Does black clothing keep you cooler?
I’m no expert in thermodynamics, but in the recent report about black vs. white clothing and which keeps you cooler I can’t make sense of the suggestion that white clothes “reflect” heat when ambient temperatures are high.
This statement, in particular, seems a bit generic:
“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.”
As I understand it (this is where the thermo-experts need to chime in) the “whiteness” of clothing only affects radiative heat, such as that from the sun or from a warm object in a relatively cooler environement.
But at temperatures above 98.6 (assuming the person doesn’t have a fever there will be no radiative heat coming from the body. All heat transfer will be through conduction or convection. That being the case, the color of the clothing will have no effect on the amount of heat escaping from the body and through the clothes. Rather, it will be the material and construction of the clothing that will make a difference (thick vs. thin, porous vs. dense, etc.)
OK, so if all of that is true, then on a hot, sunny day, white clothing should work best, no? It will reflect the radiative heat from the sun and (ideally) allow the convective (mostly) heat escape from under the clothes. Free passage of air through the clothing should also help aid in evaporative cooling (really a special case of conduction and convection). Black clothing (of the same material and construction), though, would absorb the radiative heat from the sun, while providing no benefit to removing heat from the body.
Based on this, I think the above statement should read something like this:
“White clothing reflects sunlight, but also reflects internal radiated heat back towards your body, so the net effect under conditions when the ambient air is cooler than your skin is less cooling than if you wore black. In very hot ambient air (above 98.6 or so), white clothing would provide more benefit due to the absence of radiative heat coming from your body.”
OK, experts, what am I missing?
I have edited in the link to the Staff Report. – CKDH
[Edited by C K Dexter Haven on 02-25-2001 at 11:35 AM]