Physics Question

Sometimes a simple question has a complex answer. Perhaps someone with a background in physics or meteorology can give me an answer I can understand.

Apparently radiant energy travels out from a warm surface until in encounters some object. Then part of it is reflected, partly absorbed and part of it is transmitted. So if I wear the typical three layers of clothing I am wondering what part of my heat loss comes from radiation through the layers. Also, would it make any difference if the first layer (or any layer for that matter) is white or black? I suppose someone could also ask what the balance of radiation incoming and out going is.

This was probably covered in my Physics 101 class and I would know the answer, had I been paying attention.

White reflect all visible wavelengths of light while black absorbs all visible wavelengths. A color like blue will absorb most visible wavelengths except in the blue part of the spectrum where it will reflect so that the object looks blue. The key here is the visible spectrum. All objects at a given temperature emit energy in a range of frequencies. For a body at about 100F, most of that energy is in the infrared part of the spectrum. I would imagine that at infrared wavelengths, most clothing dye colors absorb or reflect the same so it doesn't matter what color clothes you wear - all colors perform the same. In any case, the most important source of heat lose isn't thru infrared radiation but thru conduction to whatever is in direct contact with the skin. Most of the time, this is air. So clothes that trap and hold air are what you need.

Ow, my head, my head... <img src="/forums/images/graemlins/laugh.gif" alt="" />

plny
Your body is not a "hot object" sorry. <img src="/forums/images/graemlins/blush.gif" alt="" /> <img src="/forums/images/graemlins/confused.gif" alt="" /> <img src="/forums/images/graemlins/frown.gif" alt="" /> <img src="/forums/images/graemlins/smirk.gif" alt="" />

Its not hot enough to radiate much infrared, so it doesn't matter much. And then again, almost all things which are opaque in visiible light are transparent to infrared.

In terms of Kelvin temperature scale, you're just not very warm.
Jim YMMV <img src="/forums/images/graemlins/cool.gif" alt="" />

AFAIK, the only time color matters is when the sun is out. If you want your clothing to absorb heat from the sun, then your outermost layer should be black. If you want to stay cooler in the heat, then white clothing is more reflective and may assist in preventing heat absorption (in theory anyway).

Size, on the other hand, DOES matter. Insulation works by trapping air that is then warmed by heat radiating off your body. The "puffier" the garment, the warmer it is, so long as that which makes it puffy isn't too dense (thus displacing air).

Warm air is always going to move toward cold air, and in the process it will generate air currents. The air right at the surface of your body is going to be very warm, and the greater the gradient between the air temperature and your body, the more rapidly the heat will disperse. The "chill" you feel when you are cold is the movement of colder air toward your skin. The greater the distance between your warm body and the cold air (or surface), the less heat loss.

MNS

My body, on the other hand.....................

(just kidding)

Midnight,
This is a family group... <img src="/forums/images/graemlins/blush.gif" alt="" />
However The members of your avatar fan club might - no never mind... what was I thinking, see line 1. <img src="/forums/images/graemlins/grin.gif" alt="" />
Jim <img src="/forums/images/graemlins/cool.gif" alt="" />

Conduction, convection, and evaporation are the primary mechanisms of heat transfer to and from the body. It is true that radiation becomes significant on a clear day when darker colors will absorb more energy radiated by the sun. It is primarily the outer layer that matters, although inner layers will determine how much of the heat gets conducted to your skin. To a lesser degree, in the middle of a clear night radiation from the body is more significant and darker colors will radiate more. But this will be on a much smaller scale than the energy absorbed by the sun.

It's more than just color, too. Texture, density, etc. can affect radiative heat transfer. The proverbial black body is not called that because of its color, BTW.

from emedicine.com

Heat transfer to and from the body occurs via the following 4 mechanisms:


* Conduction is the transfer of heat via direct physical contact; it accounts for 2% of the body's heat loss.
* Convection is the transfer of heat from the body to the air and water vapor surrounding the body; it accounts for 10% of the body's heat loss. When air temperature exceeds body temperature, the body gains heat energy.
* Radiation is the transfer of heat via electromagnetic waves; it accounts for most heat dissipation. As long as air temperature is less than body temperature, 65% of the body's heat is lost by radiation.
* Evaporation is the transfer of heat by transformation of a liquid into a vapor; it accounts for 30% of the body's heat loss.

The body's dominant forms of heat loss in a hot environment are radiation and evaporation. However, when air temperature exceeds 95°F, radiation of heat from the body ceases and evaporation becomes the only means of heat loss. Evaporation of 1 mL of sweat results in the loss of 0.58 kcal of heat; thus, 1 L of sweat evaporated from the body accounts for the loss of 580 kcal of heat. An individual exercising in the heat easily can sweat 1-2 L/h. If humidity reaches 100%, evaporation of sweat is no longer possible and the body loses its ability to dissipate heat.

