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 to wear a hat.
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