Alaska-Yukon Expedition: Spring Conditions Terminology
Follow adventurer Andrew Skurka
as he skis, hikes, and rafts 4,720 miles through eight national parks,
two major mountain ranges, and some of North America's wildest rivers in
Alaska and the Yukon from March to October. Read his blog updates here.
In talking recently with my parents and friends I gather that not everyone is familiar with the lingo I've been using to describe the current springtime conditions. I'll attempt to define a few of the most important terms now.
In the winter and summer, conditions are relatively predictable. Winter: cold temps, dry snow, frozen rivers and lakes, full snow coverage. Summer: warm, wet, open rivers and lakes, no snow. But in the spring, conditions are something in between, and changes are happening rapidly in the transition from winter to summer. A slope may be covered in snow today, but bare next week. A river that had frequent ice bridges yesterday may be completely open tomorrow. The snow may feature a supportive crust at 8 a.m. but might be the consistency of a slushie by noon.
An unpleasant experience by which one sinks through the snowpack, potentially to the ground, because the snow is not supportive enough for the pressure being placed upon it. Postholing is especially common in the spring when a "hiker" attempts to walk across snow without using any floatation (e.g. skis or snowshoes). But in Interior Alaska, postholing can happen even with skis, snowshoes, or a snowmachine because the snow here is especially unsupportive in the Spring.
The wintertime snowpack in Interior Alaska has very little water content. It rarely contains a sun- or warmth-caused layer. And it's a shallow snowpack, consisting mostly of incoherent faceted snow crystals caused by a high temperature gradient between the ground and the air temperature. The snowpack is fluffier than Colorado's, and at the other end of the spectrum from California's sticky and wet snow. When it begins to melt in the Spring, it actually just "rots." The days get so long that it's in a near constant state of melting, and the snowpack is so porous that it can't consolidate and settle. If you get lucky there'll be a freeze crust on the surface in the morning, but the crust is often very thin (it's like skiing on egg shells) and it's supported by nothing underneath so postholing almost always ensues. Rotten snow has the consistency of a slushie.
The snowpack can obtain a surface crust due to wind, snow, or a freeze-thaw cycle. This is very desriable because it provides a supportive layer on which to travel, often at a relatively fast rate of speed since the traveler is not punching through. In Alaska, I've often found wind crusts in alpine areas and I occasionally find melt crusts on sunny snow-covered slopes early in the morning (but not once the sun has warmed it up again). Crusts are mostly non-existent in the timber or in the brush.
"Settling" or "whoomping"
Interior Alaska's snowpack is usually weak and unstable due to its faceted snow grains. On hundreds of occasions I have heard the snowpack settle beneath me (accompanied by a loud "whoomp") as snowpack's surface collapses atop the weak faceted foundation. In particularly unstable locations I can see the snow surface settle hundreds of feet away from me, creating a visual effect like how an earthquake is depicted in movies.
- Nat Geo Expeditions