"Warm air is always going to move toward cold air, and in the process it will generate air currents."

Sorta. Actually convection is powered by gravity. Cold air is denser and it falls and "nudges under" warm air, becomes warmed in due course and rises in turn as new cold air forces it up.

Hey, wait a minute! Your avatar....the sun is plainly not out yet you're still glowing in the visible light part of the spectrum.
Hmmmmm. <img src="/forums/images/graemlins/grin.gif" alt="" />

Whoooooops, my secret is out.............

Jim - go outside, naked, on a clear night. Lie down protected from the wind, on a pad. Then tell me your body isn't 'hot enough' to radiate any infrared energy to the cold, cold night sky <img src="/forums/images/graemlins/tongue.gif" alt="" />

Radiative heat loss occurs any time a body is hotter than the objects around it.

Once you start throwing clothes and stuff into the mix, you can start arguments amongst the few thermodynamics folk in physics departments <img src="/forums/images/graemlins/grin.gif" alt="" />

Stats like the ones in a post above (relative % losses) will of course vary depending on clothing, posture (a lot more lost to conduction if you are lying down, for example), and environment.

You are both right.

On a clear night, if you sleep naked, your body is a hot object.
Maybe, not Jim's however, but I doubt he sleeps naked anyway.

Sorry, but I think the above data is seriously flawed.

Radiative heat loss is roughly proportional to the temperature difference to the 4th power, so it doesn't become significant until there is a significant temperature difference between the two surfaces. A naked body in the sun will have significant temperature difference. A naked body exposed to a clear night sky will also have a significant temperature difference. Even surrounding surfaces, like ice, can be significant, but only if you are naked. As you add layers in between, which you need anyway to combat other sources of heat loss in cold weater, the radiative heat loss becomes less significant, even irrelevant.

Also, you left out transpiration, which can be a significant source of heat loss.

Here is an article which confirms your numbers, but the men are indeed naked:
http://jn.nutrition.org/cgi/content/abstract/15/5/477

Here is a better overview of heat loss and heat gain for hikers by Ryan Jordan:
http://www.backpackinglight.com/cgi-bin/backpackinglight/00184.html

Here is an interesting article on the bodies thermoregulation, which is also part of the equation:
http://en.wikipedia.org/wiki/Thermoregulation

Depends on what you mean by black and white. Something that is more of a white body to the visible spectrum can be more of a black body in the infrared spectrum. I understand ice is a good example of this. It is very effective in absorbing your heat, if you are naked.

Here is the equation for radiative heat transfer:

Q = e1 x e2 x Sigma x A x ( T1^4 - T2^4 )

where:
Q = Heat transfered from body 1 to body 2
e1, e2 = emmissiviity of body 1,2 ( black body = 1, white body = 0, 0 < grey bodies < 1 )
Sigma = Stefan Boltzman Constant = 0.174 x 10^-8 [ BTU/hr.ft^2.degR^4)
A = Surface area of body 1 exposed to body 2
T1 = Temperature of body 1
T2 = Temperature of body 2

From this equation you can see that while it can be effective to reduce the emmissivity of your outer layer, it can be just as effective to reduce the temperature of your outer layer by wearing the insulative layers you should have on anyways. I prefer dark clothing for sweaters, so they will dry better in the sun and near a fire, and light bright colours for shell materials, because they look nice. <img src="/forums/images/graemlins/smile.gif" alt="" />

Can you provide a link to the page where these statements are made? I was not able to find it on their website with a quick search.

Here is an article which more or less confirms his numbers, but the men are naked:
http://jn.nutrition.org/cgi/content/abstract/15/5/477

That's precisely why I asked for the link. In order to provide such percentages, certain assumptions have to be made because the answer will vary significantly depending on conditions.

Of all the forms of heat transfer, radiative is the most complex and least intuitive to understand. I'm also reminded that evaporative cooling is typically categorized under convection. When taking a course in heat transfer, we are taught that there are only the three mechanisms.

Well different courses can teach it different ways, but I can see how evaporative cooling and convection can be linked because they are both forms of mass transport of heat. For the human body I can see combining them, but separating them from respiration. Respiration is useful to separate because unlike the others it depends on your breathing rate, so as your body works harder to stay warm in a sleeping bag, for example, you lose more heat through your breathing, so the old scarf trick makes a real difference when its really cold. I remember seeing a site specific to heat loss and heat gain and the human body that was pretty good but I can't find it. It was a little more technical than Jason's and it included real numbers, which are always useful.

Paddy, yeah those numbers would be a nekkid person at room temp, but it's good to have those as a base. Of course they change with clothing which is more what the OP probably wanted to know.

http://www.emedicine.com/emerg/topic236.htm

And yet another straight line for MNS!

[quote]plny
Your body is not a "hot object" sorry. <img src="/forums/images/graemlins/blush.gif" alt="" /> <img src="/forums/images/graemlins/confused.gif" alt="" /> <img src="/forums/images/graemlins/frown.gif" alt="" /> <img src="/forums/images/graemlins/smirk.gif" alt="" />

Ya, my wife keeps telling me that.

