has three main parts: the starting zone, the avalanche track, and the runout zone. The starting zone is the most volatile area of a slope, where unstable snow can fracture from the surrounding snowcover and begin to slide. The avalanche track is the path or channel that an avalanche follows as it goes downhill. The runout zone is where the snow and debris finally come to a stop.
Several factors may affect the likelihood of an avalanche, including weather, temperature, slope steepness, slope orientation (whether the slope is facing north or south), wind direction, terrain, vegetation, and general snowpack conditions. However, weather remains the most likely factor in triggering an avalanche. During the day, as temperatures increase in a mountainous region, the likelihood of an avalanche increases. Although avalanches can occur on any slope given the right conditions, in North America certain times of the year and certain locations are naturally more dangerous than others. Wintertime, particularly from December to April, is when most avalanches will occur with the highest number of fatalities occurs in January, February and March, when the snowfall amounts are highest in most mountain areas. In the United States, 514 avalanche fatalities have been reported in 15 states from 1950 to 1997. In the 2002–2003 season there were 54 recorded incidents in North America involving 151 people.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.