As pointed out, a cold soaked airframe just down from FL350 can develop frost over the fuel tanks in clear air. And fine drizzle can also get stuck all over a cold airframe.
There's three temperatures to keep in mind: Air, airframe (especially over tanks), precip.
Operators in Alaska have figured out that dry snow on a cold airframe doesn't stick. But watch out for an airframe just pulled out from a cozy warm hangar with snow coming down.
Airbus and Boeing publish guidance for frost over fuel tanks on various aircraft. The performance degradation has been found acceptable in these cases. We should also remember that certain deice fluids also come with a performance penalty.