Our grandmothers said it, our mothers said it, but I’m starting to question it. It sounds like an old wive’s tale, made up to get everyone to bundle up as much as possible in the winter. I am finding it hard to believe that walking around in January with an exposed head is really any worse than walking around with an exposed arm, thigh, ass, or chest, assuming that the surface area of exposed skin is the same in each scenario. What’s the straight dope on body part heat loss?
The NYT says no.
The linked article says you do when you’re wearing winter clothing, which is when our mothers and grandmothers were telling us to put a hat on.
Of course, the article goes on to say that if you’re wearing a swim suit, the heat loss is going to be proportional to surface area. But, if I were going out into the cold with a swim suit, “wear a hat,” wouldn’t be the first thing out of my mom’s mouth.
As the other two posters have in effect said or linked to, the old tale is a corruption of a sound principle - that when the rest of your body is insulated, an uninsulated head proportionately loses much of your total heat loss. Somewhere I have a bunch of research papers from the Army where they calculate the exact amount lost from the head on a nude person, as well as a nude person with thick hair, short hair, or even bald head. I don’t have the cite handy, but my recollection is that it was over a range of 8% to 18%.
You have to take into account blood flow to parts of the body as overall temperature drops.
If you are no longer producing enough heat energy to balance loss; your circulatory system goes into a preservation mode. The extremities like fingers and toes numb out first (note frostbite injuries to climbers and hikers caught out). The circulatory system is concentrating blood flow to core and the brain (head). So the proportion of heat loss through the head will increase because the blood flow has not decreased there but has decreased on the rest of the body surface area.
Someone versed in cold weather injuries will be along shortly to explain things more elegantly but I believe my gist is correct.
The old saw about, “if your fingers and toes are feeling cold - put on a hat”, does have a basis in fact.