Tx gt -- that is SO astonishing -- I've often wondered about it -- not that I have any clearer idea now than I did before ... From what I understand after reading that piece, the reason that the Bedouin wear black is that ... they just do, and that's all, goodnight! And it's not just the Bedouin: the Rabari women, in the Thar Desert also wear stunning all-black costumes. Course, they're also covered in mirrors -- and maybe the mirrors throw back the heat/light?*
But now that I come to think about it afresh (i.e., in the light of this new research), I think the true reason has nothing to do with the physics of heat and light -- but merely convenience. Black clothes are less prone to show grime and dirt than white ones and in the desert, there's a SHORTAGE OF WATER ... right? Including for laundry. So as long as wearing black is NOT a problem in terms of heat retention (as witness black animals -- black buck in the Thar, black goats in the Sahara etc) then it certainly makes sense in terms of looking maximum macho.
*Speaking of throwing back heat/light I read the other day that Prez Obama proposed painting the tops of all buildings white as a method to help reverse global climate change and that mirror-surfaces would be best of all. Well this makes me feel maximum proud of myself coz I had this idea twenty years ago, while living in a barsati in Delhirium (such ideas are prone to occur when one is dying of heat exhaustion in a non-A/c barsati in power-free Delhi). The idea involved covering the entire top of my living area with reflective mylar -- cheaply available by the meter as a lightweight gift-wrap material. Alas, I was unable to apply this revolutionary new technique of heat-evasion coz I was too exhausted (i.e., from the heat) to put up the 500 mts of mylar I bought. Over the years I used it for innumerable wrapped gifts instead of saving the planet. Curses! Foiled again!
needless to say - its a complic8ed schema. (http://www.physicsforums.com/archive/index.php/t-65287.html)on the one hand white REFLECTS heat, so white robes could REDUCE the heating of the person wearing them. but, since black RADI8S heat well, so a person's own body heat could be RADI8ED AWAY . therefore white has the drawback of not radiating away a person's body heat, but black has the disadvantage of absorbing the sun's heat.
In fact, black robes have been found to be 6 Celsius degrees hotter than similar white robes. The Bedouin secret lies in the convection breeze set up when the warmer air inside a dark robe rises faster and escapes upward through the porous material, sucking in air from below. Cooler air from below pushes the warm air up and out of the robe top. This, in turn, creates a breeze. The breeze evaporates sweat, cooling the robe wearer, and moves the sweat-laden air out the robe top. The loose-fitting burnoose is a solar air-conditioning system. (http://www.wonderquest.com/black-robes.htm) So a black robe winds up being no hotter, and its circulating air may even make things more comfortable. gt
re idea 2 of painting the roof tops black to reduce global warming - firstly - imagine what happens to those houses in winter?? they'll be loosing precious heat! the flip side - think greenhouses that allow heat to get "trapped" - thats nice for plants but again tricky to live inside due to the heat being trapped. the varying requirements of temperature to sustain comfortable environments changes day vs night, summer vs winter so i guess any color coded solution would necessarily be variable. my recent idea with rooftops is to place solar panels embedded into "customized" roof looking materials eg u could have a "wooden, log cabin" looking plastic design with solar panels, or "red roofed" background? "roman stone" style ? rajasthani fortress backdrop? etc.... gt
Not sure why, but my "Wow ... cool" comment got posted after the second of your comments, even though I posted it immediately after the first of gt's comments. *shrug* Whatever.
I like the camouflaged solar-panel concept. I would imagine any temperature modulation based on roof-top colours/materials would require a system of vents to let air in and out. My mirror mylar was intended to bounce heat/light from above, back out -- but I didn't think about the heat trapped inside the room. I've always been interested by the use of mirror-surfaces on the inside of flasks of hot liquids -- I've always assumed that the reason for their use is that heat is a function of light and therefore is reflected internally just the same way as light would. Is that so?
the easiest way to think of heat is to recognize that the molecules that make up any matter are constantly jiggling around their average positions - and when something gets hotter - the molecules jiggle more. so believe it or not - every now and then molecules from the table, from the cup etc - do infact get dislodged and run away - just very very very few of them do that as compared to lets say molecules from your chai or another liquid.... and even more run away from a gas. and if ytou heat up any of these substances, the molecules jiggle faster and harder.
