With more data, those February lows really stand out:
The February daily record shows more detail:
I’d say the air temperature in the pit got close to freezing, but surely a stream of cold air falling through the vent hole would wash over the logger and depress the results.
Should I get powerfully motivated, I’ll strap the logger onto one of the pipes, wrap insulation around it, and have it take data from early December through late March in one session; lifting the concrete slab requires enough effort that I’m not going to do it, ah, lightly during the snow season.
The Smell of Molten Projects in the Morning 