A touch of frost

[Harry Bloomfield]

We saw our first frost this morning, just a trace, on the hut roof, during the coldest morning since the summer. The minimum I recorded overnight, was +3.7C, at 06:42.

I don't quite understand the logic, of frost appearing, when the temperature remains above freezing?

 

[Brigadier]

Because the ground / surfaces can be below freezing (due to radiative cooling), while the recorded temperature is an air temperature (which is a poor conductor of heat, and is in a shielded enclosure).