Monday January 18, 2016 Q&A

Which of these factors is the most important in assessing avalanche risk?


1 The age of the base layer

2 The depth of the layer of fresh snow

3 Temperature variation across the layer of fresh snow

Question related image
Correct answer: Temperature variation across the fresh snow layer Many factors must be taken into account when assessing the degree of avalanche risk in mountainous regions. Experts in this area — known as nivologists — have established a detailed catalog of criteria that contribute to the occurrence of avalanches as well as others that create conditions that are safe for backcountry activities. Even if it might seem counter-intuitive, a thick layer of fresh snow does not necessarily indicate elevated risk. It can even be the opposite: up to a certain point, a thick layer of fresh snow will be less likely to slide than a thinner one. The reason for this is what experts refer to as the “temperature gradient” in the layer, or the difference in temperature between the bottom of the layer of fresh snow and the air above it. This, more than any other factor, is what will influence the metamorphosis of ice crystals, which is caused by vapor flow within the snow layer. The larger the difference in temperature, the smoother and looser the crystals in the lower portion of the layer become, like salt or fine sand, and thus the more unstable the slab. If the gradient is small, however, the crystals tend to stick to each other — an effect known as sintering. Let’s look at an example: 1 meter of fresh snowfall brought by a -15°C “bise” wind, deposited on a 0°C base layer, yields a gradient of 15°C per meter. On the other hand, if only 20 cm of fresh snow falls in these same conditions, the gradient becomes a whopping 75°C per meter, increasing in turn the phenomenon of constructive metamorphism. (source : WSL) These process works mainly in dry snow. Moreover, mechanical constraints, such as settling in the layer, can enter into play in various ways, particularly in windy conditions, and are very important to take into account when assessing the level of avalanche danger. The only truly reliable way to estimate avalanche risk is by taking samples in the field and by taking into account a large number of criteria. In Switzerland, the Federal Institute for Forest, Snow and Landscape Research is responsible for conducting these estimates. The sot recent avalanche bulletins and other useful information can be found on its internal website and through its free “White Risk” app. An increasing number of training courses are also being offered to help people better understand the bulletins. The long and short of it — be careful before setting out into the backcountry on skis or showshoes! Our thanks to professor Christophe Ancey, from EPFL’s Environmental Hydraulics Laboratory. To find out more: A WSL article about the thickness of the snow pack A book to better understand the avalanches.