Windchill in the classic sense will have nothing to do with the cooling capacity of the radiator. Windchill is a combination of ambient temperature and wind speed. It is meant to take into account evaporative cooling effects of the skin. So unless the radiator is leaking or it is raining, there is no evaporative cooling effect on the radiator.

To say temperature has no effect in the ability of a radiator to reject heat is also not true. Take the extreme, if the outside air temperature was 100 degrees F and the radiator was running 100 degrees F, there would be no convective losses (still potentially would have radiation and conduction losses).

Now the real question is does humid air or dry air have more heat capacity. I want to say humid air does (I’m sure someone can chime in), which although this is counterintuitive means the radiator will reject more heat on a humid day verses a dry day of the same temperature (give all other conditions are the same).