On Dec 24, 11:32*am, Jose wrote:
What would prevent this from happening in flight?

Sublimation. The airflow around the wing evaporates deposited ice
crystals fast enough to keep visible frost from forming unless the
relative humidity is near 100% (i.e., when there's visible moisture).
Then, depending mainly on the droplet size, you'll get either rime
(frost by another name) or clear icing. Takes a hell of a wind to
cause rapid-enough sublimation, though: I've seen parked airplanes get
frosty even when gusts were in the forties.