How fast does the skin of the airplane cool to surrounding temperatures?

Ash Wyllie writes:

Actually, it can. I have seen frost on metal surfaces (like wings) when the air
temp is above freezing.

What was the temperature profile of the air during the preceding
hours, and at the exact time of the observation?

If the air is warming up and moist, metal surfaces might well be below
that temperature and below the dew point.

A clear night sky is /cold./

Yes, because large masses of water vapor have a moderating effect on
temperature, making warm days cooler and cold days warmer.

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