A few years back in YOW, I noticed a fine sprinkling of precip on the wing and pressed the CC call button to ask if the precip was liquid or frozen.
The captain called me forward and suggested that at 3C it had to be liquid. I stated that in my a/c I'd be putting a finger on it to make sure.
I abstained from discussing the physics of cold soaked wings and fuel tanks.
Some years before I observed frost over a tank on a 20+ day, but that type was approved for takeoff with frost in that limited area.
A rampie appeared shortly after and verified it was liquid.