Clearly your Harley’s engine will run cooler in winter than it does in summer, but not as cool as you may think. Most Harley Davidson models run air-cooled engines; cooler air means a cooler engine, all other things being equal.
But the amount of cooling depends on the difference between engine temperature and air temperature (remember that from high school science?)
Summer: head temp 180°C, air temp 40°C: difference = 140°C.
Winter: head temp 180°C, air temp 20°C: difference = 160°C.
So the cooling effect in winter is only 4-5% better (calculated using the Kelvin temperature scale).
Riders feel a big difference between summer and winter, but your engine won’t. I’ve been using my cooling fans in Sydney traffic even in these cooler months.