life gets a bit more complicated if you think of "absorption" - because another, different way in which you can think of approaching how to think of heat is when its represented as a "wave" of electromagnetic waves. eg visible light is usually a wave between 400nm and 700 nm..... nm stands for nanometer and is a millionth of a meter..... 400nm will give you blue colour, 700nm will give you red colour.....actually if you get even larger wavelength - you get infrared (> than 900nm) and, if you went smaller than visible blue you would get ultraviolet (< 300nm)..... remember both ultra violet and infrared can heat up stuff (microwaves are in cms range and no longer in the visible spectrum as they are in cms ranges but they too can heat up stuff)... all electromagnetic waves even radiowaves (km ranges!), x- rays (extremely small!) etc are all capable of heating stuff up - its just that different materials preferentially absorb different wavelengths. (infact all of "spectroscopy" deals with the idea that every molecule has a characteristic fingerprint that is represented by a certain characteristic pattern of electromagnetic waves).
so yes, a mirrored surfaces should keep heated stuff hot for much longer as it will not "absorb heat" as it bounce the waves off, esp as compared to lets say a black surface BUT bit by bit the molecules of the mirrored surface will also start to jiggle - and that'll slowly start loosing heat eventually. gt
on another interesting thought - leaving "heat" aside, think what happens when white light from the sun falls on a red rose? actually the whitelight (consisting of violet, indigo, blue, green, yellow, orange and red - the 7 basic characteristics of white colour)fall on the rose and all the colours except red get absorbed - so only red bounces off to your eyes! in that sense actually red is the only component that the rose petal did not eat up - it was rejected - and thats what came to your eye! is red really the "colour" of the rose? or rather, its the stuff that the rose couldnt hold onto? thats why at night, if you take a blue laser and shine it on the rose you dont see a blue rose - rather you should see it black! so what is the true colour of a rose? actually colour vision and how we percieve it is (as always) yet another fascin8ing field in biology. our eyes are "sensors" that detect blue - red 400nm to 700nm "visible" light in the electromagnetic spectrum - which we know traverses enormous sets of wavelenths. http://cassini-huygens.jpl.nasa.gov/mission/images/EM-spectrum.jpg thats why i like the quote "the universe is full of magical things patiently waiting for our wits to observe them (quoted by f jezegou).... gt
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8 comments:
why do bedouins wear black robes in hot deserts?
http://www.nature.com/nature/journal/v283/n5745/abs/283373a0.html gt
Tx gt -- that is SO astonishing -- I've often wondered about it -- not that I have any clearer idea now than I did before ... From what I understand after reading that piece, the reason that the Bedouin wear black is that ... they just do, and that's all, goodnight! And it's not just the Bedouin: the Rabari women, in the Thar Desert also wear stunning all-black costumes. Course, they're also covered in mirrors -- and maybe the mirrors throw back the heat/light?*
But now that I come to think about it afresh (i.e., in the light of this new research), I think the true reason has nothing to do with the physics of heat and light -- but merely convenience. Black clothes are less prone to show grime and dirt than white ones and in the desert, there's a SHORTAGE OF WATER ... right? Including for laundry. So as long as wearing black is NOT a problem in terms of heat retention (as witness black animals -- black buck in the Thar, black goats in the Sahara etc) then it certainly makes sense in terms of looking maximum macho.
*Speaking of throwing back heat/light I read the other day that Prez Obama proposed painting the tops of all buildings white as a method to help reverse global climate change and that mirror-surfaces would be best of all. Well this makes me feel maximum proud of myself coz I had this idea twenty years ago, while living in a barsati in Delhirium (such ideas are prone to occur when one is dying of heat exhaustion in a non-A/c barsati in power-free Delhi). The idea involved covering the entire top of my living area with reflective mylar -- cheaply available by the meter as a lightweight gift-wrap material. Alas, I was unable to apply this revolutionary new technique of heat-evasion coz I was too exhausted (i.e., from the heat) to put up the 500 mts of mylar I bought. Over the years I used it for innumerable wrapped gifts instead of saving the planet. Curses! Foiled again!
needless to say - its a complic8ed schema. (http://www.physicsforums.com/archive/index.php/t-65287.html)on the one hand white REFLECTS heat, so white robes could REDUCE the heating of the person wearing them. but, since black RADI8S heat well, so a person's own body heat could be RADI8ED AWAY . therefore white has the drawback of not radiating away a person's body heat, but black has the disadvantage of absorbing the sun's heat.
