Black garment vs white garment .

  1. Black garment vs white garment in summer . How much extra energy is absorbed with black clothes ?
    How to quantify this effect? what theory or equations to apply?
     
  2. jcsd
  3. In the order of ten times more for black over white.

    Absorbtivity in the total solar spectrum.
    Emissivity in the infrared spectrum.
    The inner surface is at human skin temperature.

    Off hand the black is ten times more effective at heating you than the white.
    Assume that black has a high absorbtivity and emmissitivity of say 90%.
    Assume white at 10%.
    The microscopic nature of the fabric weave could affect the absorbivity by trapping certain wavelengths very effectively.

    As for equations. You have solar insolation in watts/meter^2. You can get this according to time, location etc.
    You have conductivity into the skin. You have conductivity and radiation into the air.
    There are concepts such as delta T, h, k : the temperature diffrence, the boundary layer, the coefficient of heat transfer

    http://en.wikipedia.org/wiki/Heat_transfer
     
  4. Something else has to be happening, though, or there wouldn't be so much dark Bedouin clothing.
     
  5. sophiecentaur

    sophiecentaur 13,268
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    Good point. I remember trying a black umbrella as a sunshade and it was totally useless on a very hot, sunny day in the UK so black is definitely not as good, on the face of it. Something else must be happening, as you say. Perhaps the colour is to do with the available cloth. White cloth needs to be bleached and perhaps the technology (traditional, that is) did not include that facility for the poorer population - how 'black' is the 'black' cloth you see on pictures? It may just be untreated. If you Google Bedouin traditional clothing, you will see a lot of images of white robing too. Perhaps the more wealthy have white. How much whiter is the white than the black? Is the extreme black / white contrast a bit of an illusion due to the strong contrast of photos in very bright sunlight? (i.e. we'd need to do some actual measurements)
    Several layers of any colour cloth will provide insulation so perhaps the outer colour counts less than you'd think.
    The desert air is very dry and loose robes will allow a lot of circulation underneath, which will allow sweating to be much more effective than elsewhere. Keeping out of the Sun is clearly the thing to do so the lifestyle will involve being out of doors at off-peak times.

    Try Anthropology Forums, perhaps?
     
  6. Re: Sophiecentaur's post -

    As one who has spent a lot of time in the Arab world, I can say that the Bedu wear a lot more white cotton than they used to. Those who can afford it wear white cotton robes that cover them from wrist to ankle as protection from the sun and the heat. As you pointed out, the swirling of these robes as you walk produces very welcome cooling. Black locally-woven mohair is the traditional fabric, as the Arab goats are black ("Thy hair is like a flock of goats descending Mount Gilead", Song of Solomon 4:1).

    White fabrics are useful only against the shortwave solar radiation, and even then the albedo difference is nowhere near ten to one. Against the far more abundant longwave radiation, the albedo difference is negligible.

    One should not forget the insulating value of woolen garments--against both heat and cold. In my years in the Arab world, I heard far more complaints against the nighttime cold than against the daytime heat. The Bedu have a saying, "The stars make a thin blanket!"

    I once had a Bedu guide tell me, "If I knew it was going to be this hot, I would have worn my heavier robes!" It should be noted that he was sitting on a rather hot rock at the time.
     
  7. sophiecentaur

    sophiecentaur 13,268
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    @klimatos
    So I am probably barking up the right tree. That's reassuring.
    You mention the 'abundant long wave radiation". I had not considered all the stuff coming back off the hot ground.
    Re the cold nights - I understood that you can make ice at night in some desert places. The contrasting temperatures from day to night must give a worse subjective effect, too.
     
    Last edited: Jan 22, 2014
  8. With all due respect, your post has some erroneous affirmations.

    You cannot insulate the human body against heat except for extremely short periods. Without a cooling process it is only a matter of time before the inside attains the temperature of the outside.
    As for absorbitivity and emmissiivity. Aren't round numbers good enough for you? I worked in that field, did you?
    As for the colour people choose to wear. Some people like blue or red. That does not mean they are choosing it based on α or ε.
     
  9. sophiecentaur

    sophiecentaur 13,268
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    So you are saying it's all down to fashion? Are we disagreeing with you? If the difference between one colour and another of robes is not relevant then it is thickness of clothing and air circulation that will keep you from overheating.

