We’ve been having some warmer temperatures over the last couple of weeks. Warmer being a relative term, of course – most days it’s still at or below freezing, though perhaps not by much. We’ve had sprinklings of snow on a few days, though it hasn’t accumulated to much. Most days, however, have been sunny. Today is yet another sunny day. I’ve been noticing some snow melt as a result of all the sun, which can warm surfaces up above the freezing point. On our driveway and areas that receive a lot of traffic you can actually see the bare ground now. Also around the edges of the buildings where the snow doesn’t pile up as much because of drift patterns and the overhanging eaves. There’s bare ground under the evergreen trees, whose spreading boughs prevent deep accumulation of snow. And of course, there are the melt rings at the bases of tree trunks.
When I snapped these photos out in the 100-acre woods I thought this would be a pretty quick and easy post. Short, not much to say, right? There had to be dozens of posts talking about this exceptionally common phenomenon, right? Surprisingly, no. When I googled it today, most of the pages asking about it were things like Yahoo Answers, WikiAnswers, Answerbag, and similar. And judging from the great number and variety of answers posted to these questions, no one really seems to know.
The answer that seems to be mentioned the most is that the dark tree trunks absorb the warmth from the sun (even on cloudy days, some light still makes it through) and re-radiate it back into the environment; even just the slight rise in temperature from this is enough to melt the snow. Other explanations put forth were that the area under trees receives less snow to begin with; that the snow in the boughs of the trees melts and causes water to run or drip down the tree, melting the snow at the base; less grass means more geothermal heat rises from the ground under a tree than in the open; and turbulence caused by the wind breaking around the tree carries moisture away faster than in open areas.
I don’t know if anyone has done a rigorous scientific study to really say definitively. However, I found this study, which measured snow depth and ground surface temperature (where the ground and the snow meet) to a radius of 6 meters (20 feet) from a tree trunk. Their results showed that early in the season, snow under the pine tree being measured was only a third of what it was in the open, and by late in the season it was just a fifth. Probably because there was less insulative snow cover under the tree, ground temperatures closer to the tree were colder than those farther from the tree (this also meant the ground was frozen to a deeper depth under the tree than away from it).
To me it’s the re-radiation of heat from the trunks that makes the most sense, especially since in many cases the sunnier south side of the tree showed more melt than the north side. I’m surprised not to find anything definitively explaining the phenomenon, though.