White "reflects heat"?

Cecil's Columns/Staff Reports
1

Does black clothing keep you cooler?
I’m no expert in thermodynamics, but in the recent report about black vs. white clothing and which keeps you cooler I can’t make sense of the suggestion that white clothes “reflect” heat when ambient temperatures are high.

This statement, in particular, seems a bit generic:

“White clothing reflects sunlight, but also reflects internal heat back towards your body, so the net effect under identical conditions is less cooling than if you wore black.”

As I understand it (this is where the thermo-experts need to chime in) the “whiteness” of clothing only affects radiative heat, such as that from the sun or from a warm object in a relatively cooler environement.

But at temperatures above 98.6 (assuming the person doesn’t have a fever :slight_smile: there will be no radiative heat coming from the body. All heat transfer will be through conduction or convection. That being the case, the color of the clothing will have no effect on the amount of heat escaping from the body and through the clothes. Rather, it will be the material and construction of the clothing that will make a difference (thick vs. thin, porous vs. dense, etc.)

OK, so if all of that is true, then on a hot, sunny day, white clothing should work best, no? It will reflect the radiative heat from the sun and (ideally) allow the convective (mostly) heat escape from under the clothes. Free passage of air through the clothing should also help aid in evaporative cooling (really a special case of conduction and convection). Black clothing (of the same material and construction), though, would absorb the radiative heat from the sun, while providing no benefit to removing heat from the body.

Based on this, I think the above statement should read something like this:

“White clothing reflects sunlight, but also reflects internal radiated heat back towards your body, so the net effect under conditions when the ambient air is cooler than your skin is less cooling than if you wore black. In very hot ambient air (above 98.6 or so), white clothing would provide more benefit due to the absence of radiative heat coming from your body.”

OK, experts, what am I missing?

I have edited in the link to the Staff Report. – CKDH

[Edited by C K Dexter Haven on 02-25-2001 at 11:35 AM]

2

Errr… No. For a given human body (or any other sort of body of constant temperature), to a first approximation, thermal radiation is a constant, not varying with respect to ambient temperature.

3

The most plausible explanation for desert dwellers black clothing being cooler is internal convection.

As I understand the process, radiant heat energy from the sun (about 1 kilowatt per square meter) is absorbed by the dark fabric and heats up a thin layer of air next to it. A rapid convection is set up, sort of like a chimney, and the air rises inside the garment. New air continually enters at the bottom, and gets entrapped with the small amount of heated air and rises out the top.

The entire process behaves like a natural air conditioner.

The net effect is a blocking of radiation from the sun to the skin and a continual slight breeze up the body with air near to ambient temperature. Heat transfer by conduction is minimal, as is heat transfer by internal radiation processes.

You would be as equivalently comfortable as if you stood stark naked under an awning and had a small fan playing on you.

White clothing does not absorb enough energy to set up a good convection so you feel hotter.

Jezza

4

A link to the Staff Report is appreciated. Does black clothing keep you cooler?

5

The report also glossed over the fact that black objects do, in fact, radiate more heat than white objects. In fact, if you have an ideal white object, which absorbs no heat via radiation (i.e., it reflects away all radiation incident on it), then it will actually radiate away no heat, as well. Based soley on this consideration, then, which color is best depends on whether you’re going to be in the sun or the shade: Sunlight has a higher effective temperature than body temperature, so heat transfered via radiation will be into the body, but in the shade, you’ll be in surroundings at a lower temperature so radiative heat transfer will take heat out of the body.

6

Not necessarily. White and black are as viewed by our eyes. In the areas that matter, the infra-red, things that appear black to us may well be quite light colored.

You need to ensure that the body you are talking about is not white or shiny in the far-infrared before you can say that black radiates more than white.

In the daytime, even give a black body, solar radiation (~ 1 Kw / sqm) will dominate over the re-radiated energy. Factors like conduction and convection are much more important.

At night, radiative cooling is important, but conduction and convection are probably more important under normal circumstances. (As an example, on a still night in the desert in summer it is still possible to get near freezing at ground level in still conditions, but get any wind and it will remain stinking hot)

Jezza

7

I thought radiated heat was in the infra red and therefore invisible to the human eye. Therefore the colour of clothing to the eye would be irrelevant to this question ??

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That’s true. I think thermodynamicists - and physicists in general - use the terms “white body” and “black body” (as well as grey) to denote objects that behave in the infrared (or whatever wavelength you happen to be studying) like an optically white or black object behaves with visible light. I’m sure it’s quite possible to create an object that is visually black, but behaves as if it were “white” in infrared.

