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Old Snow
 
 
Old Snow
 
Old Snow Depth If a storm deposits a lot of new snow on top of a shallow snowpack, beware. Shallow snowpacks tend to grow a type of snow crystal avalanche experts call depth hoar. Depth hoar is very weak and acts as a weak layer in many avalanches, especially in continental climates (areas such as Colorado) that have shallow snowpacks. Deeper snowpacks that form in maritime snowpacks (areas that receive more snowfall and generally wetter snow, areas like the Pacific Northwest and California) tend to have less of a problem with avalanches because depth hoar formation is much rarer. http://www.avalanche.org/%7Euac/encyclopedia/depth_hoar.htm shapeimage_3_link_0
Old Snow Surface New snow will stick, or bond, better to some surfaces than others. New snow will not bond very well to rain or sun crusts for example. These crusts are very slippery and act as a good bed surface for avalanches. Also, beware of a phenomena called surface hoar that forms on top of the snow during cold, cloudless nights. Surface hoar is the wintertime equivalent of frost. It can be identified by its sparkly appearance on top of the snow. We’ll talk more about surface hoar in the next unit. As you travel around the backcountry, take note of what the snow surface is on different aspects. Next storm, you’ll have a much idea of which slopes may be avalanche prone. http://www.avalanche.org/%7Euac/encyclopedia/rain_crust.htmhttp://www.avalanche.org/%7Euac/encyclopedia/sun_crust.htmhttp://www.avalanche.org/%7Euac/encyclopedia/surface_hoar.htmshapeimage_4_link_0shapeimage_4_link_1shapeimage_4_link_2
 
Hard, slippery crusts make good bed surfaces for avalanches.  Beware when you know there is a buried crust beneath new snow.
 
Surface hoar is recognized by its sparkly appearance on top of the snow following clear, cold nights.  Once buried, it becomes
a persistent and dangerous weak layer.
Photo: Mike Jenkins
unesco Avalanche Atlas
 
Copyright 2008, by the Contributing Authors. Cite/attribute Resource. factcouraud. (2007, May 08). Old Snow. Retrieved November 23, 2009, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/avalanche-and-snow-dynamics/AvClass/Weather/Old%20Snow.html. This work is licensed under a Creative Commons License. Creative Commons License
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