frost on the wing

Recently, Jose posted:

[...] But when I was pretty much done, I
noticed there was frost on the wings, which hadn't been there before.

Which got me thinking - maybe the FAA isn't so crazy after all. If
frost can form while the plane is just sitting there, why could it
not form while the plane is flying? (yeah, there's some frictional
heating, but we could lower the temp a bit more, no?) And if frost
isn't such a good thing to have on takeoff, it's probably not so good
in flight either.

Any thoughts? Have you seen this before?

During my student pilot days, I took off solo in a Tomahawk that had a
good coating of frost on the wings. Its performance was sluggish, but not
long after I wrestled it into the air the frost dissipated. I learned a
lot that day!

Neil