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Author Topic: Avalanches in the rain.  (Read 2030 times)
surf88
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« on: April 07, 2009, 10:48:08 AM »

A common thing I hear a lot from people who dont study these things, is that if the snow is dense it is stabile.  So people assume that if its raining, the snow is wet, and is dense, and its not going to slide. 
Todays avi report has a great lesson in it:

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This is a good lesson to look back on. The only snow we had received recently was 0.6" (1.5 cm) on Saturday night. This snow was pretty dense and was able to stick in the steeps in Tuckerman. While most areas around the mountain had no new snow, the Headwall and Lip picked up slabs that were as deep as 2' (61 cm). This new snow wasn't much of a stability concern on Sunday as it didn't have significant elastic energy and was bonded well to the old surface. Numerous people skied the new slabs without any problems. Yesterday's rain did its handy work by rapidly warming the snow, breaking down bonds and providing stress and lubrication. The end result was slab avalanches in the new snow. I see two take home messages here. 1. Always be weary of rain on cold snow. 2. Don't assume there are no stability concerns when recent snowfall is seemingly insignificant. Mt. Washington's winds are well versed in depositing new snow where it counts.
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atruss
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« Reply #1 on: April 07, 2009, 01:36:46 PM »

Fantastic point to make Surf !
Reps.

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Tommy T
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« Reply #2 on: April 11, 2009, 04:32:48 PM »

That's a good reminder.  

The effect of rain depends on the structure of the snowpack, temperature and temperature gradient in the snow pack and temperature of the rain.  It is fair to say that the effect of rain on the stability of snow with any significant depth is rarely good.  

Warming and lubrication are obvious points but often the increased stress due to the weight of the water retained in the pack is the critical factor.  Weight is an especially important issue on places where the terrain is convex --  a roll-over at the top or a fairly quick increase is slope angle part way down.  These factors occur on many slopes in the Presidentials.

Tommy T.
« Last Edit: March 25, 2013, 12:10:31 PM by Tommy T » Logged

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