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Tuesday, January 3, 2012

Avalanche!!!! The Terrain Variable 



Learning to identify avalanche terrain is most important in recognizing and evaluating avalanche hazard.  It's asy to recognize where avalanches are common and where they are not. 


Slope Steepness 
The steepness of a slope is a key factor in determining avalanche danger.  It is a common misconception that avalanches occur on steep slopes.  The fact is that most avalanches occur on slopes of 30 to 45 degrees.  It is within this range of steepness that the balance between the strength of the layers of snow and the stress of gravity is most critical.  Steeper slopes tend not to hold a significant amount of snow due to gravity.  


Slopes of less than 30 degrees may not be as prone to slide, but may be as dangerous in the right conditions especially in the spring when wet avalanches occur. Unfortunately, 30 to 45 degrees also provides the most challenging ski terrain. 


Remember, too, that short slopes may be as dangerous as long ones.


For a lesson on how to use an Inclinometer for your slope steepness check out this post from Skiing the Backcountry http://www.skiingthebackcountry.com/skiing-resources/how_to_use_inclinometer


Slope Orientation 
The orientation of a slope is also an important factor.  By "orientation," we mean whether the slope is having snow blown onto it or blown off from it.  We also mean whether the slope faces north or south.  Leeward slopes, or those drifted by winds, are more dangerous because of the added depth and weight of the snow. North-facing and shaded slopes tend to be more dangerous during the mid-winter periods, mostly because of the colder surface temperatures.  South-facing slopes tend to be more dangerous during spring thaw, specially on a sunny day, due to solar heating and the introduction of water (melting snow on the surface) into the 
snowpack. 


Slope Profile 
We must also evaluate the slope profile.  That is, whether the slope is flat or curved.  Convex slopes are likely to fracture at the bulge.  Concave slopes provide a certain amount of support at the base, though they are still capable of avalanching. 


Be especially cautious around bowl-shaped slopes or those with narrow, deep gullies.  Both of these features help trap blowing snow, especially on the leeward side of the mountain. 


Vegetation 
Vegetation can be a key indication of avalanche hazard.  The first thing to look for is "ground cover."  Large rocks, trees and heavy brush help anchor the snow, at least until they become covered.  Avalanches can start even in the trees, since sparse trees can actually weeken the snow cover.  To be reasonably safe, the trees must be so dense as to make it difficult to maneuver. Equally important, yet often neglected, is knowing what the slope looks like without the snowpack.  If the slope is a grassy hill in the summer, it is more likely to slide due to the lack of anchors. Conversely, if the slope is known to have many large rocks, tree stumps or bushes, it may be more stable.  


This is true only as long as the snowpack is not so deep as to cover these natural anchors. 


Elevation 
What about elevation?  Avalanche danger generally increases with elevation.  Most large avalanche starting zones are above timberline.  This is due to the fact that there is generally a greater snow cover above treeline.  In addition, there are less natural anchors above treeline. 

Our next post will wrap up our series on the variables that help develop a potentially unstable snowcover with a discussion of snowpack. By understanding these variables, backcountry users will have a better chance of predicting avalanche danger.


For more information on avalanche safety check out the Mountain Rescue Associations public education program @ http://mra.org/images/stories/training/Avalanche.pdf as well as our Backcountry Skiing & Riding Safety Video  


Courage - Commitment - Compassion
     Mountain Rescue Association 

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