Montblack wrote:
("greenwavepilot" wrote)

snip

Michael, I am training in a Diamond DA-20 C1, incidentally, the only
composite airplane on my flight schools ramp. I am flying in upstate
SC. This morning, at 8:15 the top surfaces of the wings on the C1 were
iced significantly, as was the nose and fuselage (tail boom). Outside
air temp was 41*F/Overnight low was 40*F. Plane is tied-down, morning
sun was directly on wing surfaces, no intervening shadows. My lesson
was delayed, of course.

There can be a thermal "dip" right before sunrise, right about at wingtip
height. Duck hunters and deer hunters will confirm (and curse) this
temperature phenomenon - forget what it's called.

41F overnight? 40F at 8:15? And still ice?

So it either go down to 32F at or near your wing, or it was below 32F a
number of feet, maybe many, many feet above your wing? Or your wing was 32F
at some point in the early morning? Wonder what it was?

The temperature of a surface that's radiating heat to a clear night sky
can drop considerably below the ambient air temperature. So it's
possible for frost to form even when the air temperature never gets
down to freezing.