Three temperatures
We tend to focus solely on air temperature and many tend to believe that if it's above 0°C, we won't have icing.
But that presupposes that the airframe and precipitation, if any, are also above 0°C.
In this case the airframe was sitting overnight in freezing precip and was cold soaked, including the fuel in the tanks which had been fueled the night before.
Possibly any precip with liquid content would freeze to the wing skins on contact.
Precip well below freezing will not adhere to a wing well below freezing.
But it has to be remembered that precip in a narrow zone above and below 0°C can have a mixture of solid and liquid content.
On a sunny breezy day well above 0°C, the wing will likely remain clean after de-icing, provided that the wing temperature is well above the dewpoint.
With wing skin temperature below the dewpoint, condensation can occur, especially in the absence of wind and sunshine. That's how frost happens overnight.
Last edited by RatherBeFlying; 25th Dec 2019 at 16:08.