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.
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