Lecture 11 - Snow Avalanche Definition A mass of snow many...

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Snow Avalanche Definition: A mass of snow many cubic meters in volume that separates from a snowpack and flows downslope. Rocks, soil, ice, and debris can travel in a similar motion; however the term ‘avalanche’ is generally reserved for snow. The intensity of the hazard is dependent on slope steepness, snowpack stability and weather.
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Snow Avalanches There are two types: -an avalanche travelling as a coherent block -an avalanche that becomes wider as it travels downslope It is estimated that over 99% of avalanches are not seen by anyone. It is likely that over 1 million avalanches large enough to kill a person occur annually in western Canada alone.
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Snow Avalanches
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Snow Climatology Snowfall accumulation depends on latitude, altitude and proximity to bodies of water. Temperature decreases with altitude therefore high mountains have permanent snow cover. Snow accumulates on mountain slopes that are at angles of less then 60°.
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Snowfall in North America
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Snow Cover The probability of a White Christmas takes into account average snow cover. Snow cover maps are updated daily.
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Types of Avalanches Point-release Avalanches These begin as an initial failure after a heavy snowfall. The sliding snow then causes more failures in the adjacent snowpack causing the trough to widen.
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Types of Avalanches Slab Avalanches These occur when a snowpack fractures along a weak layer parallel to the surface. These avalanches move as cohesive blocks leaving behind a scarp. They are the most dangerous avalanches.
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Avalanche Potential New snow that has not been able to bond to the layer below is susceptible to sliding. Wet, compacted snow is less likely to slide than dry, powdery snow. A mass of snow that is above the vegetation level and above large boulders is more likely to slide.
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Weak Layers Slab avalanches require a buried weak layer. Such a layer can form from wind or from hoar . Wind Blowing snow can accumulate on the lee slope of mountains. Wind can deposit a layer of light ice crystals on a layer of more compacted snow. The boundary between the two layers could become a horizon along which failure could occur.
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Weak Layers Hoar Layers composed of hoar have less strength than the rest of the snowpack. Hoar can form deep in the snowpack (in air pockets) or on the snow surface. Hoar changes little over time; therefore overlying snow can leave the buried hoar as a weak layer.
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Avalanche Motion Rapidly moving avalanches (i.e. speeds of over 35 km/h) often generate clouds of powdered snow. Fastest avalanches have been measured at speeds of 200km/h. Some avalanches are powerful enough to climb opposing slopes.
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Avalanche Triggers Most avalanches occur soon after snowstorms. Some may occur when daytime heating from the Sun warms the upper part of the snowpack.
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