Seriously, I have learned much from this thread. It turns out the amount of radiation is proportional to the fourth power of the absolute temperature. And you are correct, at about 100 F we just are not that hot.

Slight note, that's proportional to the fourth power of the -absolute- temperature, so the ideal radiation between two 0 Celcius and 37 Celcius (32F to 98F) is really between 273 Kelvin and 310 Kelvin, each raised to the 4th! It turns out to be 219 watts, or about half of our normal output of 550 watts.

During a summer day, it's not much of a problem. But as soon as temperatures start plummeting or the night sky is clear above you (absolute zero in space), radiative heat loss starts up.

Of course, not many of us lay out under the stars buck nekkid and those that do are probably beyond help. <img src="/forums/images/graemlins/grin.gif" alt="" />

The things we do to insulate ourselves also shield us from losing heat through radiation. The simple calculation being referenced is the potential heat loss in an "ideal" situation. Real world heat loss is going to depend on many factors- clothing, a tent, and a sleeping bag completely changes the equation. If it didn't, we'd be in trouble.

Yeah, this gets right back to the original question. Theoretically a down jacket/bag warms up and radiates out heat too, on an icy day, but we all know the jacket keeps warmth in. So what exactly are the effects of clothing and insulation??

Sometimes I astonish myself with how little I contribute <img src="/forums/images/graemlins/grin.gif" alt="" />

The effects would be that the loss of heat is less than it would be without the clothing or insulation.

I have been waiting for a chance at work to answer this question, mostly because my heat transfer textbook is at work. Although some people have given questionable answers, they aren’t so bad that I need to say they are wrong. They are just a different way of seeing the same phenomenon. Mostly, I agree with Paddy Crow. She probably took the same type of heat transfer class that I took.

So, I am going to say that there are three types of heat transfer; conduction, convection, and radiation. Evaporation is a combination of more than one. The one that is the strongest depends on how prevalent the other ones are at the time. If you want to say that evaporation is a fourth heat transfer mechanism, that is fine, won’t hurt my feelings.

Let’s pretend that you are placed in a vacuum, and have no contact with any object. Convection and conduction are zero, and radiation is 100%. Or, you could be placed naked on top of a mountain with 100 mph winds. Radiation and conduction are so small because convection is so strong. Conduction is highest when there is no convection, and you are touching something with a high thermal conductivity (like copper), and it is much colder than you, and much larger than you.

Now, let’s look at real world. You are standing outside with a decent wind, and it is between 30°F and 40°F. You also have a windproof jacket with 2 inch loft of insulation. What your jacket does is create 2 inches of stagnant air. So, next to your skin, convection is greatly reduced. That leaves radiation and conduction. Because air has a very low thermal conductivity (almost the lowest), the conduction of heat from your skin to the outside is also greatly reduced. The thicker the insulation, the more conduction is reduced. So, your body loses most of its heat through radiation. This, of course, is only if the thickness of the insulation is great enough to drop the heat lost through conduction to be lower than the heat lost through radiation. If you are staying warm, then radiation, conduction, and convection are all small. I would say that convection is where the most heat is lost when you are out in the woods, especially since convection plays off the other two.

For your clothing choice, I would recommend trying to combat all forms of heat loss. They all play a role, and one shouldn’t be ignored over another.
<img src="/forums/images/graemlins/grin.gif" alt="" />

She?

I am not female. Paddy is short for the Gaelic name Padraig.

I took Heat Transfer about 26 years ago...

Now I feel stupid. <img src="/forums/images/graemlins/blush.gif" alt="" />

No harm, no foul...

would it make any difference if the first layer (or any layer for that matter) is white or black?

This is an interesting theoretical question, but I have a hunch that any actual difference this makes would be so mind-bogglingly tiny that it would be dwarfed by the heat loss from simply breathing or shifting your body.

Hate to get ticky-tacky, but did anyone else notice that 2 + 10 + 65 + 30 = 107%?

I don't recall if it was this forum or another, but I went through this whole deal about a year ago, even getting down to black-body radiation and total heat loss from respiration.

Breathing turns out to be a large heatsink for your body. Basically, the air you breath in has to go from its current temperature and humidity right up to body core temp. and 100% humidity. That's a significant amount of heat loss. There are masks, like PSolar and PolarWrap, that use heat exchangers to recover heat and moisture from exhaled air, then use it to warm and humidify the inhaled air.

I agree.

First off I say more folks should lay out under the stars buck nekkid <img src="/forums/images/graemlins/cool.gif" alt="" /> it's a real eye opening experience <img src="/forums/images/graemlins/grin.gif" alt="" />

You just gave me an idea.....

Good luck with that one. <img src="/forums/images/graemlins/grin.gif" alt="" />