You Lose Most of Your Body Heat Through Your Head

Your head is not a special heat chimney. It loses heat at roughly the same rate as any other bit of bare skin of the same area, which is to say in proportion to how much of it is exposed, around 7 to 10 percent of your body's surface. The myth seems to trace to an old military survival manual, which reported high heat loss from the head, but in those tests the subjects were bundled up in cold-weather gear with only their heads uncovered, so naturally the head did most of the leaking; it was the only thing exposed. Cover your head and leave your chest bare and you would lose more heat through your chest. A hat is genuinely useful in the cold because the head is often the part you leave uncovered, not because skulls are uniquely leaky. So wear the beanie, just not because your head is a radiator.