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We are told to wear light clothes in summer as they are better at reflecting sunshine and keeping us cool. And dark clothes absorb sunshine and keep us warm.

But is it really relavent? If I buy identical t-shirts, one in black and one in white, will I feel significantly cooler or warmer? I have noticed that black surfaces get much warmer, but do they make the person warmer too?

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The obvious answer is that yes white teeshirts really do keep you cooler, but it would be interesting to hear from anyone who has done the experiment or can point to experimental data. –  John Rennie Sep 26 '13 at 7:08
    
With light skin a white t-shirt helps (personal experience). However, the black robes in the desert don't affect heat gain because they are so thick and loose fitting. –  Kevin Kostlan Feb 27 at 2:59

6 Answers 6

This article has some relevant results based on a study of bird plumage (it also happens to be cited in the abstract of the Nature paper mentioned in one of the other answers), and is summarized in simpler terms here.

I'll attempt to summarize the summary.

Black and fluffy/loose fitting clothing is best if it is hot out and there is any ($>3 \mathrm{m}/\mathrm{s}$) wind. The black clothing absorbs both solar radiation and radiation from the body. The air in the immediate vicinity is heated, then efficiently transported away by the wind. This is slightly better than white fluffy/loose fitting clothing, which reflects more sunlight and radiation from the body. The emission from the body is reflected, so it cannot heat the air near the clothing as efficiently and have a chance to be transported away.

Tight black clothing is a terrible idea if trying to stay cool, regardless of windspeed.

If there is no wind ($<3 \mathrm{m}/\mathrm{s}$), white clothing is better since the most important thing in these conditions is to reflect as much incoming sunlight as possible.

I also have another possibility to think about. My recollection regarding loose fitting black robes in the desert is that - given a garment that is open at the bottom (robe) and top (not too tight fitting) - heating the air inside is actually advantageous to keeping cool since this drives a convection flow upward through the garment. This airflow makes cooling via sweating efficient, enough that the person wearing the garment doesn't feel as hot. Unfortunately I can't find any experimental results to validate this picture, but it seems more or less in line with the results above, at least in as much as airflow seems to be key to answering the question.

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For most circumstances this is a myth and the opposite is more true.

If you are standing in direct sunlight it would best best to wear white clothes to reflect the heat. The direct radiation from the sun would have a temperature higher than your body so it would absorb heat. A black surface can heat up to over 40 degrees in direct sunlight.

However, if you are standing in the shade on a hot day where temperature is less than body temperature of 37 degrees Centigrade, then black clothes may well radiate more heat away than they absorb from ambient heat. For most of the time black clothes will therefore be better in the heat.

In cold weather you will want to keep heat in so it would be better to have white clothes that reflect heat on the inside and radiate less on the outside. This would be true because the ambient temperature is going to be well under body temperature. Emergency survival blankets are silver on both sides to keep heat in.

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Radiation in the infrared is different from absorbing solar heat in the visible. In the infrared all things are black unless they are flat metal, so you will radiate as well wearing white as black. –  Ross Millikan Jan 25 at 3:57
    
@RossMillikan Is it really true that in the infrared all things are black? I understand the concept of a black-body, but I know of no rule saying that something must be a blackbody at its temperature. The counter position (that there is some reflectivity) would violate no law of thermodynamics. I see no reasons different materials can't have different IR reflectivity. –  AlanSE Feb 24 at 1:49
    
I don't know if it is really true. I had a project and measured the far IR emissivity of a number of things and all that weren't shiny metal were well over 0.9. They were a variety of materials and visible colors of similar materials. –  Ross Millikan Feb 24 at 2:35
    
@AlanSE - Even if IR emissivity varies a lot, there is no reason to expect that a white T-shirt would be more "white" in the mid-infrared than a black T-shirt. It could equally likely be the reverse. –  Steve B Feb 26 at 18:47
    
But the biggest problem with this answer is that emissivity is in practice almost always a tiny contribution to heat loss (much less than convection, conduction, evaporation). Even in the shade during daytime, there is plenty of visible light around for a black T-shirt to absorb. I think those silver blankets are designed to be very lightweight and totally impermeable to air and moisture; low IR emissivity is nice but not critically important. –  Steve B Feb 26 at 18:56

I wouldn't say it is a myth. Like you say, it's complicated. When you are in the sun, the predominant source of heating is from incident radiation, whereas in the shade cooling would take place primarily through convection, conduction and evaporation. When you are in the shade, you are just not that hot compared to your surroundings for radiative cooling to be very effective (10 degree difference).

