🚀 go-pugleaf

In message <7uvzn58newv.fsf@gina.research.nokia.com>, MB <*@*.*> writes
>Chris >
>> I don't think [radiative] heat loss is significant though. Any
>> tightly woven clothing will prevent most of it. Convection,
>> conduction and evaporation are far more important and with
>> these colour is irrelevant.
>
>I think that Chris got it a bit wrong here. The heat loss
>_through_ a fabric is mostly by conduction and radiation
>(assuming no forced convection, ie wind blowing through, or
>pumping air through by movement). High-density materials,
>like woven fabrics, are indeed effective in blocking the
>radiative heat loss through the material [1], being
>effectively non-transparent at thermal wavelenghts. And,
>consequently, the radiative silvery barriers sometimes found
>within such fabrics, or, more commonly, on the interior surface,
>are just wasted excess weight. (Radiative heat loss through a
>low-density insulation can be significant though, and,
>thus, radiative barriers within it, or on the interior
>surface, may be useful. So, Stephenson probably got
>it right here, although down typically is less transparent
>to thermal radiation than volumeous synthetic insulators.
>Ref. [1])
>
>But, that's not the whole story. At the exterior surface
>of the garment, rouhgly half of the thermal radiation is
>emitted into the space outside, and is thus lost. The
>only things that matter here are 1) the temperature of the
>surface, and 2) the emissivity of the surface. The Stefan-
>Bolzmann law. It's just the surface visible to the outside
>that matters, not what the material hidden within the garment
>was. The net flux of heat due to radiation at the surface
>depends on the temperature of the surroundings too, of course,
>as the surroundings emit thermal radiation too: The Sun is very
>hot, the empty space (clear sky) is very cold, and the rest
>(vegetation, snow, clouds, tent interior) are somewhere in-
>between.

I think we're talking about relative amounts here. I know there is some 
heat loss due to radiation, I just don't think it's very significant 
compared with the other types of heat loss. If enough warm air is 
trapped inside the clothing to keep the wearer warm and the outer of the 
clothing repels water, snow and wind then I don't think the radiation 
heat loss matters. Once heat has reached the outside of your clothing 
you've lost it anyway. The key is to keep the heat inside - or at least 
as much of it as you need to stay comfortable.
>
>The emissivity of the surface at thermal wavelenghts can
>be greatly reduced by making it silvery shining. In contrast,
>matt black is the worst possible choise. When emissivity
>goes down, so does absorptivity. Thus, the silvery parka,
>tent or sleeping bag is cooler in the sunshine and warmer
>under the clear night sky.
>
>Unfortunately, I don't have any figures for the relative
>importance of radiative heat loss vs the conductive and/or
>spontaneous convective ones. I'm quite confident however,
>that raditive heat loss (at the surface) can be significant,
>depending on the conditions.

Various Swedish companies like Fjallraven used to make tents with silver 
flysheets to keep out the sun during the long hours of northern summers. 
I didn't find when I tried one that it was noticeably cooler than non 
silver tents. (It was much darker - you needed a flashlight at midday). 
The coolest tents I've used were cotton ones (with bright orange 
flysheets).

Most down jackets seem to be black these days.
>
>Glenn Randall's otherwise usually most excellent book [1],
>however, blatantly dismisses silver-coloured outdoor clothing
>with - of all things - a fashion argument!
>
>Quoting directly: "While the "wet looks" may once have been
>fashionable at downhill ski areas, few people want to look
>like a walking advertisement for Reynolds Wrap."
>
>I guess Glenn wouldn't get caught wearing a skrit eihter... ;)
>
>[1] Glenn Randall, 'Cold Comfort', Lyons&Burford (1987).
>It's a secondary reference though, except for the direct
>quote.