Third mode of heat transfer - the reason wind machines exist in orchards - radiation. On open ground your near-ambient temperature tent fly is radiating its heat to space. Tent fly temperature reaches equilibrium at some temperature less than ambient air temperature, with natural convection heat transfer from ambient air pacing heat loss to space.
If the equilibrium temperature of your fly is below the dew point of the air inside or outside your tent, you'll get condensation or frost on one or both surfaces (inside first as it's more humid).
Overhead foliage shields your fly from the radiative heat sink (the sky), so tent fly temp is higher.
One side of my driveway is shaded by low cedar branches. If it's going to be clear on a late fall, winter or early spring night, I park my commuter car underneath the tree so I don't have to scrape ice on the windshield in the morning.
Sometimes setting up in the woods as opposed to an open area can markedly reduce condensation. Don't know why.
