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Not exactly a "myth", but it probably is a little overstated......
I guess that depend on what one finds to be overstated. It's believed the myth started from a study done by the US Army in the 50s and their field manuals declared one lost 40-50% of their heat through their head. A field manual I recall seeing in the 80s.

About 10 years ago, several studies took on the challenge and determined that heat loss through the head is only slightly greater in proportion to the relative skin exposure of the head. Perhaps that slight increase is due to the capillary density you refer to.

It's thought the original experiment was flawed because one's extremities (head, face, hands, ears) are more sensitive to detecting temperature change, in order to control the body's reaction.

For me, interestingly, I find that covering my neck in frigid temperatures is the first thing I do to feel warmer. But I don't think it makes me any warmer.
 

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It's also a myth that one loses more heat through their head. Look it up. It comes from the idea that one often dresses warmly elsewhere, therefore, loses more heat through their head than where they are insulated. However, surface area is surface area. Just insulating the head, doesn't make up for the rest.

T-shirts are an excellent idea to protect against sunburn. That's all.
Not sure what you are reading but when your body gets cold, it constricts blood vessels in your arms and legs. The head and core then become the primary zones for heat loss. Your core as some one has already mentioned has its own defence against heat loss which is fat which then leaves your head as the main open valved heat exchange radiator with no defence yet the need to always maintain 37C. So what happens? The body then increases the heart rate and blood pressure and shuts more stuff down to protect the brain. It is a vicious circle which is why it is important to insulate the head to conserve heat.

A "cotton" T-shirt will keep a small boundary of water close to your skin that is warmed by the body. It is not water that is trapped like a wet suit but it water that is trapped in the fabric and is harder to flush than if just bare skin. The problem with a "cotton" T-shirt is when at the surface laying flat down the water begins to evaporate and rapid heat exchange occurs. It is the exposure of the t-shirt to air which causes the cotton t-shirt to act against you but if the sun is strong then the infrared heat on a large surface area counter balances the evaporated heat loss. Cotton t-shirts are not mandatory you can of course wear a synthetic one as do I and it keeps me quite warm long enough for a snorkelling session even in nippy water.
 

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Not sure what you are reading......
I'm not going to do the work for you. Google "head heat loss". It's a myth.

Your recollection of your body shunting blood to the core is correct. However, that does not disproportionately cause one to lose more body heat through the head. Let me be clear, I'm not suggesting one should not insulate their head, only that it does not cause you to lose significantly more heat than any other similarly sized exposed area.

A "cotton" T-shirt will keep a small boundary of water close to your skin that is warmed by the body.....
That is the principal of a wet suit, with limited water exchange. Unfortunately, with a t-shirt that boundary layer will exchange continuously with water at ambient temp. Since water is all below body temp and conducts heat so well, you will lose heat without actual insulation.
 

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Incidentally, probably the best home made torso wet suit would be a cotton t-shirt with a synthetic t-shirt over the top. The cotton would act as a sponge and inhibit water flushing and the synthetic overlay would insulate the cotton.. Maybe some one could test the theory. :)
 

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I'm not going to do the work for you. Google "head heat loss". It's a myth.

Your recollection of your body shunting blood to the core is correct. However, that does not disproportionately cause one to lose more body heat through the head. Let me be clear, I'm not suggesting one should not insulate their head, only that it does not cause you to lose significantly more heat than any other similarly sized exposed area.


That is the principal of a wet suit, with limited water exchange. Unfortunately, with a t-shirt that boundary layer will exchange continuously with water at ambient temp. Since water is all below body temp and conducts heat so well, you will lose heat without actual insulation.
Reply 1: If your body closes the valves off to your arms and legs so more blood can be used for vital organs what does this tell us? It tells us that you will lose more heat disproportionately through your head because the rest of your body is not radiating heat.

Reply 2: The flush cycle is less than you think it is. I am sure you have exited a pool with a t-shirt and felt it warm to the touch. A wet cotton T-shirt only becomes a fridge when you expose it to air due to evaporation. In open water you will survive longer wearing a T-shirt than bare skin as long as you stay immersed. Considerably longer.
 

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Paul, despite your intuition, based on a few facts, this has been studied scientifically. Look it all up. You're not drawing the correct conclusion.
 

