🚀 go-pugleaf

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.

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. 

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.