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We have often used the large snow free circles that form in the spring around the trunks of large trees as camping spots. Even in an area blanketed with deep snow, there will be such bare, dry ground to camp on, although one may have a steep climb into or out of such a spot. They are like dry wells.

First, what are such bare-ground wells called?

I assume that the dark tree trunk absorbs heat and then acts as a heater to speed up melting around the trunk. Moreover, the snow was probably shallower around the trunk because of the tree branches. We observed these circles under evergreens.

Second, does this accelerated melting happen only when daytime temperatures go above freezing, or does it start earlier?

  • Former tree wells? – Martin F May 31 '19 at 23:58
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    Yes it’s almost certainly former tree wells. en.wikipedia.org/wiki/Tree_well Since there was less snow there in the first place, it will melt sooner. It’s harder to tell if there is accelerated melting due to the trunk. More likely it’s that when bare ground appears that is darker and warms up more and thus expands the bare ground. – T. M. Jun 1 '19 at 13:41
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    It could also be the tree bringing up warmth from underground as it starts "rehydrating" in the spring. But I would agree with @T.M. that it's just tree wells, having less snow to begin with, melting first. – IronEagle Jun 8 '19 at 5:08
  • @IronEagle interesting idea, it make sense it would do that. I wonder how much of an effect it would have. – T. M. Jun 8 '19 at 13:04
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    dust and bark/twig bits falling from the tree can also have a dramatic effect on the rate of snow melt. Once you have a small depression from differential snow melt, wind blowing across would flow turbulently and further gouge snow from the well even if temperatures dropped lower. – That Idiot Oct 28 '19 at 17:17
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Much of it is due to conifers -- especially spruce and fir shedding snow away from the trunk. I routinely see trees with 1/3 of the snow under their branches as in the open.

A spruce in winter appears nearly black. Sunlight enters, and bounces around a lot before escaping. Most is absorbed during a bounce.

So the crown of the tree is warmer than the surrounding air. There is some warm radiation between the lower branches and the ground.

The space under the tree cannot radiate readily on a clear night into space. So the temps at night aren't as cold.

My experience is that wells down to bare earth don't start to form until late winter or early spring. I don't think that temps have to be above freezing for them to form, but likely start when temps are near freezing.

The wiki article https://en.wikipedia.org/wiki/Tree_well describes the formation of much larger ones than I've seen. (We rarely get snow deeper than 2-3 feet here)

Here, in spring, the effect of a row of spruce is strong enough that a band of bare ground 20-30 feet wide forms on the south side of the trees, while snow is still a foot or more deep in the general open area.

I've not noticed tree wells around any type of tree besides spruce or fir. Pines generally don't shed snow outward. Cedar and hemlock may support tree wells. I don't have them here.

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