Wing ice formation is a function of distance from top of fuel to underside of wing skin or a combination of thermal conductivity (wing skin) as well as thermal convection (air above fuel). Factor in the temperature of the fuel (cold soaked) and if water vapor is present in still air.
And yes the outside air temperature needs to be considered relative to themal conductivity of the wing skin.
Upper wing ice may be likely in temperatures as high as 45 degrees F if full tanks are present.
Wing ice has been a surprise player in engine damage (DC9 series 30-80), but unseen ice from overnight precip (clear ice) has been the cause of more ice related accidents