Tale of the Thaw

 Fog forms above the snow

Fog forms above the snow

Our region in February saw an unprecedented nine consecutive days of measurable snowfall. And it was beautiful, powdery snow that invited me to explore the trails on cross-country skis, something I had scarcely been able to do during the prior winter, what with the dearth of snow. Those of us who enjoy the winter white were delighted by a foot-and-a-half of fresh snow on the ground. Here at Severson Dells we had a night hike on February 9 with about 20 of us trudging around through the drifts. A couple of folks even brought snowshoes for the hike.

Of course, it’s the middle of February and the sun is growing stronger every day. Even with air temperatures below freezing we can lose snow pack on sunny days as radiant sunlight thaws the surface of the snow. This week, tree branches high in the forest canopy were dropping loads of snow to those below.

And then we did get a real warm-up, a two-day thaw during which roofs came clean, watercourses flowed, and puddles formed. There was drizzle and fog—lots of fog.

Interestingly, we lose our snow pack faster under foggy conditions than we do under rain. Of course, air temperature, along with the temperature of the rainwater, will affect the rate of thawing, but generally the heat energy released by the condensation of water vapor over the snow (the formation of fog) is much greater than the energy transfer in a rain event.

We tend to think of water as an interim phase of H2O—liquid water. We know that water freezes into a solid we call ice (also snow crystals). And water evaporates into a gaseous form (we think of steam and water vapor). But H2O can change from its solid (frozen) phase to its gaseous phase directly, bypassing its liquid form, through a process called sublimation.

This can result in the kind of fog that we see during a thaw. And while we might not see sublimation taking place, we can see its effects. Have you ever noticed that snow disappears first from around the base of trees? Tree trunks receive and reflect solar radiation—and trees, like all living organisms, generate a certain amount of heat through basic metabolic processes. The snow around the tree absorbs this energy quite efficiently, resulting in sublimation of the snowpack closest to the trunk. You can see it in the photo.

 Snow disappears from around the base of trees by means of sublimation (direct transition to vapor).

Snow disappears from around the base of trees by means of sublimation (direct transition to vapor).

As we round the corner into the latter half of winter, enjoy the freeze, the snowfall, and the thaw—all part of the dance of moisture through the natural world.