If you stay in the deep shade all summer, by all means, wear whatever colour you like. But don't go hiking with a black shirt on.

In the winter, where body temperature might be 45 degrees from ambient, radiation will play a big role. Of course, if you're well insulated then your overall surface temperature is close to ambient, and the effect may be negligible (e.g. a white parka and a black parka may perform similarly).

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If your body temp is 45 degrees C from ambient, you are covered enough that radiation doesn't matter. It is generally true that in our environment convection and insolation dominate. –  Ross Millikan Jan 25 at 3:55

There are physical and physiological aspects of this problem. Consider this: if your fingers are freezing in cold is it better to drink a hot tea or cold vodka? Definitely VODKA! Why? because it will immediately cause your blood vessels to widen, more blood will go to your fingers and you'll have a better chance to save them. Did I make this up? No. My cousin went to a nurse school, and I read this in her first aid textbook many years ago.

Hence, we have to consider the physiological aspect here. If in a cold whether your clothes are cold, then your skin will feel cold, and that will shot down the blood supply to the skin, you'll feel colder. If you're wearing dark t-shirt it must get warmer because it will absorb more energy from Sun than a white t-shirt, so the blood supply should be better, and you'll feel warmer. That is assuming there is Sun, i.e. maybe late Spring or early Fall.

If this is Russian winter, then you're not going to be wearing a t-shirt. You'll be wearing a fur coat. It works differently. It works through conservation of heat. Its heat transfer is very low. Even if Sun warms it outside, that heat is not going inside. Your body's heat is not going outside either. Hence the color of a fur coat doesn't matter, it can be white or black, no difference. here's the proof: enter image description here

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First of all, there is no passive material that passes thermal radiation in one direction better than in the other.

The best material for thermal isolation is highly reflective (e.g. aluminum foil). Ideally, this does not emit any thermal radiation. (It also does not absorb thermal radiation, hence, it is an isolation).

The worst is black because that radiates thermally. It also absorbs thermal radiation, but if you want isolation, this is worst.

So best clothes for winter and summer would be highly reflective (Provided you want thermal isolation from the cold in winter and from the hot in summer).

But keep in mind that heat transport is not only by radiation, but also by heat convection and heat conduction.

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Although your answer is intuitive, I believe that it is incorrect. –  innisfree Sep 26 '13 at 16:26
    
I think your claim that inhabitants of deserts wear black clothes has to be proven (compare a google image search for Bedouins, where clothes are predominately white). I think wearing black clothes in a desert is actually a very bad idea. Everyone has experience with black clothes on hot days... –  Andreas H. Sep 26 '13 at 18:08
    
If you want to keep warm in the winter, wouldn't it be better to wear black clothes that absorb a higher fraction of the sunlight? –  Steve B Feb 26 at 18:58
    
@SteveB: this is only true if there is (enough) sunlight. If there is no sunlight, black is a bad idea. Just what I wrote: Black absorbs and reflecting isolates. The thing is, you cant guarantee that there is sunlight in winter at all. –  Andreas H. Mar 5 at 19:04
    
Why the downvote? Would the downvoter please explain what the incorrect statement is in my answer? –  Andreas H. Mar 5 at 19:05

Although counterintuitive, inhabitants of the desert often wear black. In fact, studies show that the color of their clothes make no difference to the heat gained. See e.g. this article in Nature.

Though this assumes that their robes are loose-fitting, such that convection is relevant.

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Well, when I do a google image search for "bedouins" I find bediuons wear all kinds of colors, but predominately bright colors (google.com/search?q=bedouins&tbm=isch). Especially those depiceted in a desert environment almost all wear white clothes. –  Andreas H. Sep 26 '13 at 17:58

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