Avalanches

Snow Avalanches

Snow falls and accumulates on mountain slopes as snowpack wherein the commonly light and fluffy flakes of powder snow pile up, become unstable in various ways, and eventually flow downhill as a snow avalanche. Because of the ubiquity of snow, and therefore of snow avalanches, in almost all high mountain regions, the science of analysis and prediction of such events has become quite sophisticated. Mountain weather, snow formation and snow pack, avalanche formation, and avalanche terrain, motion, and effects are all important elements in the prediction of snow avalanches wherein the elements of stability evaluation and snowpack observations enable better avalanche forecasting.

Avalanche-relevant mountain weather first concerns the deeper snows and relatively milder temperatures of the maritime snow climates of coastal mountains, as opposed to the lighter snows and colder climes of interior ranges, both of which types can set up different conditions of snow instability. Temperature conditions are an essential element in the consideration of avalanches—both while the snow is falling, being redeposited, or both, as well as much later in the heat exchange at and within the snowpack. The snow itself falls in many different forms of crystal type, from the light and fluffy powder flakes to the heavy, coarse graupel, or hail, with an enormous variation in between. Heat exchange at the snow surface occurs as heat enters or leaves the snowpack by conduction or convection of radiation as well as by condensation resulting from diffusion of water vapor. Metamorphism of the snow crystal results. Formation of feathery crystal of unstable surface hoar frost (e.g., frozen dew on a surface) can be a problem, especially where buried by later snowfalls. The melting and refreezing of moisture between snow crystals in contact with each other causes the formation of grain bonds, or sintering, which are also crucial elements in snow strength, so that the temperature, temperature gradients, grain geometries (i.e., orientation of grains in a deposit), and pore-space (i.e., gaps between grains in a deposit) configuration can all figure in avalanche formation. The highest crystal growth rates occur where there are large temperature gradients, higher temperatures, and large spaces between [Page 176]crystals. This produces unstable angular or faceted (i.e., grains with one or more smoothed surfaces) grains or depth hoar.

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