What are the higher heat loss areas of the body and how can I reduce heat loss in those areas?
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:
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.
It's probably worth pointing out that a lot of people reading this question may be thinking along the (commonly quoted) line that about 80% of body heat is lost through the head - which is much more of a myth than people realise (See here for details.) From what I remember, it was an experiment done with people fully kitted out apart from the fact that they weren't wearing anything on their heads... and, somewhat unsurprisingly, most of the body heat lost in this scenario was from the head.
Of course, it is important to keep the head wrapped up, but it's equally important to keep other parts warm - especially the torso. Even though extremities may feel colder, this is because your body is reserving the heat for its vitals and cutting it off more to things such as your hands and feet. Warm up the core of your body and it'll start warming your extremities as a result!
In terms of how to reduce heat loss in those areas, it's generally as simple as wrapping up as warm as you feasibly can (and as much as is appropriate for the conditions. Obviously full arctic gear isn't required for a damp autumnal day!)
In short, wrap up as best you can all over, but it's especially important on the torso. In practice though, it shouldn't come down to deciding what to wrap up in particular - if it's cold, wrap up as best you can all over!
Heat loss can occur from anywhere on the body through a number of processes:
Combat heat loss by:
As for specific, high heat loss, areas of the body, it depends:
In general, the high heat loss areas will be those not adequately protected from the various mechanisms of heat loss prevalent to the situation you are in:
here is a pictorial answer to try and figure this out! I believe this is a un-clothed male under a thermal camera (SFW)
Lots of ways to reduce heat loss: Wear a hat scarf and gloves as these keep the most important parts of your body warm. Wear warm knickers (even if you're a guy). You can get fleecy knickers but not fleecy pants. Also, you can huddle, like penguins as it reduces your surface area and conserves bodyheat.
While I personally disagree to other answers, I'd like to make a point. According to my understanding an uncovered head will loose a lot of heat. Put a hat on and no top and see what happens. I'm a hunter and when I sit, I find the cold area are the shoulders, kidney and wrist.
To answer the question directly, the three areas that lose heat the fastest are actually:
This is why those are the areas that naturally grow the most hair on human bodies. It's an evolutionary adaption to keep us warm. In those areas, there is a lot of blood flowing and it passes very close to the outer layer of skin, allowing it to cool of more rapidly and lower our body's temperature. It's all of the blood rushing around your skull, into your legs, or into your arms, respectively.
Covering your head is a no brainer. As others have mentioned and most people already know, the head it where you lose most of your body heat, so wearing a dry, windproof hat with a lot of loft is always a smart idea if you're cold.
The groin and the armpits are a bit more tricky. If you're dressed according to normal human social standards, you'll probably be fine. But if, for some reason, you're out in the cold with little or no clothing, those are the areas to focus on the most.
Keep your arms tight at your sides and cover your groin with your hands. Not only is that our natural reaction to the whole "being naked" thing, it also happens to be the best way to stay warm with no clothes on!