Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

The Straight Dope ran an explanation of why nomads often wear black clothing - it absorbs heat better from the body. On the other hand, white clothing reflects sunlight better. Is it possible to get the best of both worlds and wear clothing that is black on the inside and white on the outside?

share|improve this question
1  
Something to clarify: Cooler than what? Is white and black cooler than just black? Cooler than naked? –  endolith Nov 26 '10 at 18:41
1  
@Endolith: Cooler than wearing either pure black or pure white –  Casebash Nov 26 '10 at 21:57
1  
It all depends on whether you mean "cooler" or "kewler". –  Gordon Feb 3 '11 at 5:13
add comment

5 Answers 5

The color of a surface doesn't reliably indicate the emissivity at non-visible wavelengths. The color in the visible spectrum is more of a side effect than anything. Most thermal radiation around body temperature or room temperature happens in the infrared region, not the visible, and that's not reliably indicated by visible color:

picture of a guy holding a plastic garbage bag, in visible light and infrared light

The visibly transparent glasses are opaque to the body's infrared emissions, while the visibly opaque trashbag is transparent to infrared. So one property has no relation to the other.

The emissions of the sun occur mostly in the visible region, which is why white clothing reflects solar energy and stays cool while black clothing absorbs solar energy and gets hot:

spectrum of sunlight

But your body's thermal radiation is in the infrared, so this rule doesn't apply to the inside of clothing (unless your body is hot enough to radiate visible light, but then you have bigger problems).

Your basic idea would work, though, if you found a material that reflects visible light while transmitting infrared light (but that material would probably have the same properties on the inside and outside, and thus be visibly white on the inside, too).

For example, white paint is quoted as having an absorptivity of 0.16, while having an emissivity of 0.93. This is because the absorptivity is averaged with weighting for the solar spectrum, while the emissivity is weighted for the emission of the paint itself at normal ambient temperatures. ... The white paint will serve as a very good insulator against solar radiation, because it is very reflective of the solar radiation, and although it therefore emits poorly in the solar band, its temperature will be around room temperature, and it will emit whatever radiation it has absorbed in the infrared, where its emission coefficient is high. http://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation

share|improve this answer
add comment

Since outside interacts with the radiation both ways, it would not help. But, there's something that would:

You need to have clothes which are Black (totally absorbing) in visible light and White in Thermal-infrared wavelengths (totally reflecting). This color is called Red (well, to some extent: if you shift the spectrum into visible terms).

share|improve this answer
2  
Could you provide a source to this? –  aitchnyu Sep 26 '13 at 9:52
add comment

Direct sunlight falling on people walking in the desert calls for white or light colors.

The coolest sun shades are white, also sun umbrellas. Coolest clothing, black versus white, is light colored. The reason is that in direct sunlight the energy of the sun, could be 1200 watts/metersquare all falls on the cloth. White reflects the visible light, which is most energy from the sun, so much less is absorbed to reach the shaded tables underneath .

nomads

Desert nomads of Cholistan in Punjab come to the annual festival at Channan Pir.

As one can see the dominant dress is colored white. Black is the color of mourning and it might be that some women wear black even in the sun, and one will see light colored or brightly colored clothing occasionally due to the human tendency for decorating the self, but white is predominant. I checked on google and the statement in your link that Tuareg wear black is not correct. Most photos are white or light colors (similar arguments). Maybe they wear black for camouflage at night if on the war path?

Flowing robes also can be seen , no tight clothing, so that the air circulates and by convection cools the body as it perspires.

So the question becomes: would a black ( black in the infrared, totaly absorbing infrared material ) lining inside a white robe help in cooling the body by removing infrared radiation faster? My guess is yes, but cannot guess how much faster. The infrared radiated from the body would be better absorbed and transmitted to the outside but the main mechanism would still be convection.

share|improve this answer
    
"would a black lining inside a white robe help in cooling the body by removing infrared radiation faster?" There's no way to tell, since the visible color has no relationship to the infrared reflectance/transmittance/absorption. –  endolith Jun 26 at 11:37
    
@endolith clarified what black I expected . –  anna v Jun 26 at 12:44
add comment

Since both absorption and radiation depend on color in the same way, this would decrease the energy that is radiated off into the surroundings. So if radiation were the only way to lose heat color wouldn't matter, and in the long term you'd have the same temperature as the incoming light.

But since nomad's don't live in vacuum there are other ways of heat transportation. What color is best depends on which forms of energy transportation dominate.

Since I guess that you lose more heat to other sources than radiation, while a large part of the incoming heat comes from light, making them white on the outside and black on the inside could make sense. But there are complicating factors such as wind, vaporization of sweat,...

share|improve this answer
    
This is totally wrong. –  Anixx Jun 26 at 11:40
add comment

If the temperature inside clothing is the same as outside, the wear will not do anything.

This is because white surface worse adsorbes light but also it worse emits infrared radiation. Conversely, dark surface better adsorbs, but also better emits.

In the case the outside temperature higher than inside, such wear will help to be cooler, but even better wear would be that one which has the both sides white. Similarly, if you want to keep heat in winter night, you would prefer the all-white wear because it better keeps the difference in the temperatures.

Conversely if you want your temperature to be as close to that of environment as possible, you should prefer all-black wear.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.