It’s more like crystal growing. What little water vapor that is present in the atmosphere might not fall as snow but condenses out directly onto an existing ice substrate.
“condenses out directly onto an existing ice substrate.”
You mean like rime ice?
I have no feel how water acts at 7 millibars of atmospheric pressure, so do not have a clue.
Hmmm, thinking about it, rime ice forms on the carburetors of airplanes at extreme cold temperatures and low vacuums. You probably are right.
.......Bob