The northeast vent on the well pit grew an ice rim during the same night that frosted the roofing nails:
The other vent remained clear, so the prevailing breeze definitely goes in one hole and out the other:
Well Pit Vent Frost – inlet
A waterproof Hobo datalogger hangs from the string.
The Smell of Molten Projects in the Morning 
Comments
2 responses to “Monthly Image: Frost Growth”
Do you think it’s possible that the string served as a kernel for frost formation?
I think not, because the frost grows symmetrically around the pipe.
Yesterday, the IR thermometer reported 19 °F below the “inlet” pipe and 29 °F below the “outlet” pipe, so the pit really does vent the way I expected: the frost shows where moist air condenses. That’s way cold for a pit with a water pipe in mid-air, so I plugged the vents to see how the ground temperature stabilizes.
It’s been that way for over half a century, though, so if the pipe hasn’t frozen yet, it probably won’t; this is mostly curiosity.