On Dec 24, 11:07 am, quietguy wrote:
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.

Frost forms when the metal radiates its heat into the clear
sky faster than the surrounding air, so that it gets cooler than the
air and the moisture condenses on it in the form of frost. In flight,
the air moving around the wing keeps it at the same temperature as the
wing and frost won't form. Other forms of ice will, in the right
conditions, but they're not frost. They're impact ice, supercooled
water droplets that freeze when they hit the wing and anything else in
the way. Frost forms directly from vapour to solid without going
through the liquid phase.

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.

That's hoarfrost, related to impact icing. Not sublimation. It's
the same supercooled water droplets found in ice fog.

Dan