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?