Thanks. Both explanations make sense (and are complementary with cool air on a still night settling on the plastic surface with radiant cooling ensuing).
Regardless, the forcast low tonight is in the mid-forties which could mean 40 for a couple hours in my greenhouse. I'm not in this to make money, I'm in for early tomatoes. I threw a little electric heater in there. Right now, it's 51 outside, 62 in my greenhouse.
Small greenhouses are like small fish tanks. They are more unstable than the big ones.
But, fun!
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