Jan 242010

Back in December, I went tobogganing with the kids.  It was -20°C, and windy.  No problem, just don’t face the wind.

Me on a sled

Me on a sled

Today I went tobogganing again.  It’s -5°C and windy.   I got cold.   OK, maybe I wasn’t dressed as well as I should have been, and I probably was dressed correctly back when it was -20°C.  But this makes me wonder about the claims we make about high humidity making it feel colder.   Alberta has this ‘dry cold’ which doesn’t feel as bad, supposedly.  I know that when I go to Germany in winter it feels much colder than I expect it to be.

So how could this work?  Here are some hypotheses:

  1. the extra mass of water in the air causes an increase in cooling rate
  2. the water infiltrates clothing and decreases the insulation value
  3. it should actually feel warmer, since the higher the humidity, the less heat you lose from evaporation
  4. some other mechanism.

OK, lets start with the first option.  Well, sorry, it doesn’t seem to work.  At a dew point of -5°C, there’s not much water in the air.   This is the dew point you would have at 100% humidity and a temperature of -5°C.   We’re talking about 2g of water per kg of air, which is about 1m3 of air.   It seems unlikely that this little bit of water in the air would actually drain so much more energy as it gets warmed by our skin.

Vanja Disappearing

Vanja Disappearing

Ok what about option 2.  Well, unless we actually have 100% humidity, the water from the atmosphere isn’t going to condense in our clothes.  But the vapour that continuously leaves our skin at our body temperature of around 37°C will raise the humidity in our clothes far above the saturation level, especially if the air is already humid.  This will then not leave the fabric but will condense.  This makes the clothing damp and more likely to conduct heat rather than to insulate.  This seems like a reasonable candidate.

What about the third option?  It’s a myth.  We all know that hot humid climates can be uncomfortable – we perspire in order to cool off, but the water fails to evaporate so we drip and don’t get cooled.   So even at -5°C, if the relative humidity is 100% then moisture won’t evaporate, so we won’t cool!

Ok, so option one seems a little sketchy, option two seems to have some foundation in physics.  Option 3 does as well – it’s just a matter of whether 2 outweighs 3 or vice-versa.  It seems likely that both 2 and three happen at the same time, and given the number of people who assert that damp cold weather is more uncomfortable than dry cold weather it would appear that it’s likely that option 2 outweighs option 3; evaporative cooling decreases, causing our clothing to get damp instead.  Damp clothing conducts heat better, so we feel cold.

It would be nice to find some scientific evidence to this effect.  What I have come across is somebody who’s confused about how evaporative cooling works in cold weather claiming that you notice the evaporation more when it’s cold and humid. I’ve also come across a few answers on one page.

Well, there’s still option 4. I don’t have any other ideas. Anybody else?

It seems that it should be reasonably easy to prove scientifically what the humidity does to your comfort level:  set up a temperature controlled object that has a porous surface.   Keep its surface temperature at say 35C, and pump moist (ie 100% RH a t 35C) air through it at a constant rate.   Now change the temperature and relative humidity of the room and monitor how much power needs to put into the system to maintain the temperature.  Try with putting clothing on the object, etc.  Somebody must have done this already.

 Posted by at 10:09 pm