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Why do bedouins wear black robes?

by ffthink
Tags: bedouins, black, robes, wear
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ffthink
#1
Feb28-05, 06:12 AM
P: 2
google finds some explanations (see quoted text below).
i wonder, if the explanations are true, why the dress code in saudi arabia is white!?

I had heard (via my high school physics teacher) the black clothes will keep you cooler than white clothes. Is this true?

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.
If you are packing for a trip to the desert would it be better to pack light or dark clothes? The answer is not a simple as you might think, as Don and Yael discuss.

D: Hey, Yael, check out my new white linen suit. It's going to keep me cool on my vacation to the Mojave desert.

Y: That is one snazzy suit, Don.

D: Oh, I'm stylin'. Plus, everyone knows that white reflects heat and black absorbs it. Yes, if you're out in the sun, you're better off wearing white.

Y: Not always, Don. After all, Bedouins, the nomadic people who spend their entire lives in the desert, wear black robes.

D: But that doesn't make sense. Dark surfaces get warmer in the sun than light surfaces. You'd think the Bedouins would have figured that out by now.

Y: Don, Don, Don. Things are never that simple. You're right that the air underneath black fabric warms up faster than the air underneath white fabric. At the same time, though, black fabric provides more shade than white fabric, and this decreases the amount of light that directly reaches the skin. Plus, a lot depends on the type of clothing you're wearing. You see, warm air rises. And when it does, it's replaced by cool air. And if you happen to be wearing a robe, all that movement of the warm air creates a breeze that sucks up cooler air from the bottom of the robe and pushes it out the top.

D: So wearing a black robe is like having a suit with a built in fan.

Y: Exactly. But again, the key is that the robe is loose-fitting. Otherwise, there isn't enough room for the air to circulate. So if you're going to the desert in a tight-fitting suit, it's better to wear white.
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HallsofIvy
#2
Feb28-05, 07:02 AM
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There are those of us who think 98.6 is an intolerable temperature. However, if you are out on the desert and the air temperature is over 100, you might be very happy to hold 98.6 degrees (body heat) against your skin as a barrier against the higher temperature air.

My father spent much of World War II in the battery compartment of a submarine. He said that the important thing was DON'T MOVE so you didn't disturb the layer of air over your skin where the temperature was ONLY 98.6!
ffthink
#3
Feb28-05, 09:48 AM
P: 2
do there exist any experiments about this topic (or about a cooling-technique that uses a black surface)?


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