Expert: logictest - 9/16/2004

Question
Thank you very much for the information.

There didn't seem to be a definitive physical answer. This question's been bugging me for a long time! Please could you tell me what colour clothes you personally would choose to wear if you had to live a few days in the desert (ignoring every aspect (e.g. social convention, material availablity) other than heat?


Thanks


-------------------------
Followup To
Question -
Hi,

I hope you can help answer something that's been bothering me for a while.

It's about heat. In photos of people in deserts, it seems some people wear all black, and others all white. I don't understand. I realize white reflects heat, so could reduce the heating of the person wearing them. I also realize black radiates heat well, so could radiate a person's own body heat away. White has the drawback of not radiating away a person's body heat, but black has the disadvantage of absorbing the sun's heat.

Which is best? Is it the case that people in the shade are better wearing black, but people in the sunlight are better wearing white?


Thanks very much
Answer -
Here is an interesting discussion that answers a similar question:


http://www.newton.dep.anl.gov/askasci/gen01/gen01173.htm

QUOTE:

Ask A Scientist
General Science Archive  

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Black Clothing in the Desert
2002042

name         Jon
status       student
age          16

Question -   hello,
We all know that white reflects heat while black absorbs most heat.  In
the desert wearing a black robe would absorb more heat than a white one
(naturally), and wearing a black robe could increase temperatures inside
by about 6 degrees.  Why do people wear black robes in the desert?
------------------------------------------------
Some assumptions need validation:
1. Who says few/some/most/all people in the desert wear black robes -- or
have we been taking "Lawrence of Arabia" too seriously?

2. "Black" and "White" apply to the absorption / reflection of visible
light. It says nothing about the absorption / reflection of infrared, which
might be a more important factor.

3. It is assumed that the "black" and "white" fabric are the same type
material woven in the same way, etc. This point may not be correct. A black
loosely knit fabric may well be "cooler" than a tightly knit white fabric.
It would allow the fabric to "breathe" better.

4. Other issues than the coolest garment may be important. For example: Are
there religious or cultural reasons for wearing one color vs. another? If we
believe the newspaper and magazine photos, it would appear that most people
in the desert wear turbans rather than loose fitting headgear. Certainly
this practice is not consistent with keeping cool since most body heat is
lost from the head.

5. If the color of the dress is important, then certainly the color of the
skin is equally or more so. However -- although this is a generalization --
people living in hot climates tend to be dark skinned, and people who live
in cold climates tend to be light skinned.
This does not make sense, does it?

Need more data before any conclusions can be made about this inquiry.

Vince Calder
========================================================
I am going to guess...These things are usually not just for one reason.
First off some wear white , some wear black...even in the same tribe.  Here
are some factors that will come into play:

Availability of color fabrics
Cost
Personal preference
Custom
Thickness of the fabric...layers


I recently saw a special on desert nomads of the African desert and some
wore white while other wore black.  Also, I am not at all sure I would
accept the 6 degree estimate...It seems possible that the color will matter
much less in the multiple air layer that the robes assume.  The outermost
layer would certainly absorb more heat but then it would have to be
conducted to through the inner layer(s).  Here is a scenario.  I wear a thin
tee shirt on a sunny day..if it is black I notice the difference form white.
If I then wear a tee shirt and a black shirt over it...it makes less of a
difference.

Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy
=========================================================

END QUOTE.  

Answer
There is something called the emissivity found in almost any general physics text that says darker objects absorb more light but also "emit" more by the same token.

So, in a hot climate it depends on whether one is in the sun or in the shade;  with dark clothing dark clothing will heat up faster in the sun but cool faster in the shade.  

This seems to be why polar bears are light-colored:  they don't see too much sunlight, and their light coats allow them to radiate _less_ heat, keeping in more of their own body heat.