Avalanche Control in Whitehorn IIPosted: December 30, 2009
Skiers visiting Lake Louise this week were treated to blue skies and sunshine, thanks to a high pressure system that’s settled in nicely in our area. And while skiers get to enjoy beautiful weather and great visibility on the mountain, this has also meant that our avalanche control team has been able to venture into and spend a lot of time in Whitehorn II.
Good visibility is always welcome during avalanche control work, and sometimes will be the deciding factor on whether the teams go into a certain area. Especially in a place like Whitehorn II, which is a huge piece of steep terrain with ridges, rocks, and cliffs, good visibility provides an extra measure of confidence for the crews traveling there. Stable weather generally results in little change to snow stability, allowing control teams to dedicate more of their time in places they had previously not spent much time in.
Looking back a bit, the week leading up to Christmas saw a few significant weather events, starting with about 20cm of snow that fell with little to no wind. As the snow petered out, the high pressure system moved in, announcing itself with west/northwest winds that redistributed most of that snow. This wasn’t the first time this season that winds came from other than the usual direction, and skiers familiar with how our mountain usually develops would have noticed drifting and scouring in unusual places, and Whitehorn II was no exception.
As a result of strong winds that blew in before the cold snap earlier in the month, the fans (areas at the bottom of slopes where avalanches “fan” out) of Whitehorn II had all their snow blown away, and it was only with the new snow before Christmas that these places had a chance to recover. The more recent winds resulted in hard slab conditions higher on the slopes, and because they came from the west/northwest, snow was distributed across the slope from the side, rather than down from above as is usually the case.
Wind slabs are the result of wind-transported snow. As the delicate snow crystals are blown across the surface, they are broken into tiny pieces. This means they can be packed into a tighter and denser layer of snow, a result increased by the packing action of the wind. Wind slabs often form the most destructive avalanches since they are harder to break apart than softer snow, and must be treated with caution.
One problem that can arise from a thicker and denser slab is that it can be for the most part supportive under the weight of a skier, which can give a false sense of security in the stability of the slope. Features on the slope (rocks, etc) can create weak spots, and even when the rest of a slope seems supportive, one turn in the wrong spot can cause the whole slope to avalanche. In the case of Whitehorn II, this slab was especially stubborn, due to its thickness and density.
When making snowpack observations, avalanche technicians use a scale to describe snow density. Rather than a number scale, as is used to describe sizes of avalanches for example, snow density is described by how easily a certain object will penetrate the layer horizontally (vertical penetration may go through more than one layer of snow, resulting in a variable and inaccurate reading). From least to most dense, the scale goes fist, four finger, one finger, pencil, and knife. The slab in Whitehorn II is pencil, which is about as dense as it gets before you get ice or a melt/freeze layer (knife). In the photo below, you can see that the patrollers’ ski cuts are barely visible at the top of the slope, due to the density of the snow and the inability of skis to penetrate that top layer.
While these slopes have reacted to explosives, the avalanching has not been widespread. In other words, rarely would one bomb be enough to get the whole slope. Snow that should have avalanched but didn’t is called hangfire, and enough hangfire remained in all gullies that many shots were required to clear each slope.
So – where does that leave us in Whitehorn II? It’s not ready to open yet, and while it may be easy to say we’re one or two snowfalls away from opening, there’s more to the picture than just snowfall. Wind and temperature have already played a big part in how the mountain has developed this season, and will also have a role in deciding what it’ll take to get Whitehorn II open. In the end, it’s a big piece of terrain, and lots of work by avalanche control teams will be needed no matter what the weather does.