By Lawrence J. Winship For the Gazette
As we move deeper into winter, once again it becomes evident that living at 42 degrees north latitude has one certain consequence: We trade the bright long days of summer for the short, dark days of winter. Lower sun angles, later dawns and earlier sunsets are all caused by the tilt of our planet’s axis away from the sun. We spin at an angle of 23.5 degrees relative to our orbital plane around old Sol. Facing towards the light and then away in a perpetual annual cycle, we don’t turn our backs completely on the sun, as they do above the Arctic Circle — but it does get pretty dim! Season in and season out, Earth continuously radiates heat energy back to the cosmos, so the reduced solar input in winter throws our soils, forests and lakes into negative heat energy balance and they freeze — water like a stone.
Heat gain and loss in winter are not uniform across the landscape. The heat that penetrated deeply into the ground in the summer takes a long time to make it back to the surface. Even as the ground surface freezes solid, temperatures in deeper soils don’t fall below 32F, creating winter hibernation places for mammals and amphibians. Some perennial plants do best in well-drained soils because freezing of soil water lifts the upper soil layers, which would tear their roots. And, because water expands when it freezes, we place our building foundations below frost to avoid damage.
We regularly see the seasonal movement of heat as we deal with the changes in liquid water, especially on roads and the windshields of our cars. But winter offers us another way to think about heat loss and gain — snow. A fluffy layer of snow changes the soil heat balance dramatically. An “open” winter without snow allows frost to penetrate deeper, delaying the thaw in the spring and making it harder for farmers to get their crops going. Voles depend on snow cover to create “snow warrens” warmed by heat rising from the soil and protected from cold winds.
Patterns of snow accumulation and loss can also present us with fun and informative “snow puzzles.” For example, have you noticed narrow vertical stripes of snow on some house roofs the day after a light snow? It’s an example of differential melting. We’re seeing the loss of heat from the floors below, up through the attic and into the roof — the rafters are acting like insulation, preventing melting where they contact the roof deck.
One of my favorite “snow puzzles” in the natural world concerns the bare rings of soil that form around the base of trees. It looks like some creatures may have been digging out a snow fort, or could it be that the tree trunks are somehow melting the snow? We know that all living things, as they turn food into metabolic energy, inevitably lose heat. Some plants, like the skunk cabbage, definitely heat up in the early spring, and some tree buds are thought to produce enough metabolic energy to melt thin coats of ice as they begin to expand. But what’s happening with tree trunks?
I’ve noticed that tree stumps, posts and other nonliving objects can also form melt rings, so something besides metabolic energy is at work. The heat balance in the snow at the base of the tree might be altered, perhaps by heat radiated from the darker trunk. Recently, just for the fun of it, I took my non-contact infrared thermometer (a ray-gun-like device I use to check woodstove and hot-water temperatures) out to the woods to explore the thermal environment. Sure enough, at noon the sunny south side of the trunk of a large red oak was 54F, while the north side read 26F, the same as the air temperature. The ground around the tree was 26F in the shade and 29F in the sun — frozen solid.
We know that trees contain a lot of water and that water has a high heat capacity — for a tree trunk to increase in temperature from 26F to 54F it has to absorb a lot of heat. That day it was practically windless, so the exchange of heat from the trunk was mainly by absorption and re-radiation of energy; wind-chill was not a factor. Because there’s a temperature gradient of 28F across the trunk, certainly some amount of heat is flowing from the sunny side to the shady side, and also down into the ground.
I might learn more if I kept an hourly chart of measurements through a few days and nights, further detailing the rate of heat flow in and out of all sides of trunk, related to solar intensity, wind and air temperature — school project, anyone? But I think we can make some predictions to test as the winter progresses. No sun? No melt rings. Darker trunk (oak) compared to lighter trunk (birch)? Perhaps a slower, less deep melt with the birch? Should the melt ring form faster or get bigger on the south side? We need time-lapse photography! What about a dark fence post versus a light fence post?
Let me know what you find out, and enjoy winter and “snow puzzles.”
Lawrence J. Winship is emeritus Professor of Botany at Hampshire College.
Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 845 West St., Amherst, appears every other week in the Daily Hampshire Gazette. For more information, call 413-256-6006, or write to us.Click here to return to full list of Earth Matters articles.