Last night was interesting weather, perhaps the kind where a high tunnel would get colder than outside. My forecast low was 34. At 1 AM, it was 36, but the grass was crunching under my feet and there was a thick layer of frost already. Inside the high tunnel, my plants are on a bench over horse troughs of water. The thermometer read 42 by the plants. But there was a thick layer of ice on the inside of my plastic, which I guess was frozen condensation.
It does seem counter-intuitive - if heat rises, why do I have ice only on the ceiling? I think it is an example of atmospheric radiative cooling.
As a reference, a cold front had just moved through - clear skies and dry air. I think the conditions for frost at least correlate with the radiative cooling, if not entirely the same.
Looking at the NWS site, my nearest station recorded a low last night of 29. They were off by 5 degrees in forecasting the low.
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