A well known clothing manufacturer (can't tell you their name, but their initials are REI) claims that you lose 75% of your body heat through your head. They sell hats, you might guess. A wetsuit manufacturer claims you should wear a rubber hood because you lose 80% of your body heat through your head. Don't try it on the subway, but it might help some in the water.
Think about it, though, if all that was true you could get naked, put on a hat, and be warmer! Research is under way...
According to Guyton & Hall's Textbook of Medical Physiology, a nude person in dry, calm air loses about 60% of their body heat simply by radiation through their skin. Makes you hot, er...cold, just thinking about it, right?
Actually, you radiate much like a light bulb, but infrared energy, not visible light. In fact, you radiate as much as a 100 watt bulb when you're sitting still. You're even brighter (light-wise) when you exercise, not so much if you have poor circulation.
You lose another 15% of your body heat through conduction to air. Your fast moving little skin molecules rub up against slower (colder) air molecules and warm them up while you cool down. Add a breeze to move all that warmed up air out of the way so you can warm up some more, and you can lose another 15% through convection. Wind chill happens!
Couch potatoes take heart, just sitting there contemplating Vanna's navel, you're burning calories and losing heat when water evaporates from your body surface and even when you just breath, for that matter. You lose 16 calories an hour and over a half a quart of water a day through your skin and lungs. What goes in must come out?
When you sweat profusely you can remove heat up to 10 times faster, you can produce over 3 quarts of sweat an hour (where's your 7-day deodorant pad?), and you can lose as much as 1/4 cup of salt a day.
So if you're all bundled up, yeah, put on a hat. But all the rest of your body needs covering too. Please.