Thanks for the article Rex. Voice recorders, now thermometers. . . I feel like I should out-geek you with an article on the best backpacking slide rules.
>"I felt colder on some nights that were several degrees F warmer than others, according to the max-min thermometer. I think the difference was a slight breeze, despite using the same sleep system."
I notice I feel a lot colder despite the same dry-bulb temperature when in a creek drainage. Cold air can be falling downslope, generating a breeze near a moderately-sloped stream.
The other big variable I find is cloud cover versus clear skies. That makes a 5F, even 8F difference for the same air temperature. We are close to equilibrium with clouds - we radiant away infrared heat, the ground, trees, and clouds radiant infrared heat back at us. But on cloudless night, grass, trees, tents, and people are radiating infrared heat away into space while getting almost nothing back. (Overly technical point: while deep space is 3.2K (-453F), and clear air is mostly transparent to IR, the water vapor in the atmosphere does radiant a little. I've measured sky temps by IR of 30F on a 70F clear day and -45F on a cool, 30F winter day.)
>"I question laying the thermometer on the ground could that not effect the reading if the ground temp is different from the air temp"
You can tweak what you are measuring by how you place the thermometer. If you wanted a good air temperature, I'd place the thermometer in a tree, a few feet off the ground. That avoid the radiant losses discussed above. If you want to include those radiant losses on a clear night (so the thermometer reports what your tent or bivy is experiencing), then place it in the open. Rarely would the thermometer be in good thermal contact with the ground, so the big factors are if it is in the wind it will record close to air temperature. If it sees open, clear skies, it will record low due to those radiant losses. Putting under a tree or "shading it" from the clear sky with a branch, stone, or pot lid would get it back to recording close to air temperature.
Bringing two thermometers (or playing at home as I do) and placing one on top of your car while another is under your car, for instance, and you'll see the magnitude of the radiant heat loss on different nights. Windy and cloudy - little effect. Still air and clear skies - huge difference.