In fact, black robes have been found to be 6 Celsius
degrees hotter than similar white robes. The Bedouin secret lies in
the convection breeze set up when the warmer air inside a dark robe
rises faster and escapes upward through the porous material, sucking in air from below. Cooler air from below pushes the warm air up and out of the robe top. This, in turn, creates a breeze. The breeze evaporates sweat, cooling the robe wearer, and moves the sweat-laden air out the robe top. The loose-fitting burnoose is a solar air-conditioning system. (http://www.wonderquest.com/black-robes.htm) So a black robe winds up being no hotter, and its circulating air may even make things more comfortable. gt
re idea 2 of painting the roof tops black to reduce global warming - firstly - imagine what happens to those houses in winter?? they'll be loosing precious heat! the flip side - think greenhouses that allow heat to get "trapped" - thats nice for plants but again tricky to live inside due to the heat being trapped. the varying requirements of temperature to sustain comfortable environments changes day vs night, summer vs winter so i guess any color coded solution would necessarily be variable. my recent idea with rooftops is to place solar panels embedded into "customized" roof looking materials eg u could have a "wooden, log cabin" looking plastic design with solar panels, or "red roofed" background? "roman stone" style ? rajasthani fortress backdrop? etc.... gt
Wow -- now THAT's cool! Literally ...
Not sure why, but my "Wow ... cool" comment got posted after the second of your comments, even though I posted it immediately after the first of gt's comments. *shrug* Whatever.
I like the camouflaged solar-panel concept. I would imagine any temperature modulation based on roof-top colours/materials would require a system of vents to let air in and out. My mirror mylar was intended to bounce heat/light from above, back out -- but I didn't think about the heat trapped inside the room. I've always been interested by the use of mirror-surfaces on the inside of flasks of hot liquids -- I've always assumed that the reason for their use is that heat is a function of light and therefore is reflected internally just the same way as light would. Is that so?
the easiest way to think of heat is to recognize that the molecules that make up any matter are constantly jiggling around their average positions - and when something gets hotter - the molecules jiggle more. so believe it or not - every now and then molecules from the table, from the cup etc - do infact get dislodged and run away - just very very very few of them do that as compared to lets say molecules from your chai or another liquid.... and even more run away from a gas. and if ytou heat up any of these substances, the molecules jiggle faster and harder.
life gets a bit more complicated if you think of "absorption" - because another, different way in which you can think of approaching how to think of heat is when its represented as a "wave" of electromagnetic waves. eg visible light is usually a wave between 400nm and 700 nm..... nm stands for nanometer and is a millionth of a meter..... 400nm will give you blue colour, 700nm will give you red colour.....actually if you get even larger wavelength - you get infrared (> than 900nm) and, if you went smaller than visible blue you would get ultraviolet (< 300nm)..... remember both ultra violet and infrared can heat up stuff (microwaves are in cms range and no longer in the visible spectrum as they are in cms ranges but they too can heat up stuff)... all electromagnetic waves even radiowaves (km ranges!), x- rays (extremely small!) etc are all capable of heating stuff up - its just that different materials preferentially absorb different wavelengths. (infact all of "spectroscopy" deals with the idea that every molecule has a characteristic fingerprint that is represented by a certain characteristic pattern of electromagnetic waves).
so yes, a mirrored surfaces should keep heated stuff hot for much longer as it will not "absorb heat" as it bounce the waves off, esp as compared to lets say a black surface BUT bit by bit the molecules of the mirrored surface will also start to jiggle - and that'll slowly start loosing heat eventually. gt
on another interesting thought - leaving "heat" aside, think what happens when white light from the sun falls on a red rose? actually the whitelight (consisting of violet, indigo, blue, green, yellow, orange and red - the 7 basic characteristics of white colour)fall on the rose and all the colours except red get absorbed - so only red bounces off to your eyes! in that sense actually red is the only component that the rose petal did not eat up - it was rejected - and thats what came to your eye! is red really the "colour" of the rose? or rather, its the stuff that the rose couldnt hold onto? thats why at night, if you take a blue laser and shine it on the rose you dont see a blue rose - rather you should see it black! so what is the true colour of a rose? actually colour vision and how we percieve it is (as always) yet another fascin8ing field in biology. our eyes are "sensors" that detect blue - red 400nm to 700nm "visible" light in the electromagnetic spectrum - which we know traverses enormous sets of wavelenths. http://cassini-huygens.jpl.nasa.gov/mission/images/EM-spectrum.jpg thats why i like the quote "the universe is full of magical things patiently waiting for our wits to observe them (quoted by f jezegou).... gt
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