Understanding how to make good decisions in the backcountry is difficult.  Sometimes the signs of stability or instability are obvious and it's an easy go or no-go decision.  Most of the time it’s difficult to decide how stable the snowpack is.

The first step in understanding avalanche hazard and managing terrain is to take an AIARE 1 Avalanche Course.  These courses provide a baseline of information on all things avalanche and are the perfect blend of theoretical understanding and practical application.

Once you have completed AIARE 1, your journey has just begun.  Avalanche education continues throughout your career as a backcountry skier and should evolve on every tour you go on.  Choose mentors who can help shape your mindset around evaluating the conditions and terrain features you travel through.  Understand the elements that make both the conditions and terrain safer or more hazardous.

One critical part of understanding the stability of the snowpack is making critical observations and interpreting snowpack tests.  While we don’t always need to dig to understand the snowpack, there are times when it is essential.  In the cases when you might want to dig, it’s nice to have a baseline for both unstable snowpacks and stable snowpacks.  Digging in both of those conditions will provide a foundation for understanding all of the “in-between” results that you’ll get on a majority of your days in the backcountry.

Snowpack Test 1: Obvious Signs of Instability

Both Whumphs and Shooting Cracks are obvious signs of snowpack instability that we saw in the field today at Mt Rose. When you see or feel these you know there is a problem.  These indicate a very fragile weak layer stressed nearly to the breaking point.  When your weight tips the balance, the weak layers fail and you get shooting cracks, a whumph or if the terrain is steep enough, an avalanche.  If the result is a crack or whumph, pat yourself on the back.  You chose to be in terrain that was not steep enough to slide!  On top of that, you just got keyed into a serious problem in the snowpack, and a sign to use extra caution in choosing terrain moving forward.

Snowpack Test 2: The Mitt Pit

This is an informal test that is quick and easy to do while you’re skinning.  If you have an extra couple minutes, you can carve out a block and see how much force it takes to fail on the weak layer and the shear plane it leaves behind when it does fail.  Both the amount of force and the quality of the shear have a large bearing on how stable the snow is, so make sure to pay attention to both.  In the case of basal facets/ Depth Hoar, the shear plane may not appear perfectly planar.  That is not because the shear plane has bonds on both sides of the layer (as the case might be when it is another type of weak layer), but because the faceted weak layer is very weak and fairly thick - it just crumbles apart!

Snowpack Test 3: The Compression Test

So we’ve made a couple of observations that have opened our eyes to the instability in the snowpack.  We want to understand the layers more and quantify what we’re seeing.  This data will help us put the problem on a scale and weigh it against other snowpacks we’ve seen and will give us a baseline for comparing our results to tests we make on this same layer later in the season.  This test has 4 loading steps: isolation of the block, 10 taps from the wrist, 10 taps from the elbow and 10 taps from the shoulder.  The progression is Very Easy (failure on isolation), Easy (failure on wrist taps), Moderate (failure on elbow taps), Hard (failure on shoulder taps). The quality of the shear is also important, so when we get a failure we note how planar the shear plane is or the failure type.

In this case we got a CT1 SC at 0 cm on basal facets (Compression Test, 1 tap/Easy, with a Sudden Collapse of the weak layer, where the ground was the weak later) result.  The test was repeated and produced the same result.  This consistency means the result was not anomalous and that this is likely representative of the snowpack in other areas as well.  

But what does this tell us?  Who cares if we got a CT1 SC @ 0cm ?  Well, this tells us that we are likely to initiate a failure in the snowpack.  This could result in an avalanche if we travel in steep enough terrain for the snow to slide.  

Given all the data we’ve collected so far, we’re pretty sure we can have a failure on the weak layer.  We’ve seen 4 different examples of it failing.  The next thing we need to know is: is the slab cohesive enough and the weak layer weak enough for a fracture to propagate across a slope?  This is where Snowpack Test 4: the Extended Column Test comes in.

Snowpack Test 4: the Extended Column Test

To test whether a failure can propagate through the snowpack as a slab avalanche across terrain, we need a bigger block.  Snow scientists Karl Birkeland and Ron Simenhois developed the Extended Column Test in 2006 to assess the fracture propagation potential across an isolated slab.  It has become the standard for understanding if slab avalanches have a high likelihood of happening in terrain steep enough to slide.  The good news is they can be performed in low angle terrain without affecting the outcome of the test.

In our snow pit on Mt Rose, we got a moderate result (on the 11th tap) in which the failure propagated across the weak layer in a sudden planar failure.  This indicates that not only can an avalanche be initiated, but it can propagate across the terrain and cause a slab avalanche.  The weak, sugary grains at the base of the snowpack are a real attention getter!   

Now compare the snowpack tests in the first video with a snowpack test from the week prior:

North Lake Tahoe Backcountry, December 12, 2023.