Snowfall at Lake LouisePosted: November 18, 2008
What a difference a week makes!
We’re starting to see a few more smiles on the faces of our avalanche control team and trail crew as they wander around the upper mountain of Lake Louise, thanks to our recent snowfalls. With over 20cm received last weekend, and more over the last few days, white has replaced brown as the dominant colour in the alpine, and with more snow in the forecast, we’re becoming optimistic that further terrain openings are not far off.
While the pervasive warm temperatures of late have put a damper on our snowmaking efforts, they have done a great job of settling our upper mountain snowpack, and so far this is a year where the snowpack is building right-side-up, but I’ll get to that in a moment.
Up to now, our avalanche control efforts have produced a number of results, with many avalanches sliding on a hard crust that formed during a rain event on November 2nd. This crust was hard enough to support the weight of a jumping skier, and provided a good sliding surface for whatever snow lay on top. Now, the warmer temperatures have resulted in a few things. Due to heat from the ground and the air, the crust has become moist, and is breaking down to the point that it will no longer support the weight of a skier. It is also becoming less of a distinct layer as it joins those layers above and below it. Only in places where there has been significant wind loading did we get any big avalanches. In other words, it currently takes a lot of snow on top of this breaking-down crust to produce results, and even then only after explosive charges are placed on the slope. A good example of this today was in the Heart, which is a feature to the skier’s right of Paradise Bowl, and avalanched about as big as anyone could remember – all on a day that otherwise produced few noteworthy results.
Now, about that right-side-up snowpack. Ideally, as a skier you want your snowpack to have its densest, strongest snow on the bottom, and then get gradually lighter as you approach the surface. This makes for great skiing, as you have soft snow to ski in on top, with a firm base on the bottom to keep the snow in place on the slope and you off of whatever ground features may be lurking below. In the avalanche world, weak over strong is good, and strong over weak is bad.
A (very) quick lesson in snow and temperature: Basically, weak layers develop when the snowpack is of insufficient depth to deal with a significant difference in temperature between its surface and its base. In this part of the world, ground surface temperature remains within a degree of 0 degrees C all winter long as a result of residual heat built up over the summer. Snow needs about 10cm of depth to cope well for every degree of temperature difference between surface and base (referred to as temperature gradient). For example, if your snowpack is 100cm deep, the temperature at the surface can be -10C or warmer without the snowpack suffering any ill effects. If the snowpack is shallower or the temperature colder, the snow can no longer maintain a gradual change in snow crystal structure from top to bottom, and weak layers begin to develop. (Many volumes of snow science have been published over the years, so obviously this is about as brief a summary as you’ll find on what can be a very complicated subject!)
A typical start to the season in this area involves a little snow mixed with cold temperatures, and any weaknesses in the snowpack that develop during this time tend to linger ominously throughout the winter, unless of course they are taken care of by avalanche control or warm temperatures. You’ve probably heard the term hoar, and there are two kinds – depth hoar and surface hoar. One type of hoar crystal can be those big feathery crystals you sometimes see next to a stream on a very cold day, covering not only the snow surface, but branches, fences, and the like. They are very fragile, and collapse as soon as you try to pick them up or disturb them. As the names imply, surface hoar happens at the surface, and depth hoar below the surface of the snowpack. Depth hoar happens when the temperature gradient is steep (shallow snow and cold temps) and surface hoar happens on cold days when there is a nearby source of moisture (not really relevant to a ski hill scenario), or during a cold, clear night. Depth hoar is a concern since the overlying snow is not well-supported. Surface hoar is not a concern as long as it is the top layer, but once it gets buried by new snowfall, it becomes a weak and unsupportive layer, just like the depth hoar.
Phew – enough about avalanches for now. Besides avalanche control, we also had a long day today of flying snowmaking equipment into place for the upcoming World Cup races, and my next post will be all about that, complete with photos. See you then…