[LRflex] Re: OT: Dark Desert Clothes



>From: Colin Howarth <colin@xxxxxxxxxx>
>Reply-To: leicareflex@xxxxxxxxxxxxx
>To: leicareflex@xxxxxxxxxxxxx
>Subject: [LRflex] OT: Dark Desert Clothes
>Date: Mon, 27 Nov 2006 03:05:15 +0100
>
>On 26.11.2006, at 18:52, David Young wrote:
>
> > Aram offered:
> >
> >> Sorry, I am behind in reading.  Maybe someone already said this.
> >> I read an article long ago about dark desert clothes.  It seems
> >> that the
> >> dark clothes absorb the light and get hot, and this heat cause the
> >> air
> >> inside the clothes, which are loose fitting robes, to get and hot
> >> and rise.
> >> This draws in cooler air underneath and creates a sort of air
> >> conditioning
> >> system - convective cooling.  Of course it does not work with a
> >> black three
> >> piece suit.
> >
> > Now that's the best (and the only good) explanation for black robes
> > in the desert that I've seen!
>
>This is so much more fun than commenting on photography or pictures....
>
>OK. How would you define "best" and "good" explanations? I would hope
>that the "best" explanation was correct.
>
>I think I read something like Aram's explanation is some google
>search (or wikipedia article) on bedouin robes.
>
>Now, I can follow the argument as far as "this heat cause the air ...
>to get and HOT and rise."
>
>The bit about "draws in cooler air underneath" is true I suppose, but
>misleading. You don't have cooler air inside the robes - we just said
>it was hot. It gets heated up. You might have cool ankles, but a hot
>neck :-)  (cool in the sense of normally hot desert air)

It has been many years since I read that published article, but sounds like 
you found something akin on the web.  I assume the chothes are loose enough 
to allow for the hot air to vent out the top somewhere allowing the cooler 
(relative to the inside) to circulate up to take its place.  That is what I 
meant my convective cooling.  Cooler air relative to the heated air inside 
the clothes.


Aram

>
>I suppose that if there were some sort of air circulation, then that
>would act like a breeze, which would help the sweat evaporate quicker
>which would draw heat from the skin. Still - it would be a pretty
>warm breeze. I believe the article mentioned above said that the
>scientists had found that there was no significant skin temperature
>difference between light and dark-coloured robed subjects.
>
>That might just mean that the mammalian temperature regulation system
>was working - the people in the dark robes sweating more.
>
>I'm not sure why there's a term "convective cooling". You can
>transfer heat energy (e.g. cool down) by radiation or by conduction.
>Convection is what happens to fluids as a result (of non-uniform
>density).
>
>I wonder what this thing about bedouin robes is. Everyone seems to
>assume that it's a really clever trick. Just because lots of people
>do something, doesn't mean it's a sensible thing to do - does it ? :-)
>
>Now, there might be a really good reason for the dark robes - but it
>would be good to hear from a real bedouin (with internet connection).
>
> > Unfortunately, black also radiates the heat better, which is what
> > causes the M8's "magenta twist" - especially with synthetic fabrics.
> >
> > Does your science wizardry extend to explaining why synthetic fabrics
> > would cause more problems than, say, cotton or wool?
>
>Maybe they get hotter because they don't conduct heat as well (to the
>air) or to the inside of whatever. Maybe they don't let air pass
>through them so easily. Maybe they really are a bit blacker and
>absorb sunlight better? Maybe they don't even cause more problems? :-)
>
>Friends, real science is tricky and best left to scientists. A lot of
>it involves tedious, careful, controlled experiments. Armchair
>science was fine for the ancient greeks - but didn't actually tell
>them much about the real world.
>
>I look forward to the definitive "correct" explanation. :-)
>
>colin
>
>(more scientist than photographer, I'm afraid)
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