Written by: Joey Vosburgh
Phantom Team Rider
Interpreting snow observations can seem like a big mystery. There are clues everywhere, but you need to figure out what they mean. It can seem overwhelming and confusing to interpret the results of your investigation, but it doesn’t need to be complicated. Here are a few things I focus on when collecting snowpack information to try to answer the critical question: will this slope avalanche?
Surface Hoar Reactivity Avalanche
My investigation starts before I leave the house. I’ll check the weather stations available online to see how much new snow we got, how strong the winds are and what direction they came from, as well as what the air temperatures are. As I’m driving to the trailhead, or starting up a valley, I’m looking for evidence of recent avalanches. I look at the trees to see if the snow held in branches is uniform, or has it been blown off in some areas which can give me an indication of which aspects might be wind loaded.
Pay attention to the state of the snow surface. In my perfect world it would snow every day; not only would every day be a powder day but the snowpack would also be free of persistent weak layers. Sadly, this isn’t the reality and the breaks in snow allow various layers to form. It might be a crust from a sunny period or facets from a cold snap. Ideal conditions for surface hoar are clear, calm and humid. With experience you can recognize these conditions more easily. Crusts are pretty obvious, faceted snow seems sugary, and you’ll learn to recognize the beautiful sparkle and the sounds of surface hoar while you ski. Pay attention as you travel, making note of where you find these conditions, which aspect and at what elevation you see them. It might be incredible skiing and they might not be a concern now, but once buried you’ll be happy you have a bit of a mental map of where they exist.
As I travel I often use hand shears to quickly assess surface instabilities. https://www.instagram.com/p/BdAgKyFDct1/?igshid=1c8683v971nis Interpreting hand shears isn’t calibrated to skier triggering like compression tests (keep reading to learn more), but they are easy and efficient. Things I pay attention to: did the block fail while I was cutting it? How hard did I have to pull to get it to fail and was it clean and easy to move, or was the failure sticky or uneven?
Ski pole probing is another quick way to constantly assess the upper snowpack. By using your pole upside down and pushing it into the snow, a general idea of layering can be felt. This is not precision but it is effective when tracking a crust layer or firm bed surface. Doing it in many locations throughout the day can help understand distribution of those layers. I also pay attention to and compare my ski penetration (Ps) to foot penetration (Pf). For example, if I’m skinning along and the trail breaking is only 10cms, but when I take my split off I sink to my waist, I’m getting a big red flag. This shows me that the upper snowpack has stiffened but below is lower density, often referred to as “upside down”. Basically this is a clue that the upper snow has formed a slab.
Some clues are glaringly obvious. Anyone who has experienced the stomach dropping sensation of a big whumph, or observed cracks shooting out from the tips of their boards likely instinctively recognized that the snowpack was giving them a big heads up! But you have to get off the uptrack to make these obs! Even if it’s only for a short burst of trail breaking, the information that can be gathered is invaluable. Whumphs are a clear indication of a weak layer, shooting cracks indicate that the snow is cohesive and reactive; you’re essentially triggering a layer but it didn’t quite avalanche. When I get whumphing or cracking I will often either adjust my plans, or collect more information by testing the snowpack.
Compression tests (CT) are one of the most common tests, and if you learn how to interpret them they can give you important clues as to whether or not a skier will trigger a slope. I could try to explain it, but I’d rather let the master do it. Watch this excellent video by Bruce Jamieson where he highlights the importance of the fracture character (triggering is more likely if the fracture is sudden) and correlates test results to skier triggering. https://vimeo.com/30996756 I watch this video every year to calibrate my loading steps and remind myself of the nuances between different fracture characters. When I do a compression test I am trying to decide if a slope is safe or not. I’ll try to find a safe slope with a similar aspect and elevation for my test pit so it is indicative of that slope.
Where the CT gives an indication of initiation (the likelihood of triggering an avalanche), Extended Column Tests (ECT) and Propagation Saw Tests (PSTs) will give an idea of the likelihood of propagation (the likelihood that if triggered it will propagate across a slope into a bigger avalanche). These tests are a bit more involved but I will typically use them when I’m uncertain about a persistent weak layer. If I get an easy to moderate sudden CT result and my ECT or PST show propagation is likely, I’m going to be worried about triggering large avalanches and I’m really going to reign in my terrain use.
Extended Column Test (ECT)
Like any good mystery, no matter what the clues hinted at, the ending can still be surprising. Pay attention to the evidence around you, take the time to investigate, and critically assess what this information hints at; will this slope avalanche? Whatever the answer, use terrain to stack the odds in your favour. But that’s a whole other topic… to be continued…
Terrain UseFor those of you that are interested in learning more about testing the snowpack (and many other avalanche related topics) I highly recommend checking out the library of videos available at https://www.snowline.ca/videos4recreationists.