I do not agree that frost can form on an aircraft in flight. The many references I can find to the formation of frost refer to calm, still, or static air being necessary for formation. My experience over four decades of Canadian winter flying has never shown "frost" forming on a moving plane - only on a motionless one, in calm air.
I support this with the knowledge that to prevent formation of frost on valuable crops, helicopters are flown over low, or fans used to disturb the air. It must work, it's been done for years. I can also assert that an aircraft flown with frost in non critical areas at takeoff, will land back with that frost gone due to sublimation in flight - moving air.
I have seen frost form on the fuel tank skins of a "wet wing", but not otherwise.