The Changeable World of Fallen Snow
While digging out the woodpile from under snow the other day, I got to wondering about the cold white stuff filling the shovel over and over again. Except for a thin layer on top, the white stuff wasn't recognizable as snowflakes. I seemed to be moving mounds of little ice pellets that looked sort of like thumb nails.
With some help from Geophysical Institute glaciologist Carl Benson, and the doctoral dissertation of his former student Matthew Sturm, I now know a lot more about the strange world within the snowpack, where flakes become ice globs. Of course, to Benson and his colleagues, none of this is strange at all-but them, glaciologists often seem a bubble or two off plumb to those of us who can't muster much fellow feeling for snow and ice. The stuff to which they've devoted their lives---frozen water---behaves strangely. For example, almost as soon as a snowflake hits the surface, it abandons its elegant shape as quickly as would a half-hearted dieter given a box of jelly doughnuts. Lying on the surface with a bunch of its kin, the snowflake's complex, feathery form soon starts to change. Molecules on the surface of the icy flake begin to migrate from the delicate points toward concavities in the crystalline structure, slowly transforming the flake to a more rounded grain.
Then too, water in its vapor form behaves unlike just about everything else in the north; it moves from warmer areas toward colder ones. Thus, molecule by molecule, snow sublimes---the contained water goes directly from solid to gaseous form---and the released water vapor migrates upward, away from the comparatively warm ground toward the often miserably frigid air.
The upward-moving vapor leaves behind a course-grained layer of frost crystals known as depth hoar. This crystalline layer can have a hard time keeping it together, so that in some places---say next to warm building wall, as an easy example---it may vanish entirely, leaving empty spaces where small animals can run about without having to do much energy-consuming digging or tunneling. (A thriving colony of voles has taken advantage of that effect near the University of Alaska Fairbanks art department for several years. Spring always exposes vole highways and storerooms at the edge of the building's foundation just while students are working frantically to complete final projects, thus ensuring that the species are too busy to hassle one another.)
The moving water vapor leaves some former snowflakes eroded and partially decomposed, while it freezes onto and builds up the former flakes, now approximating crystalline blobs. Sturm's dissertation illustrates five distinct layers that this winter-long process produces in the subarctic snowpack.
The dissertation also documents that the snowpack in the Fairbanks area is more air-permeable than, say, the typical alpine snows in Colorado's high country. Possibly related to that peculiarity, and the core of Sturm's work, was his establishing that convection goes on within the local snow cover. The vapor-bearing air can scoot upwards at rates of about 0.2 mm/sec, eventually carving tiny chimney-like structures in the snow, and even horizontally, spreading water and warmth along a snow layer.
Even so, the Interior's snow makes a good insulator. Benson's crew has measured a temperature gradient of one degree centigrade for every centimeter of snow depth. No, that doesn't mean that with really deep snow, such as we've had this year, the voles at the bottom of the snowpack are enjoying Hawaiian temperatures. It does mean they are comparatively snug, enjoying temperatures neat the freezing point while we humans trying to get our cars started at the surface are suffering in forty-below air.
All in all, though it looks peaceful and still, the fallen snow is busy stuff. It's constantly changing, as each particle metamorphoses into something else. Reminds me of politicians after the election...
