Persistent Weak Layers

Weather is the architect of avalanches. Snow isn’t the only creator of avalanche hazard. More often it is the weather events that happen between the snowfalls that create instabilities….rain, wind, frost, cold temps, warm temps… Some of these layers morph and bond within a few days….some layers persist for many weeks. Every year the blueprint is different. What is the blueprint this year??
CLICK HERE to read the report, ‘Persistent Weak Layers and the Winter of 2007-08’ compiled by CAC forecaster Karl Klassen.
It is a detailed look at the deep weak layer problems that can be a problem in our snowpack.

_Note: While the blueprint discussed in Karl’s report is from a 2007-2008, the info is all relevant.
A good discussion of frost and terrain use is included.
Be warned….get your reading glasses and save this report to your hard drive! This is a detailed review!!
Should you have any questions after reading this report, feel free to shoot us a quick email for clarification._ :)

The video clip below was shot by Genevieve Chabot for the Gallatin National Forest Avalanche Center (Montana, USA)
The discussion relates to a snowmobiler triggered avalanche at Buck Ridge ~ Feb 16/08. The slide involved a very dense windslab sitting on a layer of FACETS. At the deepest point the crown fracture measured 7 feet.

Another example of a persistent weak layer is a layer of surface hoar or hoar frost. If the frost is not destroyed by sun, wind, warm temps, rain, human activity… AND is buried by a new layer of snow, it can remain as an unstable layer in the snowpack for a long time. Frost tends not to bond to itself or the other snow layers and is resistant to metamorphism or structural change once protected within the snowpack.

Frost freshly formed on the surface.

This picture is from Chappel Creek, near Valemount, BC on Febuary 17, 2008. (Considered the Feb 25 layer as this is approximate date that the frost layer was buried.)

Frost preserved and sandwiched between snow layers.

During our avalanche course on March 3/08 a snow pit was completed approx 12 km up the Quartz Creek trail (very near the old sign-in box.) In this pit the frost layer buried on Feb 25 consisted of very large crystals sitting on a 2cm ice crust from previous rains and warm temps. A terrible combination.

On March 3, 2008 this extremely weak layer was only buried by a few cms of low density snow.

Our sleds, footsteps and and the jump test in the video below, were simply crushing right through the fresh new snow and into this frost layer. These types of activities help to mix up the layers and punch holes in near surface weak layers, in effect, introducing small pockets of stability.

Here is Zac’s Tracs first attempt at a YouTube video upload. You can see that during this test on Mar 3/08 we don’t see any action on the February 25 frost, but we do pop out the January 26 weak layer.

For a slab failure we need a slab. Because all of the fresh storm snow was so low density we did not see the February 25 frost layer react in the big block test in the video…only the January layer. Other snowpit tests, like the compression test and the burp test were used to show the instability of this near surface frost layer.

It is always very educational to complete an avalanche class during periods of elevated avalanche danger. There is always something dramatic to demonstrate and the participants really ‘get it’.
Just a little curious?! CLICK HERE to view our current avalanche course schedule.

CLICK HERE to see what is happening in the snowpack this season.
Canadian Avalanche Centre’s Public Avalanche Bulletins.

CLICK HERE for Avalanche.org to access US Avalanche Bulletins.

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