Tuesday, June 5, 2012

Color of clothing to wear during the summer

This post is a bit out of the norm for my blog, but a while back io9 posted and article on why you should wear black during the summer to keep cool. They linked to another article from The Straight Dope. These have been bugging me for a while. You can go out and do the experiment yourself and see if you feel hotter in white or black under the summer sun. I strongly expect the white will win. So why is the "physics" explanation saying otherwise?  My take is that it is because the physics they present has been oversimplified to the point of being wrong. They also look at a scientific study that has no bearing on clothing, unless you happen to wear clothing made of bird feathers.

Their argument misses one extremely critical point, your body does not emit thermal radiation in the same part of the spectrum as the Sun's primary energy emission or the color you see in (and yes, black-body radiation is a perfectly valid term for this despite what The Straight Dope says) . When you describe a shirt as white or black, you are talking about the reflectivity in the visible part of the spectrum. That is only a narrow part of the spectrum, but it happens to be where peak emission is for our Sun. For that reason, the color matters critically when it comes to how much solar energy is deposited in your clothing. A black shirt will absorb a lot more sunlight than a white one. That absorbed sunlight will be thermalized, increasing the temperature of the shirt.

Where their argument goes astray is when they start talking about the radiation from your body that tries to cool it. All object radiate energy away based upon their temperature. The term black-body radiation is often used because if you had a perfectly black object, which absorbed all incoming light, reflecting nothing, this is what you would see. It just happens that stars are pretty darn good black-bodies and their spectra fits the expected shape perfectly except where elements high in their atmosphere have absorption bands.

You, as a human, have a temperature much lower than a star. This means that you radiate a lot less energy and that it is at much longer wavelengths. The peak wavelength for your emission is roughly 10-5 m. For the Sun it is closer to 5*10-7 m. You are seeing your shirt and calling it white or black based on the way it interacts with light in that shorter wavelength. However, when it comes to cooling, the radiative component depends on the longer wavelength. To really know what that means, you need to have a far IR spectrometer and look at the absorption spectra of your shirt using that. I haven't actually done this, and if anyone has, please comment and correct me if I make a mistake here, but I have a feeling that light t-shirts are going to be mostly transparent and even if they aren't, their "color" in the far IR will have virtually no correlation to their color in the visible. The end result is that I expect the thermal radiation cooling your body doesn't depend at all upon the color you see your shirt to be. It depends a lot more on thickness, materials, and style of fabric.

As the article says, if there is any wind, then the dominant heat transfer mechanism will be the air moving heat away. That process too is independent of color. Perhaps the wind will take more heat from a black shirt, but only because the black shirt was hotter to start with so it had more thermal energy to give up. The white will still be cooler in the end.

My conclusion, wear white during the summer, and if you really are worried about whether your shirts are making you too hot, send them to a lab for some far IR spectroscopy to see which materials really let your body heat back out.

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