    Didn't we deal with the cooling due to evaporation, helped by air circulation? What is your point here? Did you want to add any information, to make a correction or just to be grumpy?

    You could suggest that the Bedu guide was talking rubbish but he is probably more aware of the best thing to do - even if he may not have done any Physics at School.
     
  10. http://www.nature.com/nature/journal/v283/n5745/abs/283373a0.html

    Brief summary of the findings: http://www.theguardian.com/science/2012/aug/19/most-improbable-scientific-research-abrahams
     
  11. Any desert dweller will agree with you that radiation and conduction from the hot ground is something to be protected from. For the Earth as a whole, this amounts to some 372 Wm-2. Tropical and sub-tropical desert heat loading from the surface will be much more than this. I have fried many an egg on a hot rock to the amusement (and hopefully, edification) of my students.

    I was thinking of heat loading from above. The Sun contributes some 133 Wm-2 of shortwave radiation to the Earth's surface, while the atmosphere contributes some 242 Wm-2 of longwave radiation. Again, these are global averages assuming normal incidence.

    For anyone interested in the Earth's heat budgets, I suggest starting with the classic "Physical Climatology" by William D. Sellers (1965) and progressing to the contemporary: Kiehl, J. T. and Trenberth, K. E., 1997: “Earth’s Annual Global Mean Energy Budget”, Bulletin of the American Meteorology Society, Vol. 78, No. 2, February 1997.
     
  12. Once a person has used envrionmental heat stress monitors in the context of sunny construction sites, hot metal, air and furnace exposure in smelters and steel mills one gets up to date with the recommended health and safety regulations and protective measures to be taken.
    http://www.bing.com/images/search?q=envronmental+heat+stress+monitor&FORM=HDRSC2
    These monitors integrate the wet, dry and black bulb temperatures to give an idea of stress on human beings.

    In general loose, light coloured clothing and head gear are recommended. And this is what western people wear. In some places (a steel mill) walking around like that is not allowed. You need wool flame retadant clothing and the tappers are wear aluminum coated, flame proof insulkated head to toe gear.

    Sure a polar bear is white with black skin and his hairs act as in a way to trap heat waves (so it is claimed as I have only briefly read this years ago).

    I worked in the selective coating of solar absorber fins. 85% absorbtivity in the solar spectrum and 30% emmissivity in the infrared spectrum. Black fins with a microporous microstructure that trap radiation. It was a phosphoric acid anodization followed by a nickel sulfate AC aqueous process.

    Look. You can beat this bush all you. Sure if I wear a thick white parka on a sunny day versus a black net tea shirt, my ability to sweat cool myslf will be different. Nuns' habits were often light coloured and dark coloured (I know black and white are not colours) depending on season and climate.
    A dark cotton t-shirt versus a light cotton t-shirt. Kids may not know the diffrence but most adults know what to wear when it is 35C and sunny.

    As for bedouins here is what they wear according to
    http://www.bing.com/images/search?q=bedouin+clothing&qs=IM&form=QBIR&pq=bedouin+&sc=8-8&sp=4&sk=IM3
     
  13. sophiecentaur

    sophiecentaur 13,268
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    The black / white original beduin thing is a different issue to modern fashion design (Bing). They would have used what was traditionally available to them and would have made the best of what they had or what they could afford. It worked well enough for them to have survived, with or without the bleaching process for producing white(er) fabric.

    The modern factory situation is very different and no one would be allowed to work in the UK in 'desert conditions' - not without regular breaks in a cool rest-room. Health and safety even gets enforced in remote construction sites, too. (As those pictures of tricorders and sonic screwdrivers demonstrate. :wink:)
    @pikpobedy You seem to be interpreting all this as a bit of an argument, rather than a friendly discussion. That's a shame.

    @ klimatos
    I thought the solar energy reaching the ground was about 1kW/msqu. That's the number that's used for calculating output of PV cells, at least. Where is the rest, in your more detailed breakdown?
     
  14. My apologies. To everyone here. Sincerely. As for the mentor that PMed me? Less is better than too much,
     
  15. sophiecentaur

    sophiecentaur 13,268
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    Spoken like a gent.
     
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