I assume you’re referring to the Stefan-Boltzmann law? You are of course correct, but I was talking about heat transfer DUE to radiation. I wasn’t clear enough and I apologize. The more I think about it, though, if the ambient temperature is higher than body temperature, NO heat transfer (from skin to air) due strictly to radiation, conduction, OR convection can occur. The air would, in fact, make the skin hotter, since heat transfer occurs only from the “hotter” object (or fluid) to the “cooler”. Evaporative cooling due to perspiration, however, would still help, the drier the air the better.

Incidentally, the use of “Errr… No.” is pretty rude. If you think I’m a complete idiot, just say so and don’t resort to insinuations. I asked a polite question, clearly indicating that I was unsure of my conclusions and seeking help from someone in the know. You may not know how, but it is possible to correct someone without being condescending.

9

True. jezzaOZ, I should have made that clearer. Being a physicist, and a theoretician at that, I was of course referring to an ideal blackbody (and an ideal whitebody) which behaves as black or white throughout the entire electromagnetic spectrum. Of course, there’s no perfect blackbodies on Earth, but something that appears black in the visible range is likely to be a pretty good approximation to black in the infrared, as well.

By the way, radiative heat transfer can be any frequency of light, depending on the temperature of the objects involved. The only reason we associate infrared with heat is because that’s where most of the radiation is from a typical animal body or a relatively cool fire.

Besides, it’s obvious that black clothing keeps you cooler. What color is the Fonz’s leather jacket? I rest my case :slight_smile:

10

Nope. The longer the wavelength the shinier things get. Shinyness is directly related to the surface rougness compared to the wavelength. The longer the wavelength hitting a particular surface the more it will be reflected.

You have wavelength related absorbtion and re-radiation in particular atoms and molecules (atomic spectra), but for good old black body radiation, roughness compared to wavelength is the driving factor.

This is not to say that carbon black is white in the infra-red, but more that you must be very specific about a surface finish and chemistry before you can say whether it is white or black at a particular frequency.

Jezza

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Boid said:

Since when? I always thought “Er, no” was a nice way to ease into correcting someone. As opposed to, say, “You flaming idiot, where did you come to that silly notion, Mickey Mouse U?!” Or “Nope, you’re wrong, dipshit.”* I’m not saying that John W. Kennedy can’t be condescending, but I don’t think he was really trying in this case.

*Note: Those are examples only, not to imply they apply to any particular poster in this thread. :wink:

12

I remember once watching a game show in Japan where the question was about why zebras have their stripes. The answer, they reported, was that the alternating black and white stripes provided better cooling than an solid color. Apparently, it creates convections or something that draws the heat away or something.

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So would black underwear and white outerware give one the best of both worlds? Also, is this connected to the strange markings of various animals (pandas, the high frequency of “Sylvester” type fur on cats, etc.)? Perhaps these animals have black fur where the air moves freely, and white where it doesn’t.

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I would suggest that the body does not radiate heat in temperatures above 98.6 degrees fahrenheit. We human depend on the evaporation of body fluids (sweat) to extract excessive heat from the body.

Once again, we find our foreparents were right without having the science. White underwear to reflect body heat back, and dark outerwear to absorb external radiation now makes a lot of sense in the winter .

15

No matter what the ambiant temperature, any object warmer than absolute zero will radiate energy. The key is that the surroundings are radiating, also, and if they’re hotter, the body will not have any net energy loss (or a negative energy loss, if you prefer, or a gain) that way.

16

Well, saw this thread and just had to register to post…:slight_smile:

I’d like to offer some armchair grassroots explanations of blackbody radiation to help clarify things, as anyone that hasn’t formally been through a college physics course likely won’t have any exposure to what we’re talking about when we toss about terms like “blackbody” and “stephan-boltzmann”.

I’m going from memory, so please excuse any vagueness regarding persons/places/times :slight_smile: I can lug the textbooks from my attic and give more detail should anyone desire.

Some brief history ensues…

It was observed quite early that objects reflected light in varying amounts. “white” and silvered objects were obviously more reflective than black. It was also observed that black objects tended to absorb and radiate heat to a greater extent than whiter objects. It was also observed that no objects were perfectly reflective. Also, black objects got hotter faster than white objects.

A conundrum arose. Light and heat are energy. Objects absorb light and no object is perfectly reflective. What happens to the light that gets absorbed, where does it go? One would expect that an object that is constantly absorbing light would have do get rid of the energy in some way, else the object would just keep absorbing and absorbing, eventually melting or burning under the vast amounts of energy it consumed. This conundrum is dubbed “The Ultraviolet Catastrophe”

So, logically, the object must be discarding the energy somewhere…but what happen’s to those photons that get absorbed? They get re-radiated.

