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Tuesday, April 24, 2012
Does black clothing keep you cooler?
Does black clothing keep you cooler?
February 21, 2001
Dear Straight Dope:
I had heard (via my high school physics teacher) the black clothes will keep you cooler than white clothes. Is this true? (I cannot quite remember, but I think he mentioned something about black-body radiation, an obsure theory of Einstein's.)
Not black-body radiation--simply radiation of heat from your body. But the more important factor, believe it or not, is wind speed.
An extensive and detailed study (Walsberg, Campbell, & King, 1978. J. Comp. Physiol. 126B: 211-222) examined different colors of bird plumage under different temperature conditions--with the added wrinkles of examining whether the plumage was fluffed or flattened, and varying the wind speed.
Under cold conditions with no wind speed, black, flattened plumage held in heat the best (though barely, compared to fluffed black plumage). Under hot conditions with no wind, white, fluffed plumage let heat escape the best. Both pretty logical findings.
But once the wind picked up, the results changed dramatically. With even a modest wind (anything above 3 m/s, or about 7 m.p.h.) fluffed white plumage exhibit the lowest net heat loss. This explains the large number of arctic animals that are fluffy and white. It's not just camouflage.
At high temperatures, as I say, white is best at not transmitting solar/ambient heat to the skin when windspeed is zero (only barely better when fluffed). However, with an increase in windspeed (again anything above 3 m/s), fluffed black plumage is the best at reducing the amount of heat transmitted to the skin. Flattened black plumage is the worst in terms of heat gain no matter what the windspeed.
What this means is relatively straightforward: black clothing absorbs sunlight and the heat radiating from your body, but if it is loose-fitting, and there is wind, the wind convects the heat away faster than it is absorbed. White clothing reflects sunlight, but also reflects internal heat back towards your body, so the net effect under identical conditions is less cooling than if you wore black. While it's true you don't often find fluffy black animals in deserts, you don't find many white animals, either--typically you find animals that blend into the background. So it appears that if heat gain and camouflage are in conflict, the need to avoid predation outweighs other considerations. On the other hand, desert-dwelling nomadic people such as the Tuaregs wear loose-fitting black clothing, and have been doing so for a very, very long time. If there were an advantage to wearing white clothes, you'd certainly expect they'd have figured that out by now.
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