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.
seabrookeleckie.com 
I almost told E.g. yesterday, when we were out for a walk, how I had read in your blog about the warmth of trees melting the snow around them. There must be a glitch in the Matrix.
That’s a funny coincidence, Lavenderbay! The magnetic hill’s out in Moncton isn’t it? I was going to suggest perhaps it caused time to flow backwards sometimes, too…
Yes, I agree it’s mostly re-radiation, but if there’s any drifting while the snow is still loose, the wind can quite quickly carve out little craters around each trunk.
It’s funny, Dave, in all those theories that just simply wind blowing the snow the way you see drifts form around houses didn’t even cross my mind.
Seabrooke — I hadn’t given much thought as to why the bare rings existed around trees in the snowy winter. I always thought the area surrounding the trunk of the tree was a warmer climate than further out from the trunk due to the temp of the trunk compared to the ground. I think? you came to a conclusion similar to this? Anyway it was fascinating to travel down this puzzle with you — Barbara
P.S. Shouldn’t one start with bare ground just before it snows and then measure the rings over a day or so to prove any theory. It seems I remember that the rings grow larger over time. Oh see what you started in my mind!
I bet there are all sorts of things one could do to prove or disprove the theories, Barbara. Measurements would be one, some sort of infrared sensor that detects small amounts of heat radiation would be another. The easier things would perhaps make an interesting science project for a kid.
Talking to my friend that’s going to school for ecology, the answer does pertain to heat. Essentially, since the tree doesn’t freeze solid (it is living and all), it is slightly warmer than the snow, causing the ring to form around it (it absorbs heat like anything else when the sun shines on it). As for the south side “shadow” that forms, that’s due to latent heat. As the sun shines more on the south side, the tree reflects the heat, creating what seems like an inverse shadow.
But I do find it entertaining that I stumbled onto this blog because I refused to put up with the wikianswers/yahoo answers and their obvious inaccuracies. :P
Thanks for the info, Stu! That’s essentially what I suspected, but I couldn’t find anything that confirmed it.
It’s funny that there’s no moderation or screening process for answers on Wiki/Yahoo, and those asking are left to use their own judgement about the validity of the answers.
This blog and thread was an amazing find for me. When my friend Jim and I tried to figure this out while hiking yesterday, I assumed the answer would be readily findable through a very quick Google search. Not so. And I still haven’t heard a satisfactory explanation. It’s not the sun, because the rings are always symmetrical, not more on the south side. Could it possibly be metabolic heat from the tree? Hard to believe.
Here in the Northeast we just had a heavy snowfall overnight. Almost 2 feet onto bare ground. No sun yet. All the trees have rings; all symmetrical. Rings are visible around telephone poles also. The rings are showing sharp borders and are about 1 foot out from the trunks. The trees are oaks and maples with no leaves, so the snow had opportunity to fall close to the base. The winds are swirling the snow around, and snowflakes don’t fall straight down anyway. How about this: If the snowflakes fall in various directions, then the trunk or pole blocks a certain percentage of those coming in on an angle, and if the angle of fall is randomly distributed we get a “shadow” of lesser depth of snow around the tree. The sunshine effect might make the rings asymmetrical as time passes, with the ring wider on the sun side, but this hypothesis doesn’t depend on the trees or poles being warmer to start with.
The hypothesis would predict that if snow were driven from one direction by strong winds that the rings would be asymmetrical, tailing away on the down-wind side. With heavy, wet snowflakes the rings should be closer to the trunks.