by slparker » Mon 28 Jun, 2021 10:55 am
"Umm radiation works from a hot body to colder things.'
I don't think that this is the way it works. All bodies radiate (emission) irrespective of ambient temperature but heat absorption from radiation will be greater than loss if directly exposed to high radiation (i.e. the sun). A body will still radiate heat away even when exposed to direct sun: this is the property of radiant heat loss.
But, if bodies are in direct contact than heat will transfer from a hot body to a cooler body. This is the property of conductive heat loss.
In practice, we lose generated heat (and that gained by radiative absorption) mainly by evaporative heat loss.
IN hot weather we are best served by preventing heat production(slowing down work output) and doing whatever potentiates evaporative cooling - I agree that preventing radiant heat gain is an important point, though.
What is the role of clothing? I am not convinced either way - black garments radiate heat more quickly than white garments; so black clothes (if loose) have a theoretical advantage over white clothes in shedding gained heat (and a practical advantage if you ask a bedouin) but heat from solar gain will be greater as it reflects less.
I am no physicist but heat gain from air temperature should be effectively unaffected by garment colour (as it is about conduction) and so will evaporative heat loss.
So black loose clothes may be better but certainly less practical for bushwalking and if you wear form fitting clothes then white may be a better choice; but irrespective of this, promoting evaporative cooling and producing less body heat is essential. A parasol, if practical, is clearly the best option if absorption of radiant heat is a concern and loose clothing (promoting the chimney effect) is also good for promoting evaporative heat loss.
How well this works in practice has never been studied, I believe, since that 1992 paper about the Bedouin.
Personally, I haven't noticed a lot of difference in heat gain from the colour of my clothes but that is purely subjective. I tend to wear favour loose fitting clothes and avoid walking in the middle of the day either way.
Addit: in practice, my clothes are usually wet with sweat when I walk - water evaporates more quickly from a dark surface -is there a role in dark clothes if they are saturated?
My point being that there are more variables than just: white reflects more than black so therefore white is cooler.