The Spring Snowpack
Backcountry Essentials: Isothermal snowpacks, cornices, and glide avalanches
As we enjoy a prolonged spring and continued snowfall in much of the Northern Hemisphere, our snowpacks are undergoing myriad changes. When the pack deepens, the temperature gradient begins to stretch out and eventually disappears. This process takes time and usually reaches its apex in mid-late spring. This lack of gradient is referred to as “isothermic” or one temperature. The snowpack at this point is more or less a homogeneous mass. Of course there are always exceptions. Having a thorough understanding of your seasonal snowpack history and paying attention to trends is the most important piece of information. Verifying avalanche forecasts by poking around and digging in the snow will give you further insight into what is going on in the mountains you visit.
Isothermal snowpacks are generally quite stable and are the perfect candidate for supportable corn skiing. Paying attention to overnight temps is critical to determining when the skiing will be good and when it will be dangerous. Three nights with out a refreeze is a big red flag. Additionally, if you are making your way up a slope and sinking in above your shins, it’s definitely time to turn around and find a cooler place to ski. The old adage “On top by 10, down by 2” holds true, especially with a new load of snow from a spring storm
Cornices that have lasted to this point of the year are probably quite large and overhanging. Giving these yawning boxcars a wide berth is imperative, as they tend to loose their strength with heating and can calve off on approach. The best way to deal with these is abstinence. However, if you must get near to one of them, ensure you are well over solid ridgeline and use a rope if need be. Cornices can be effective slope testers, but make 100 percent sure that no one is below you when dropping one.
A rare but extremely dangerous type of slide is the glide avalanche. These monsters are a result of free water percolating down through the pack to ground level and lubricating what ever surface may lie underneath. Then, all at once, a catastrophic release of the entire season’s snowpack comes careening down the mountainside. Prolonged nights without a refreeze are a good indicator that these may become a hazard. Often times large glide cracks will form on the surface of the snow and can be very deep. Stay away and out from underneath these cracks at all costs. When the forecast mentions a possibility of glide avalanches, avoid drainages where you know the snow sits on top of very steep, slick surfaces like rock slabs.
As always, keep an open line of communication amongst your group and don’t be afraid to call it a day when all the Indian signs are pointing to danger.
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