Snow temperature near avalanche. Beginning students tend to focus too much attention on air heat, possibly because it’s such an easy number to feel and measure. But it’s snow temperature we are concerned about, not air temperature, and radiation balance usually plays a larger role than air temperature. For this and other reasons, temperature is a complex factor in the avalanche game.
Snow temperature near avalanche
Most of the time, air temperature is a minor player, especially compared to loading. Once again, we return to our mantra: snow does not like rapid change. Dry snow near the melting point is highly sensitive to rapid change mostly mechanical change and, to a lesser extent, thermal changes. What is important to know is how snow temperature affects the mechanical properties of the snowpack.
Everything happens faster at warmer temperatures than at colder temperatures. Car batteries work better, trees grow faster, paint dries faster. All chemical reactions including the metamorphosis of snow work faster at warmer temperatures than at colder ones.
Warm snow deforms more quickly and easily than cold snow, affecting creep. An increase in the creep rate of snow will also increase the rate of shear deformation along buried weak layers within the creeping snow and this increase means avalanches.
Once in Alaska we had several weeks of cold, clear weather that turned the entire snowpack into depth hoar. Then we got a couple of light snowstorms, which layered snow on top of the depth hoar with no avalanches, and temperatures remained cold. Finally, a warm, wet air mass from the Pacific pushed into the interior, warming up the surface of the snow for the first time in several weeks, and localized avalanches started even before any precipitation fell.
The warmer temperatures had caused avalanches by affecting the properties of the slab, not the weak layer. A commonly accepted explanation is that as a slab warms up, its settlement and creep rates increase, which can accelerate shear within the still cold and brittle weak layer.
Another important effect
Snow temperature is that snow is stiffer at cold temperatures than at warm temperatures. This is especially important when a slab is bridging a weak layer below it. A cold slab will bridge someone’s weight more effectively than a warm slab. So, people might get away with crossing a slope in the cold morning, but as the slab warms up, more of their weight is transferred to the buried weak layer under them and can initiate a fracture in the buried weak layer.
Having said this, as a backcountry avalanche forecaster, I’m always suspicious of the first sunny day after a storm, not so much for the increase in temperature I just described but mostly because that’s when powder fever runs amok at the same time that buried weak layers have been recently loaded two important factors that, in my experience, are much more important than air temperature.