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All good skiers, especially those who go off piste skiing, are like pilots or sailors, and have a healthy respect for the medium they are moving in. Whether in the air, on the sea or in the mountains, they all know the dangers. The more knowledge they have about their surroundings, the better they will be able to achieve their goals. For a skier to understand snowcraft and snow - the stuff of his dreams and occasionally his nightmares - there is no better way to do this than to talk about avalanches. What happens after a snowfall in the mountains? Cold fresh snow consists of beautiful little crystals with sharp edges and points that act as hooks. After these pretty little things have touched down they get cosy with each other (but not immediately), and start to interlock under the influence of gravity - unlike humans, who rely on mulled wine and a friendly smile. So this tempting carpet of powder snow is lying there, all joined up, waiting to be vandalised by the likes of us. Here"s an example: sixty centimetres has fallen during a cold night on to older well settled snow that is similar in texture to the new fall. The slope gradient averages 25 degrees with a large bump at the top, dropping away quite quickly, and then gradually sloping up towards the bottom. It looks great. The weather is cold and sunny, and the snow all fluffy when you kick it. Do you go? You bet you do. But will you die? Well, you should be all right. A second example that explodes into life and only brings death, could be sixty centimetres of snow during another cold night, but this time the new fall has dropped onto old snow that has been blown by the wind into a smooth crust. The slope is the same, and looks as good, but do you go? Not likely! And why not? Because if you do, you may not be around to finish this skiing article... In each example the fresh snow looks exactly the same, but why is the second one so dangerous? In the first case all the snowflakes nestle into each other, and into the older snow beneath, but in the second example the new snow does not lock into the old. Also, in both cases, there is a point on top of the bump where the snow is under tension. The convex shape of the bump tends to stretch the crystals apart. Lower down as the gradient slopes upwards, the snow is not under tension, and this concave shape tends to prevent the crystals from separating. In the first case a skier can cut the snow under tension, and some of it may come away, but because it"s tied into the old snow underneath, it may hold. In the second example, however, there is no interlocking with the old snow, so when skis cut a swathe through the new stuff, especially where it is under tension on the bump - CRUMPH -
the lot goes, thundering down on the slippery base. As it comes to a stop near the bottom it builds up enormous pressure, and anyone unfortunate enough to be carried down and buried underneath it can be locked solid, unable to move even a finger. This kind of avalanche may not necessarily go just when or where it has been cut. It may wait until you are half way down and having a breather. Even noise and vibration can set it off. And it doesn’t need to be sixty centimetres. Sixty centimetres is about twenty inches; even four inches can be lethal. Both stable and unstable snow conditions depend on many factors, including weather, altitude, temperature, surface structure, gradient, and depth of snowfall. Wherever you ski, you can think about these things and take a look around you. Check the gradient, look closely at the snow. See where it alters composition in and out of the sun. All this will advance your snowcraft, and at the same time help you to avoid avalanches. As a final piece of advice, avalanches are like mushrooms so if you"re not sure, don"t touch them... Simon Dewhurst has taught downhill skiing in North America, Scandinavia and the European Alps for 35 years. His book "Secrets of Better Skiing" can be found at http://www.ski-jungle.com. If you have any comments about the above article, he will be happy to answer them. |