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Goodness sakes, I posted what you should google above. i just did it myself and it provides an entire page of links that refute your conclusions on heat loss through your head.

I'm sure I can do the same for wet tee shirt as insulation. When cotton becomes wet, there is no more insulation, ie a thermal gap between your body and the water. This is typical achieved by air pockets in some materials, as air conducts heat at 5% the rate of water. No air left in wet cotton.
 

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Why should I google it Minnewaska if you are privy to this info about heat loss from the head? Just post the link of what you viewed and then we will be on the same page. It will take me years to find what you are looking at by chance.

Re: The t-shirt I said the t-shirt is holding a boundary layer of water that has been warmed by a body in the cotton weave. Not between body and material. Water against the skin has a different viscosity as that against cotton hence the flush effect with a t-shirt is less. I said nothing about trapping air for warmth.

"When fluids encounter solid boundaries, the fluid in contact with the wall is at rest and the viscous effects thus retard a layer in the vicinity of the wall." A-Z Index

You must have experienced pulling a wet tee shirt away from your body having been swimming and feeling the warmth of the water as it drains out! Well if you did not have that t-shirt on that energy would be lost to the pool.

The t-shirt is much better than nothing whilst you are immersed. It is when you come out of the water that the t-shirt acts like a fridge because of evaporation.
 

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Believe what you want. The evidence is there for others to look at. Cut and paste the google search term "head heat loss" and click any hit. While google customizes a bit, my first hit was from LiveScience, the second from WebMd, then the BBC, then Guardian, etc, etc. All more reputable than a sailing forum, me, you or anyone else.

As for the tee shirt, I completely disagree with your conclusion. The boundary layer of water will simply extract heat from the body more quickly, because that's what water does. That's why your t-shirt is warm when you get out of the water, as you described. It didn't start warm, therefore, heat was removed from your body. Without something that insulates that warm water in your t-shirt from the surrounding water, it will also transmit heat and then draw more from your body.

No point trying to tell me what I "must have experienced". I was paid to dive in an earlier life.
 

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Minnewaska.

T-shirts: The viscosity slows the water down and provides time for the boundary layer to be heated up by the body. This in turn means that because the boundary layer is warmer than open water the potential difference for heat exchange between body and water at the boundary point is less. Imagine a river flowing past a wall. The water at the wall(boundary) will be flowing slower and will be warmer than the flow of water in the middle of the river. Now imagine naked skin as the wall and a t-shirt as reeds at a river bank. The water at the the boundary of reeds would flow even slower allowing even more time for the water to heat up because the water at the bank is flushing less. I sent you a link to a thermodynamics bible and even found the correct page for you that gives you the formulation of "BOUNDARY LAYER HEAT TRANSFER" How can you argue against it? Its proven physics.

Heat loss from head: I will try to find your references tomorrow and have a read.
 

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Believe what you want. The evidence is there for others to look at. Cut and paste the google search term "head heat loss" and click any hit. While google customizes a bit, my first hit was from LiveScience, the second from WebMd, then the BBC, then Guardian, etc, etc. All more reputable than a sailing forum, me, you or anyone else.

As for the tee shirt, I completely disagree with your conclusion. The boundary layer of water will simply extract heat from the body more quickly, because that's what water does. That's why your t-shirt is warm when you get out of the water, as you described. It didn't start warm, therefore, heat was removed from your body. Without something that insulates that warm water in your t-shirt from the surrounding water, it will also transmit heat and then draw more from your body.

No point trying to tell me what I "must have experienced". I was paid to dive in an earlier life.
I believe I found your references but see no revelation. I don't think anybody believes that under normal conditions the head exchanges more than any other equivalent surface area. What is being said in this thread is that cold water(The body regards cold water as anything less than 35C) will cause vascular constriction which is not possible on the scalp so this will result in a disproportionate heat loss from the head when snorkelling because blood flow has been restricted to other parts of your body.. Here is a link for you that explains it. What Is Peripheral Vasoconstriction? | LoneSwimmer
 

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If you look at this person Minnewaska, the body surface has cooled to 17c because the body has become its own insulator. In effect become its own wet suit and that red area at the top is the only place heat can be exchanged from.
 
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