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From: "Steve Harris" <>
Subject: Re: human body heat emmision
Date: Sat, 21 Apr 2001 05:26:04 -0600

"Norman Yarvin" <> wrote in message
> In article <9bi403$t5p$>,
> Steve Harris <> wrote:
> >Total heat output (radiation and advection) at the skin must
> >be about the same as the metabolic heat.
> >Take kilocalories per day, multiply by 4184 cal/J, divide by
> >86,400 sec/day. Typically 100-140 watts.
> >
> >Now, usually total conductivity this is given in terms of watts/m^2/C.
> >So divide 120 watts by 2.2 m^2 or so and the 6 K temp gap between
> >your skin temp and room air, and you come up with about 9 watts/m^2/K.
> >Of this, about half is radiation and about half is advection (in still
> >air that's all natural convection). So long as things don't get over
> >comfortable temp, sweating doesn't have to be factored in.
> But what about evaporation in the lungs (or nose, or wherever most of the
> evaporation is)?  There's a fair bit of cool, dry air going in and warm,
> moist air coming out.

There is, but if you do the heat capacities, you find that the air you
breathe has so little that it doesn't add that much either way. If it did,
you'd freeze in the arctic. You get a little more from the water vapor you
breathe out, but only if you pant a lot <g>.

> >Anyway, the answer to your question for naked skin of "average" color
> >in a room with comfortable still air, is that your skin temp difference is
> >sufficient to lose you about 4 to 5 watts per square meter per degree,
> >or something like 60 watts total for a 2.2 m^2 adult.
> >
> >If you backcalculate from the Stefan-Bolzmann law for these
> >powers, figuring an environmental temp of 310 K and skin
> >temp of 304 K, you get skin emissivites of around 10%,
> >which sounds about right.
> Not to me.  Infrared emissivities of most things are near to one.  Only
> polished metal surfaces have emissivities like 10%.  This refers only to
> thermal infrared, of course.  If this weren't true, infrared thermometers
> wouldn't work nearly as well as they do.
> But if you factor in the need for conduction through the skin -- the
> surface of the skin isn't 310K (98.6 F) -- then maybe it does make sense.

Naw, you're right-- emissivity of 10% is reflectivity of 90%, and you'd have
to look like the murdered woman in Goldfinger for that.  Or one of those
kids on that Outer Limits episode where they get the SETI message that turns
them all into metalheads.

I screwed up a division somewhere.

Your emitted IR power P at 90% emissivity at skin temp Ts to 298 K
surroundings is.

P = 0.9 *5.67e-8* [Ts^4 - 298^4]

If we have 100% view factor for our naked person <g>, and P is the needed IR
cooling of 30 watts/m^2 then Ts comes out at is 303.4 K, which is a skin
temp of 87 F or so. Not unreasonable.


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