Let's kick this off with a little role-playing game: You... are not you. "You" have never traveled outside the tropical rain forest you know only as "home." There's no electricity in your village, the outside world is a sort of bizarre fleeting concept... and then one day a stranger shows up. An anthropology grad student. And you fall madly in love. When they ask you to leave with them, you can't suppress your exuberance when you say "Yes!" And this new love of yours gets a quizzical dancing look in the eyes, and says they'll show you a place where boiling water freezes in mid-air... a far, far away land called "Northern Minnesota," where the world is white many months of the year.
You get to Minnesota as summer is drawing to a close, and people warmly welcome you with you tips about how to dress in winter... and you inwardly smile a bit smugly, because you're wearing the sweater that a friend used to wear in the mountains. In fact, you're thinking about taking the sweater off right now, and it doesn't seem likely that temperatures could change THAT dramatically. But as the weeks start going by it gets COLD! And your new friends smile, and say something about the leaves all still being on the trees... and you start to listen to all their snippets of advice. Before long you feel almost dizzy with all these ideas on how to dress for "winter," and although you think there must just be some connection you're missing, a bunch of those ideas seem to contradict each other. What should you do?!
Let's start with a look at physiological need... completely aside from your comfort and not wanting to feel cold, why does your body need to stay warm? What is it trying to do? Why do some areas seem to get colder than others or, conversely, some parts seem sweaty while others seem almost numb from cold? Why does it seem like lots of clothing doesn't seem to make much difference on some parts of your body, but worn elsewhere even something thin can make you feel much warmer? In a word: survival.
As rude and cold and calculating as it might seem, your body is quite aware that some parts of it aren't critical for survival. Conversely, if other parts of your body shut down you'll die... so your body is "wired" to sacrifice some parts of itself to save other parts. Long before you actually get to the ragged edge of survival, though, your body lavishes attention on its critical parts: your brain, heart, & organs. The thing is, your body can only produce a finite amount of heat... like a small furnace in a big house, when things get really cold you have to close off some rooms. If your body senses that critical areas are getting cold, it shunts heat away from your extremities and brings it to your core. That's why our toes & fingers can so often feel cold when we otherwise feel comfortable.
Imagine, if you would, a house with well-insulated walls... and no insulation in the ceiling or roof. That'd be nuts, right? (It's pretty common for houses in northern latitudes to have two or three times more insulation overhead than in the walls.) Consider that analogy as it relates to winter apparel: If you're wearing a big poofy down parka, it wouldn't make sense to not also insulate your head. You would lose a wildly disproportionate amount of heat through your head.
Let's use a cooking example along a similar vein: You're broiling some steaks and smoke billows from your oven. The smoke alarm pierces your ears. You get out a fan... and close all the doors and windows? No, of course not. The smoke couldn't go anywhere. You open the windows and doors... you REMOVE THE BARRIERS to airflow. You put as little between the outdoors and your smoke-filled kitchen as you can, because doing so will get rid of all that air that would otherwise linger in your kitchen for days.
The point is that "creating" warmth really means RETAINING warmth. Your body produces heat; it's your job to keep that heat near your body, which we accomplish by creating DEAD AIR SPACE. Great insulation is sort of a nightmare-ish labyrinth for gleefully excited warm air. We make it more difficult for warm air to escape the microclimate over our skin. If you insulate some parts of your body and not others, the parts that are not insulated will, obviously, lose a lot more heat... which is why you can be wearing a big down coat and be cold, but add a hat and stay warm. By extension, if you wear a super-thick, wind-resistant jacket and counter it with a thin beanie with an open weave... you're obviously not as well insulated on your head, and you will minimize the efficiency of your big down jacket because you're blowing all kinds of extra heat out through your poorly-insulated head.
This is partially where the old saying, "If your feet are cold, put on a hat," comes from. Your head can lose proportionately more heat for its given body surface area, precisely because of mechanisms that shunt blood from your periphery to feed the core... there's more blood flow there, and it's more exposed.
Thermoregulators throughout your body constantly collect data and forward it to your main computer. If your temp is dropping, your peripheral blood vessels will constrict, which will help shunt warm blood to your core. Conversely, if you're overheating, your peripheral vasculature will dilate, maximizing surface area exposure and radiant heat loss. Incidentally, if you're wearing clothes or footwear that constrict or compress, it can have a similar effect of diverting some blood flow away from the periphery and toward your core. And this, this seems like a good place for us to pause. Next time we'll talk about what the whole concept of "layering" really means, when it works best, when not to dress that way, and how to make it work effectively when you do.