Walking and Climbing

I have always been glad & impressed to see Andy put so much into researching the science of comfort in the outdoors and applying it to Furtech. And there's a lot that's correct about his assessment; wet clothes do conduct heat much faster than dry ones (even synthetic despite the claims) and the key effect of stuff that keeps you warm when wet is that it lets the water out faster than it accumulates to free up the airspaces.

But Andy's numbers compare proportions of dry air & liquid water... not water vapour. It's well known that liquid water is a much better heat conductor than dry air... but what about water vapour & by inference, "wet" air.

I'm sorry to say that I haven't been able to find any evidence - and believe me I've looked over the years - to justify the claim that "wet air" conducts heat faster than "dry air" (at least when we talk about normal temperatures of human existence and not hundreds of degrees C in chemical engineering plants!).

First, my assumptions - we are talking about trapped air in clothing that is either "wet" (high partial pressure of vapour - and probably high relative humidity also) or "dry" (low partial pressure of vapour and probably low RH). We aren't talking about fog, drizzle or any other kind of 'wet' air... just the maritime weather we get in the UK. The reason I say 'probably' is because warm air holds more moisture and can therefore have more absolute moisture vapour but lower RH. Really we don't care too much about this as we are trying to stay warm when it's cold... The point about being trapped is that I am not going to think about whether wet or dry air behaves differently from a free convection point of view.

OK... so here are some numbers that are readily found online:

Thermal conductivity of dry air is about 0.024 (Andy's site gives 0.025 W/(m.K) which is totally fair as it depends on your definitions)

Thermal conductivity of water vapour at standard temp & pressure (298K, sea level) is about 0.0179 W/(m.K)

Neither vary much with the temperatures we deal with in the UK, though both DECREASE as it gets colder. Moreover, air at 100% relative humidity actually holds a very small mass percentage of water vapour (as can be seen from this chart if you can get your head round it !  ).

In basic terms, there's not much water in the air anyway to mess up the basic dry air conductivity (100%rh at 4C means the air holds approximately 0.5% water by mass).

So, if you interpret that information, the more water vapour there is in your dry air, the BETTER it insulates (as the water vapour conducts less well than the dry air). It's a tiny effect in theory because there's not much water to scew the numbers for dry air, and I can't find any proper data as I've said. I've seen similar 'lore' floating around on the internet about how air with higher RH conducts heat faster but I haven't seen any hard science to support it; certainly not at human-habitable temperatures.

I'd invite Andy or anyone else to demonstrate that I've got it wrong, as it seems to go against my intuition about the feeling of cold in damp weather. It's not an area where I claim any expertise and I'd be glad to acknowledge my error if anyone can show me where it lies.

That all seems to be right, but I'll add one more thing, which may or may not have relevance (I'm not sure on what 'scale' this effect can be seen - if it doesn't work on this scale then it serves for revision for me and it can be ignored!):

Water is 0-1% of atmosphere by volume, agreed, and this will change with conditions. Water (vapour, though it also applies to liquid) is the most important greenhouse gas - ie. its ability to absorb infrared at critical wavelengths multiplied by its concentration in the atmosphere. As such it warms the surrounding air significantly when losing this absorbed infrared heat. A lower concentration of water in the air would therefore mean less insulation, fitting with what you've concluded, John. By the way, that graph is the most ridiculous thing I've ever seen - no idea what's going on! 

I would just turn the heating up a bit if it was realy cold

nonetheless rogB i think you might be on to something; after all we are talking about the perception of a cold feeling.

it seems to me while moist air may conduct heat less efficiently (as john demonstrated) a higher relative humidity would alter the concentration gradient for the evaporation of water from our skin and clothes (similar to how plants in dry climes have sunken stomata to reduce water loss)- making it harder to evaporate water. I certainly find that i sweat when out and about at even relatively low temperatures and with our climate, and therefore a higher rh, this would require more energy to evaporate the sweat and thus remove more heat from my body- making me feel colder.

 i except this may be quite a simplistic view of how this works but it seems logical to me

Rog, Andrew,

as you notice, both I and the OM article avoid the subject of evaporation/condensation cycling. But it can't be ignored...

I am trying to work out, as a 'thought experiment' what happens when the ambient RH changes. Don't forget that the air close to the skin can have a local microclimate that we can consider to be approximately independent of the outside air. Temperature & RH inside the baselayer will depend on what your skin is outputting in terms of moisture and heat. You have to consider a temperature & humidity gradient between the skin and outer layers of clothing.

Let's compare the simple case of two equal temperature gradients of -30C across the clothing (36C Skin to, say, 6C outside). In the UK case, the ambient RH is high; in the Canadian case the ambient RH is low. I don't care to much about the numbers.

If the ambient RH is high, then moisture will condense at a higher temperature as it reaches saturation point. That implies that it is more likely to condense inside your clothing. Which (while releasing a little heat in the process) will result in net cooling 1) the conductiviry increases & 2) you'll have to evaporate the moisture eventually.

So we are back to the problems of damp insulation combined with evaporative heat loss. Maybe that is enough to explain why we get the impression that our insulation is less effective in damp-cold, even when we stay 'dry' (don't allow liquid water to get at our insulation).

I think this is illustrated in the graph (which is actually crammed with info if you can just work out how to read it). I'm no expert with psychrometric plots like this, but you can see what I mean if you:

Pick a pair of RH lines. Say 60% for continental winter & 80% for maritime winter. At 100% RH the wet bulb saturation temperature is equal to the dry bulb as there's no possibility to lose additional heat by evaporation. But that also means that any additional water evaporated at high skin temperature will soon condense into the clothing. And it means that the RH of the local microclimate around the skin will rise so more sweat will be dumped into the baselayer in liquid form before it can evaporate. All adding to the effect that Robert has succinctly added to the thread in the time it took me to put this together!

John

The other question this raises is what's the best thing to do with pit zips...

By opening them up, you reduce the temperature of the microclimate close to the skin... and hopefully reduce sweat rate & overheating....

but you also bring the dew point further into the clothing layers and so what moisture there is in the microclimate will tend to condense sooner!

The first will result in immediate cooling which should reduce the amount of moisture being produced at the skin.

The second will result in delayed cooling for the reasons we have been looking at above. By the time you feel cool enough from the immediate effect, you might have condemned yourself to chilliness for some time to come...

It's a tough one... personally I don't mind using pit zips as I am utterly convinced that no membrane (or even non-membrane analogy) is capable of the permeability rates needed to get rid of my excess moisture. It's far better not to create it. And if that means feeling a bit cold to control sweating then I'll put up with that. But thinking about all this has confirmed my belief that it really is worth removing layers when overheating as pit zips can encourage water to accumulate in your insulation through condensation.

it really is worth removing layers when overheating as pit zips can encourage water to accumulate in your insulation through condensation.

Yes, but of course there are practical limits.  If I have a shell and a base layer and it's windy and rainy then I'm probably better off overheating a little. 

Or if I'm attached by crampon points and an axe to a big climb then undoing a pit zip may seem like a better option right there and then than taking my jacket off!  Of course, pit zips originated on climbing jackets where the luxury of adjusting one's layers can be decidedly compromised!  If a crappy short-term solution is the best you've got then it's still the best you've got...

Pete.

Peter Clinch wrote (see)

If a crappy short-term solution is the best you've got then it's still the best you've got...

Pete.

bloody pragmatist...