Backcountry Essentials: The spring giant
Spring is in the Northern hemisphere, meaning the days are longer and the coverage is better. Weak layers from the early season are still around in some remote areas, but most have either gained strength, been scrubbed out by full-depth releases, or are too deep to be triggered by most skiers. It’s time to shift focus towards a new enemy in the backcountry—wet slides. These types of avalanches generally occur after the sun does its work on freshly fallen snow or when we enter extended periods of warm weather and poor refreezing. At this point, remember that everything happens faster at warmer temps.
The anatomy of a wet snow avalanche is very similar to that of a dry snow avalanche. The main difference is that in a wet slide, the weak layer is being further weakened rather than overloaded.
This weakening is the result of free water in the snowpack. It can come from a warm storm where rain falls on snow, or when intense sun bakes the snow. The free water percolates down through the snowpack and saturates the slab, causing it to settle and creep downhill with the force of gravity.
Additionally, the water that has reached the weak layer has begun to dissolve the critical bonds between the snow crystals. As seasoned avalanche guru Bruce Tremper would say, “Imagine a bunch of grapes. The grapes are snow crystals and the grapevines are the bonds.” If the bonds dissolve, you are left with the grapes held loosely together by the surface tension of water.
Roller balls and pinwheels on steep slopes and under rock bands are a sign that you are nearing the point of collapse and an avalanche. If temps stay above freezing for 24 hours or more, the next step would be point releases that fan out and entrain more wet snow as they descend. These can be detrimental to your health if you happen to be in the way of one of these errant sluffs. However, wet slabs are the most dangerous springtime avalanche due to their size and forecasting difficulty.
Wet slabs often respond poorly to slope cuts and when they do release, they move as one giant mass barreling over everything and gouging the landscape. Like a dry snow avalanche, wet slabs often feature stout crown lines that can cross an entire slope. Mature forests don’t stand a chance and entire villages have even been wiped out from these gargantuan lava-like flows.
The takeaways here are to remember that, like most things in life, it’s all about timing. Early starts and following the sun around the compass from east, to south, to west is the best bet for finding prime spring riding conditions. After a recent snowfall this is especially important due to rapid warming. If you are wallowing up to your thighs in soggy snow, you are already in the red zone and it’s time to get off the slope immediately. Shady northerly aspects will often be your best exits on these types of tours.
During spells of high pressure and a solid melt-freeze cycle, corn skiing will be the order of the day as the sun softens the snow. Paying attention to your local weather stations and keeping an eye on overnight temps will aid in your trip planning and keep your team out of harms way.
|Another backcountry tip: The Human Factor →|
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