I suspected that it was something like that.
We have had 3 frosts so far where I live and only one of those frosts involved temperatures as low as 31°. The other two, it was 40° and 36°.
My thought was that evaporation had something to do with it, but what you discribe makes sense......I guess.
I have confidence that my thermometer is not off by anything like 8°, but it is mounter under the roof portion of my back deck and the frost collects on the roof and the windshields of cars and so forth.
What still confuses me is how the water droplets are able to lose heat, but the surrounding air is not. Perhaps water being a better conductor?
And yes, on all occasions it was under a clear sky.
And lastly, my apologies to those who are Celsius users, but converting Farenheite to Celcius is not all that tough a calculation.