Out on the hill the other day with
Andy
Davidson from
FurTech
- that's him perched precariously on a gritstone outcrop - and in the
middle of the Peak's white crusted snowiness, we got talking about
insulation and humidity.
Insulation's
pretty straightforward really - generally it works like this, your
clothing traps air which your body heats up to prevent further heat
loss, then, in order to stop wind and air movement from stripping that
warm layer of air away from your body, you need some sort of wind-proof
or wind-resistant element to hold it in place.
Simple stuff, but in reality what's doing the insulating isn't strictly
the clothing. It's the air that it traps. That's all well and good, but
guess what, air is variable. In particular, its moisture content varies
according to local conditions and that, in turn impacts on how well the
air insulates. In simple terms, moist air transmits heat more readily
than drier air. And that, in turn, means that the same sleeping bag can
feel significantly cooler or warmer depending on the moisture content
of the air.
That's why a sleeping bag that kept me toasty warm at -25C in the cold,
dry atmosphere of the Andes at 5,000 metres plus, suddenly felt almost
marginal in temperatures of -10C in Scottish winter. The bottom line
was that the sleeping bag was the same, was trapping roughly the same
amount of air, yet that air was a significantly less efficient
insulator because its higher moisture content, allowed it to transmit
heat away from the body at a faster rate.
How much faster? Well, it turns out that Andy's done some
rough
calculations which show just how quickly the
insulating effectiveness of the airspace in your clothing falls away
with the addition of just small amounts of water. According to Andy's
table, just adding 10% of water to dry air reduces its insulation value
to around 30 per-cent of the dry value.
Water conducts heat some 24 times better than air... which explains a
lot. And that's not taking into account the effects of evaporation, it
take a lot of heat to evaporate water far more than simply to raise its
temperature.
Okay, so it's rough science. But for you, me and other Brits, what
really matters is that our moist climate means that clothing simply
doesn't keep you as warm. I've climbed waterfall ice in the Alps
wearing a softshell jacket and trousers at, say, -10C, and felt fine
and comfortable. Try the same clothing in Scotland and I'd be shivering
within minutes...
So if you've ever come back from the cold, dry, high mountains of the
Alps, Andes or Himalayas and thought, streuth, it feels ridiculously
cold at home on British hills, even if it's notionally much warmer,
then that'll be why...
John Burley