It was later observed that an object in isolation (i.e. not reflecting outside energy) radiates energy at a specific peak wavelength that was purely dependent upon its temperature. The higher its temperature, the shorter the wavelength of light (equivalently “heat”, as heat and light and radio and infrared and ultraviolet are just different wavelengths of the same particle, the photon) it emitted. Also, this energy wasn’t a fixed discrete wavelength, it was spread out across the spectrum in a bell-curve distribution. Interestingly, a very cold object (a rock at room temperature) versus a hot object (a stove burner) versus a very-hot object (the sun) all had a bell-curve spectrum distrubution of exactly the same shape, varying only in where along the spectrum the peak wavelength was located.

What was most important was, the nagging “Ultraviolet Catastrophe” paradox was resolved, as upon measuring the total energy that the objects were radiating discovered that the energy output exactly equaled the energy input, only that the output was in a different wavelength than the input.

Then a whole lot of statistical mathematical mumbo jumbo theories happened, papers were published, prizes awarded, grant parties thrown.

And the end result being, your white shirt on a hot day keeps you cooler because it is reflecting away more energy than a black shirt (thereby absorbing less energy than a balck shirt), which means that it re-radiates less energy back to your skin than a black t-shirt. In other words, your white shirt has a black-body radiation peak of a longer wavelength than does your black shirt.

The explanation I heard for zebra stripes wasn’t about skin-cooling (otherwise why aren’t elephants striped?) but that it confused predators when they were in a herd because all of the zebras visually sort of blend together…

Hello everyone!

17

You’ve missed the point by concentrating on radiation. In the real world, convection and conduction are equally as important.

Loose black clothing a la Bedouin is cooler because it absorbs lots of energy and then uses this energy to set up a heat driven convection engine between the clothing and the skin. The process results in drawing in relatively cool ambient air from below and expelling it out the top of
the garment. The incomimg dry air rapidly evaporates the sweat on the skin, giving an immediate cooling effect.

The convection does not accumulate lots of humid air next to the skin, which is what actually makes us feel hot.

If there is not enough energy absorbed (white clothing) the convection process does not establish and the wearer gets to feel quite hot and sticky inside.

Jezza

(presently sitting in 108 degrees @ 20% humidty without an airconditioner and feeling a trifle warm)

18

quote:“You’ve missed the point by concentrating on radiation. In the real world, convection and conduction are equally as important.”

I agree with their importance, but I disagree with the assertion that the clothing being black is the major contributor to convection. It is far more important that the clothing be loose. It is just as common (from what I’ve seen in media, I haven’t visited) for people in hot arid contries to be wearing white or tan clothing, especially Egypt and India. Other countries, particularly muslim ones, seem to wear predominately black. I suspect this is a cultural thing as opposed to a heat-survival device.

By the logic above (hotter clothing would increase convection), the heat radiating from the human body would also be creating convection currents to cool the body, so an overheating body would create the same effect as if the hotter clothing were doing so. While the convection difference between black and white clothing may exist, I think it is insignificant when weighed against the radiative and reflective benefits of the color, as well as the convective effects specific to the weave of fabric, layer, weight, etc. The positive increase in convection from the black clothing would be outweighed by the negative effect of it reradiating the sunlight it absorbed back into the wearer. This is opinion however based on my experiences, I could very well be wrong :slight_smile:

19

So, I have question, and I’m getting a little confused. I want to make a tent. I want to be able to use it in winter and summer- I want it to stay as cool as it can in the summer, and as warm as it can in the winter.
I was thinking about using two-ply cloth, one side a shiny black, or dark color, and one side a flat white, so it’s reversible.
The black side out in the winter, and the white side out in summer.

Is this good reasoning, or is it faulty?

I really do want to do this, been designing it for a couople of years now, so any input is appreciated…

20

The best ever is aluminized fabric. The key driver is heat radiation, either incoming solar radiation in summer or outgoing radiation in winter.

In Summer run the tent with some vents at either end - usually flyscreen material. This allows free circulation of air and you will be as hot - or cool - as ambient

In winter close it all off the venting and let your body heat keep the place warm. You will lose very little heat to the sky by radiation.

The same principal applies to aluminized survival blankets and emergency shelters for forest fires.

If you don’t want to go aluminized, then impermeable white tents are best.