Ideally, the weather would stay just at 80.6 degrees F. At that temperature, the naked, resting human body is "thermoneutral": the heat it generates matches the heat lost to the air around it.
But rare is that kind of Eden, so the human body adapted. In the heat, the evaporation of sweat cools us off. In the cold of northern latitudes this time of year, blood vessels near the surface squeeze tight so blood is shunted deeper into the body. That transfer has several effects. The brain, the heart, and other important organs stay toasty, albeit at the expense of the now blood-depleted hands and feet. The lack of superficial blood flow greatly increases the insulating properties of the skin, the subcutaneous layer of fat just underneath it, and nearby muscles. And, of course, there's shivering. It may not be comfortable, but those shaking muscles generate extra heat.
But we're not as thermostatically sensitive above the neck as we are below it. Blood vessels in the surface of the head constrict very little in response to cold, which is a good thing because the brain needs a steady supply of blood. There's little subcutaneous fat for insulation. As a result, even if the rest of your body is nicely wrapped up, if your head is uncovered you'll lose lots of body heat — potentially up to 50% of it — in certain cold-weather conditions. What's more, a cold head can trigger blood vessel constriction in the other parts of the body, so it can make your hands and feet feel cold even if you are wearing mittens and warm socks and shoes.
The solution, of course, is a hat and, if it's really cold and you want to really stay warm, maybe one of those face-covering balaclavas. Wool is a good insulating fabric because it traps air, but not if it gets wet. These days, warm hats are made of polyester fleece that repels water. Some of the warmest have some protection against the wind around the ears but allow moisture to evaporate through the crown.