Safety Week: Assessing the Snow

Heading out in the backcountry in the winter requires doing your homework. Besides checking the avalanche and weather reports, being able to identify what's going in the snow under your feet is crucial to having a safe and fun day in the mountains. The snow we ride is in a constant state of metamorphosis, with layers of different density and cohesion forming all season long due to constantly-changing mountain weather conditions. When these layers create cohesive slabs that overlie weak layers of faceted snow and a smooth bed surface in terrain over 30 degrees, we have the recipe for an avalanche - a dangerous combination that can be quite hard to identify. Reading your up-to-date local avalanche forecast is the best way to understand the avalanche risk you may be dealing with, but to get a more detailed picture, or when riding in an area without an official forecast, TGR athletes and crew know to dig into the snowpack to figure out what's going on.

A particularly dangerous type of avalanche problem is the Persistent Weak Layer (also called persistent slab). PWLs are the "sleeping dragon" of avalanche problems, and are one of the most difficult problems to forecast. You'll often hear that this is the kind of problem that keeps forecasters and professionals up at night. Persistent slab avalanches can be stubbornly hard to trigger, and the weak layer can stick around in our snowpacks for weeks, months, or even all season long. While it might be tempting to step out into tasty-looking avalanche terrain (especially if it has other rider's tracks all over it already) on powder days when a PWL issue has been around for weeks, the consequences of triggering an avalanche in that terrain remain the same. PWLs are by definition persistent, so the best way to deal with the issue is to simply avoid it - you can "outsmart" it by giving it a wider margin than usual and finding terrain that's under 30 degrees.

There are many other forms of avalanche problem and by taking out our shovels and probes to dig down through the various layers in the snowpack, we can identify which ones are of concern, and whether we have that potentially deadly combination of slab and a weak layer on a given slope. Snowpack analysis can take many forms, whether it's simply looking for the softest surface conditions to ride in a bomber snowpack, or probing for weak layers and digging a hasty hand pit while on the move, or excavating a full profile to determine stability and structure trends throughout the season. Digging into the snow not only gives us a picture of the snowpack's structure, but also gives us a chance to perform stability tests to check snowpack strength. In the end, a solid, supportive structure and strong, well-bonded layers are what make up a stable snowpack. Here are two ways to help quickly identify what's going on in the snow while you're out in the mountains without having to dig a full test pit. Of course, if you are looking for a specific weak layer and want to document trends, break out your tools and do a full Hand Hardness profile, Extended Column Test, Compression Test or Propagation Saw Test.

One very important note: Pit test results should never be used to "greenlight" a slope, they should only be used to gather information and are only one part of the multifaceted decision-making process.

POLE PROBING

We can gather a surprising amount of information about snowpack structure simply by sticking our ski poles into the snow. If you're aware of a weak layer, crust, or other suspect density change lurking in the mountains, go ahead and probe around to see if you can find it. Take your pole, flip it over, and probe it handle-side-down into the snow to see if you can feel the different layers in the snowpack. A crust will be obvious - your pole will slide in smoothly with even resistance until you hit a hard spot that's harder to break through. If it becomes easier to insert your pole the further you go, that can be a red flag - it's a sign the snowpack is upside-down, with stronger, more cohesive snow on top of lighter, less cohesive layers. That's another recipe for an avalanche in steep enough terrain.

While pole probing isn't a perfect method, it's a great way to get a large number of data points. Because it only takes a few seconds, you can be doing this on all manner of slopes across a variety of aspects and elevations to help paint a rough picture of how the snowpack differs - crucial in determining where persistent weak layers may lie.

QUICK SKINTRACK HANDPIT

A second, slightly more involved tool comes in the form of digging quick hand pits. These can be done next to the skintrack or bootpack on the way up - and can give you some vital data in determining strength of the upper snowpack. They can demonstrate how well new snow is bonding to old snow, or even help in identifying weak layers in the upper part of the snowpack. Using just your hands and perhaps a ski pole, you can quickly excavate the upper 30-50cm of the snowpack to get a peek of what's going on. This is a particularly useful tool to use if you already have a general idea of the snowpack structure, but want more localized data. Again, these are very quick to dig and can be done on the move, so they can be used to gather many data points in a short period of time.

Identify a location to dig and mark out a 30x30cm square in the snow. Use your hand to clear the downhill side and use your pole to cut out both the back and the sidewalls as deep as you can. Now, take your hand and gently pull the isolated column downhill, noting what happens and how much force it takes to shear the block, if at all. There is no hard and fast rule for interpreting results from this kind of test. However, if the isolated column very easily shears off on a planar layer, that's a red flag and likely indicates you have a slab on top of your weak layer. Take a moment to identify if there are facets on the surface of where your column broke. Repeating the test on a few areas of the same slope to see if you get similar results can help paint a clearer picture of what you might be dealing with.

EXTENDED COLUMN TEST

If you've got the time and energy to dig a full snowpit, the Extended Column Test is a way to test for whether the snowpack has the ability to propagate a crack across a slab - an essential ingredient for an avalanche. Once you have a pit site selected, a hole dug, and hand hardness test done, it's time to test any weak layers you may have found with an ECT. Remember, this test was designed to check for the propensity of a weak layer to propagate a crack. Propagation is bad, so we want to dynamically load the snowpack in a controlled space to essentially "simulate" a rider on a slope creating an avalanche.

To effectively perform this test, you'll need a shovel, a snow saw, two probes and an 200cm+ long ECT cord.

Isolate a 90cm wide, 30cm deep, column on the back wall that is deep enough to expose the weak layers of concern. There are several ways to do this, depending on the tools you carry, but here's a quick way to do it cleanly. Stick the two probes vertically into the snow, 90 cm apart and 30cm back from the wall. Using the probes as a guide, cut the vertical sidewalls of the column using a snow saw. To better isolate the column, you can cut a "pizza slice" when you cut sidewalls, so there is some space between the isolated column and the surrounding snow. Then, run your ECT cord behind both probes and cut out the back wall of the column. Remove the ECT cord (put it right back in your pocket so you don't accidentally lose it).

Now you're ready to begin the test. Take your shovel, and place the blade upside down flatly on one end of the 90cm-wide column. With one hand lightly holding the handle for stability, use your other hand to tap the blade slowly 10 times from the wrist, 10 times from the elbow, and finally 10 times from the shoulder, keeping count and observing what happens to the column while you tap. If you observe cracking or propagation during any of the taps, stop immediately and note the number of taps you were on, whether the cracking propagated across the column, and how deep the failure occurred. 

If you plan to share these results, record them using the American Avalanche Association's SWAG notation. This is the standard way avalanche professionals communicate information about snow conditions, weather, and avalanche observations. For example, if you saw propagation on your third tap from the elbow on a layer 33cm down from the surface, record it as "ECTP13 @33cm down."

Without diving too deep into specifics of what each result means, any failure that occurs with propagation is an obvious sign of instability on a slope and indicates that a slope has a likelihood of avalanching. At the same time, an ECTX result (no failure or propagation observed) does not indicate general slope stability. All it means is that you could not generate enough force to create an avalanche or the weak layer was not present or reactive on that particular spot of that particular slope.