24 April 2012

Wear black clothes in the summer?

That counterintuitive recommendation comes from The Straight Dope, via io9:
We're all encouraged to wear white in summer, since white clothing is supposed to keep us cool — but it doesn't. In fact, black clothing is the best way to keep cool in the heat. It's basic physics. And biology...

This means that white clothing reflects a great deal of wavelengths of energy coming in. This means it should reflect the sun's rays back, instead of letting them cook us. And that's perfectly correct.

Except that this explanation is also incomplete. Heat is not just coming in off of the sun. It's also coming off a person's own, sweating, warm-blooded, mammalian body, which is a lot closer than the sun is. When all that body heat hits the white clothing covering it, it gets reflected right back towards the body. When we wear white, we cook ourselves.

The best color to keep cool in the heat, it turns out, is to wear black. Black absorbs everything coming in from the sun, sure. But black also absorbs energy from the body instead of reflecting it back.
I think this is oversimplified and perhaps misleading, but there is additional discussion re wind and atmospheric conditions at the link, though apparently no discussion of hot sunny days vs hot cloudy days.

22 comments:

  1. I don't buy it, and I agree that this argument is over-simplified. Mythbusters should do a show on this. :)

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  2. "When all that body heat hits the white clothing covering it, it gets reflected right back towards the body."
    That is true if you glow in the dark. Personnally, my body doesn't create its' own visible light, so this argument is quite invalid.

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    1. https://en.wikipedia.org/wiki/Infrared

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    2. Exactly why i talked about "visible light".

      Look through infrared camera/goggles/whatever at both black and white clothes : they look the same. Thus, this colour argument about body generated heat is still invalid.

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    3. I wonder, do Eskimos dress in white?

      According to wikipedia about 2/3 of heat dissipated by the human body is by radiation when there is no wind. And the peak emission is about 9700nm == 1030 wavenumbers. That's the mid IR. If you look up the absorbance of cotton fiber in the mid IR you find it is peaked around 1000 wavenumbers:
      http://ddr.nal.usda.gov/bitstream/10113/8608/1/IND43907875.pdf see fig 3

      While the absorption is variable, it is much more absorbing than in the visible. Thus Cotton should appear dark in the mid IR even if it is not Dyed. It might be that dyes will make it even darker but I'm inclined to believe it is mostly black in the Mid IR.

      If that is correct then your argument about the visible seems correct. Sunlight is peaked in the visible though it does have strong IR components. Black cotton will maximally absorb there. the body radiates in the mid IR and the cotton strongly absorbs there regardless of color.

      It is a puzzle.

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    4. Aha. I tracked down the "correct" explanation of this. accoding to the straightdope If there is zero wind or the black clothingis in thermal contact with your body it is worse than white. But if there is wind >3m/sec then loose fitting black clothing is best.
      http://www.straightdope.com/columns/read/1886/does-black-clothing-keep-you-cooler

      The physics works like this:
      if the wind is blowing and the clothing is not touching your body then the clothing will be the temperature of the wind. This is true if the clothing is black or white. Now on the inside of the clothing there is no wind. So the black is absorbing your radiation better than white. it then conducts this to the exterior of the clothing where the wind carries it away. White clothing reflects your radiation back so it is not carried away as fast. If the clothing were skin tight then you, then you would be the primary conductor of the sunlight heating the clothes not the wind.

      Okay that sounds good, but I raise the following objection:
      in the mid IR, where you radiate, white cloth should be very dark. So this explanation seems wrong. Secondly if you still buy the argument, it seems like skin tight white clothing would be the best since this would let the wind cool you by conduction even faster, while still reflecting the visible. But again, that would be wrong if cotton is black in the IR.

      Thus I offer yet another explanation:
      You want the wind to cool your skin, so being naked would be best except you don't want the sun hitting your skin and warming it up. Thus what you want is clothing that 1) is thin enough to admit the wind but 2) dark enough to block the sun.

      thin white cotton would not block the UV or Visible light. But thin black cotton could.

      THus the coolest would be thin black loose fitting cotton.

