On 7/4/2014 11:54 PM, Jeff Liebermann wrote:
On Fri, 04 Jul 2014 00:17:36 -0400, rickman wrote:

On 7/3/2014 9:19 PM, Jeff Liebermann wrote:
However, if the system leaks and is therefore not adiabatic, the story
is a little different. Heat will be introduced into the room through
the ceiling and windows. The result with be a temperature gradient
where the ceiling and window air are warmer than the air near the
floor. The only temperature of importance is your head, which
radiates most of the waste heat from your body.

I would love to see a reference for this factoid. It is an often
perpetuated myth that half you heat leaves your body through your head.
Simply not supported by the facts.

As usual, it depends on the circumstances. At rest, the head radiates
about 7-10% of the body heat. When exercising, the increased surface
blood flow increases this to 50%. Presumably, the OP is not
exercising under his ceiling fan:
http://skeptics.stackexchange.com/questions/2839/do-we-lose-most-of-our-body-heat-through-our-heads

Ok, I found this discussion and clicked the link for the first reference
on the exercise issue...

Not Found

The requested URL /12401.php was not found on this server.
Apache/2.2.3 (Red Hat) Server at researchfrontiers.uark.edu Port 80

The second section refers to a reference that is a Wilderness Medicine
Newsletter. I have not heard of them and they do not give references
to the work of the people they say they interviewed. So I'm not sure
what to believe of the conclusions. They say...

.... discovered that we do indeed lose heat through any exposed part of
the body and the amount of heat we lose depends on the amount of exposed
surface area. The rate of heat loss is relatively the same for any
exposed part of the body not simply the head. You do not lose heat
significantly faster through the scalp than any other portion of the
body with the same surface area.

Later...

As you begin exercise, cerebral blood flow increases due to increased
cardiac output and the percentage of heat lost through the head accounts
for about 50 percent of total body heat loss. As exercise continues,
more oxygen is directed toward muscle and blood flow to this tissue
increases. Core temperature has to be maintained and as body heat
increases, the skin arterioles expand, or vasodilate, redirecting blood
flow to the skin which cools the blood. Hence, total blood flow to the
brain is decreased and the percentage of total body heat lost through
the head is reduced to about 10 percent. The percent lost through the
scalp returns to 7 percent after sweating begins.

So depending on the phase of exercise they claim 50%, 10% and 7% but in
contradiction to the initial statements that there is little if any
difference in the different parts of the body regarding heat loss.

Earlier in this discussion a post is made the references an old US Army
training book, US ARMY SURVIVAL MANUAL, BASIC PRINCIPLES OF COLD WEATHER
SURVIVAL. The info in this book has been widely misinterpreted where
they talk about wearing a survival suit but with no head protection;
then the head does loose 50% of the heat from the body.

Regardless, your claim is about a person nominally at rest I would
assume. If you are referring to a person exercising the 50% number only
applies during the initial portion of the exercise before they warm up.




http://outdoors.stackexchange.com/questions/681/what-are-areas-of-the-body-which-lose-heat-more-quickly-and-how-can-i-reduce-th

This link discusses (again with no verifiable references) that there are
parts of the body with higher heat loss per square inch than other areas
when not vasoconstricted. But nowhere is there info to support the
statement, "The only temperature of importance is your head, which
radiates most of the waste heat from your body."

So in the words of Mythbusters... BUSTED!


Because it is closer
to the ceiling than the floor, a fan blowing downward will heat your
head instead of cooling it, as you observed.

This statement shows no understanding of human physiology. Even
ignoring the issue of perspiration and evaporative cooling, the human
body is nominally at 98°F and will be cooled better in an airstream of
even 90°F than in still air.

Correct, although I suspect the cooling is mostly from evaporative
cooling, not convection. The part of my rant that you trimmed
indicates that this was in reference to the relative merits of the fan
blowing air down or up.

I objected to this statement...

"The only temperature of importance is your head, which radiates most
of the waste heat from your body."


I indicated that blowing down would be more
effective for cooling because the head is closer to the air source and
therefore has a higher air flow rate. Further away would be less
effective. If the fan it going to cool the head, the optimum location
would be closest to the fan (unless the OP is into standing on his
head).

However, the OP didn't specify the temperature of the air near the
ceiling. If the ceiling air were hotter than body temperature, a
downward blowing fan will heat the cranium instead of cooling it. It
would be like trying to stay cool using a hair dryer blowing from
above. I can believe that blowing HOT air downward might result in
heating instead of cooling.

The point is any reference of cooling the head vs. the rest of the body
in invalid. The more important part of the issue is that a ceiling fan
blowing up will only cause an indirect, dispersed air flow to the person
with nearly no effect.

To talk about the hot air next to the ceiling assumes that that hot air
is not dispersed in the first minute of use which is not valid.

The relative cooling has to do with the airflows, not with the thermal
emission of the head.

--

Rick