What are areas of the body which lose heat more quickly, and how can I reduce that loss?
My original answer to this question sparked a surprisingly intense debate, so I'm rewriting it to clarify a few points and offer a more holistic answer.
Let me start by saying that every square inch of skin on the human body is capable of allowing heat to escape. That is to say, if you wear a jacket with no pants, your legs will lose more heat than your torso -- but if you wear pants with no jacket, your torso will lose more heat than your legs. Both areas are capable of losing heat -- and will do so more quickly when exposed. Therefore, any part of your body that's more exposed to the cold than another part has the potential to lose more heat than other than parts that are less exposed.
This is fairly obvious, but I mention it for completeness' sake. This generally means that you should try to cover yourself as much as possible, especially larger skin surfaces like the torso.
However, the point that I think the OP is trying to get at is that -- all else being equal -- every square inch of exposed human skin does NOT lose heat at the same rate. That is to say, some areas of skin lose heat faster than others. If you were to stand in the cold totally naked, some parts of you would lose heat more quickly than others. This is an indisputable, biological fact. Allow me to explain:
Human blood serves many functions -- one of which is thermoregulation. As an evolutionary adaption, humans have areas on their body that are especially suited for releasing heat, in order to cool us off when we get too hot. In these
areas, there is a high volume of blood flow, and that blood passes very close to the skin. These areas are:
- The skull
- The groin
- The armpits
- Some might also say the extremities, like fingers and toes.
In these areas, there is a lot of blood flowing around, and the skin is fairly thin, allowing heat to escape from the blood more quickly than in other areas.
Therefore, if you are in a desperate survival situation where you risk hypothermia, these are the areas of the body you should focus on keeping the warmest.
Armpits are fairly easy to keep warm, just keep your elbows down and they'll be insulated by your torso and upper arm.
It's important to keep warm, dry underwear in order to keep your groin warm as well.
And yes, YOU SHOULD WEAR A HAT. While you might not lose the "majority" of body heat through your head, it is one of the areas of skin that loses heat most quickly and it should be one of the main things you focus on keeping warm.
The extremities are also important to keep warm. I have friends that go out jogging in the winter wearing shorts and a t-shirt. They don a warm beanie and gloves right before they go out, and those are all it takes to keep them warm in freezing temperatures.
In summary: If you have clothes or ways to protect every inch of skin on your body, you should do that. I thought this was basic common sense, but people were squabbling so I'll reiterate.
However, if you only have limited means of protection and you need to focus on warming only a small area of your body, you're best off focusing on the four areas listed above.