      This last theory could be disproved if it were the case that desert dweller robes are heavy.

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  3. "Black body" radiation (not obscure at all) has nothing to do with color. It is the relation of heat (of a body) to the light or other EM radiation it gives off.

    Black clothing clearly gets hotter than white clothing - do the experiment with a magnifying glass and white or black thread. The black thread burns, the white one doesn't.

    Yes, white clothing does reflect more heat back in, but that is a minor effect.

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    1. No you are quite mistaken. Blackbody radiation has a pre-factor known as the Emissivity. For there to be an equilibrium the emissivity must exactly equal the reflectivity. SO the more reflective the less the black body radiation emitted.

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  4. That one's easy: Look at what people living in deserts wear. Not the rich oil magnates, the normal people. They all wear dark colors.

    The color is only half of the reason, though. They wear several layers of wide, wavy clothes to further mitigate the heat.

    I can't cite any sources, but I honestly assumed this was common knowledge.

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    1. Exactly. As a photographer, I visited Bedouin territory many years ago and I was astonished to see everyone dressed in black or very dark blue (indigo), loose-fitting layers of clothing. When I asked my host why they didn't wear lighter-coloured clothing, he looked at me like I was crazy (that happened a lot during my time there!) and told me they didn't feel like dying of heat exhaustion. I tried it, and was amazed at the difference. Yes, you're still hot (after all, it got upward of 120 degrees some days), but you definitely feel cooler wearing dark clothing.
      I asked him about all the people i saw in the cities wearing white robes, etc. and he said it was a status symbol - a sign that those people: a) are not "of the desert" anymore (an important social distinction in some countries), and b) could afford air conditioning, hence the white clothing.

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    2. Basing this solely on my own experience in Saudi Arabia, but wearing black is absolutely miserable there. Men get white cotton robes, women get black polyester robes, invariably.

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  5. if you wear loose-fitting white clothes they reflect sunlight and let your own heat vent out. you can wear black clothes in 100+ heat if you want but i hope you don't have far to walk.

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    1. You try walking through the Tanezrouft, like the Tuareg have done for thousands of years, wearing white and see how far you get.

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  6. Now I have some science to defend wearing my goth clothes in the summer! ^_^

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  7. I don't mind wearing black clothes in the summer, well my cooking clothes (double breasted chef jacket) are black! And i love wearing my cooking clothes when I need to, so I'll wear them in the summer for sure! Even though they are black. I love them.

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  8. The Physics here is suspect to me. The heat the body gives off to the clothes is generally direct conduction or convection, not radiant heat, so I believe the color would not change the amount transferred from the body to the clothes. But the heat from the sun is primarily radiant, and that's where the white is better over the black.

    Cheers,
    -Chipper

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  9. Don't forget that white clothing lets through a lot of light, and therefore sun rays are still cooking YOU. In fact, they make white shirts that you can tan in.

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  10. Ok. So black on the inside and white on the outside should be better than either?

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  11. All I know is that I cook like a suckling pig if I sit in the sun wearing black!

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  12. Black is a very efficient radiator under certain conditions. The SR-17 Blackbirds were painted black to help dissipate the heat generated by the friction of moving through the atmosphere at 33 miles per minute. But humans operate in thermal regimes that have very little to do with the heat of friction. Car radiators are painted black for the same reason, but they are still far outside of the human heat envelope. Aerospace and reciprocating engine thermodynamics aside, human body radiation is not black-body radiation.

    I found reference to a physiological study published in 1978 that looked at black versus white and windy versus calm conditions with respect to bird plumage.
    (link: http://www.straightdope.com/columns/read/1886/does-black-clothing-keep-you-cooler ). Hot with no wind: wear white; hot with wind: wear black. I suspect the desert-dwelling humans who are known to wear black are onto something.

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  13. Lol ok , I am a Roofer, If i am in a black shirt i will only last 30 mins working in the sun , however if i am in white i can work all day in the sun.
    30 years roofing in these conditions

    I hope this helps :)

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    Replies
    1. You're talking about working in the sun. What if it's a totally overcast/cloudy day with no sun?

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