I have an untested hypothesis regarding why wool is warm in winter/cool in summer: Unlike some other fibers, wool is a little wirey (famously scratchy) and so the hairs stick up, presenting more wetted surface to the air (even when humidity is high, this is still an improvement over, say, cotton or dacron) and so there is more evaporative cooling. Conversely, during cold weather the deeper the napp, the thicker the insulating layer of still air between your bod and the outside world. However, I suspect I'm somewhat allergic to it.