Sunday, November 30, 2008

Psychology Of Water Vapor

Now here's another fascinating tidbit I came across in my cold-adaptation research: Did you know that the "relative" in "relative humidity" is "relative to how much water the air could hold, at the current temperature"? Hot air can hold more moisture than cold air. So if you keep the actual amount (mass) of water in the air the same, higher temperatures give lower relative humidities. And since the relative humidity determines how fast damp objects (such as our bodies) dry out, this is what we perceive. From a human perspective, the absolute amount of water vapor in the air is fairly meaningless, unless you know the temperature.

From the limited data I can find out there, humans seem to feel physically and psychologically best at relative humidities between 40% and 60%. More than that starts to feels "muggy" in hot weather and "dank" in cool weather.

At the other end of the spectrum, people around here often complain that the "air is too dry in the winter", causing dry eyes, chapped lips, etc. But it seems that to at least some degree, this perceived "too dry"ness is primarily indoors, manmade, and a result of (guess what?) artificial heating.

Take this morning, for example. In the Smallish City right now it's cool and cloudy outside. A small sleet-storm is edging its way towards us for later in the day. The temperature is 28F (-2C) and the relative humidity is 78%. Walking around outdoors, you certainly don't feel the air is "too dry". The cold air can't hold much absolute moisture, but it's holding about as much as it can-- so your body isn't drying out quickly. If anything, it borders on feeling "dank" out there. And when the sleet starts, it will surely feel downright wet.

Here in my kitchen, however, I have a space heater running to keep the temperature at a toasty 51F (10.5C). The actual amount of water in the air is about the same as outside. But at this indoor temperature, the air could hold a lot more water vapor. The relative humidity drops to 32%-- feeling just a bit on the dry side, but still fine for me.

I look out at Accordion Neighbor's house, though, and notice that he's got his woodstoves cranking. He probably has it up to 68F (20C) in his house, like "normal" people. At that scorching temperature, the relative humidity in his place right now is a parching 17%. The water is being sucked out of his family's bodies into the hot air. He probably has chapped lips, his kids might have bloody noses and dry throats (indeed, I haven't heard them scream in several days), and they are all probably going through hand-moisturizer at a rapid clip.

A.N. is probably trying to counter this problem with an old trick: keeping a kettles of steaming water on top of the woodstoves to get some extra humidity into the air. (A similar old trick is used around here by those with radiator heat: narrow buckets of water that hang from the radiator, getting warm & evaporating moisture into the air.) This trick isn't necessarily a bad idea, for human comfort-- but from a standpoint of energy-efficiency it's very much a losing battle. In an old house, it won't be long before that water vapor makes its way back outdoors. And, as we learned in a previous thought-experiment, evaporating water costs you temperature, and energy. Every gallon of water you purposefully evaporate costs you a couple pounds of nice firewood, or cup of fuel oil.

So, call me a crackpot, but: one way to keep yourself feeling dried out in winter is to keep your house cooler.

7 Comments:

Blogger Johanna said...

Me, I am not swayed by all the arguments for sitting around feeling cold, and I am still trying to wrap my head around this concept of not turning on the furnace to save energy while managing to convince oneself that using an inefficient electric clothes dryer makes economic, environmental or common sense.

I am waiting for the day you actually turn on the heat and focus your thoughts on preventing heat loss, because that's information I could apply.

11/30/08, 12:56 PM  
Blogger Turboglacier said...

Oh, I'm focused on preventing heat loss, too, inasmuch as that is reasonably feasible. I've gone around and caulked all the window cracks. I am putting additional plastic insulation on some of them. I injected foam in many baseboard cracks. I put foam behind drafty electrical outlets. I'm sealing up holes in the basement through which cold air came in (such as the big hole formerly used by the dryer vent.) But, short of a multi-thousand dollar jobs such as re-insulating the walls or replacing all the windows, this is all I can do for the moment.

As for the electric dryer: I have to disagree with the common sense. The dryer is basically just a big electric heater. It's electricity (in fact, ANY electricity used in the house) is ultimately turned, with 100% efficiency, into heat (with the exception of a miniscule amount that may escape as light.) So, as long as you manage to keep that heat in the house, you're maintaining perfect efficiency.

(Making the electricity in the first place, of course, is not a 100% efficient process-- but neither by a long shot is running the furnace. Depending on the source of the electricity, it's arguably "greener" than burning oil here at home.)

11/30/08, 1:33 PM  
Blogger C Neal said...

Remember that warm, dry air is actually perceived by humans to be less warm than warm, moist air. A room with a thermostat set at 60 and with 80% relative humidity feels just as warm as a room where the thermostat is set at 65, with only 10% RH. See this chart from NOAA:

http://www.infoplease.com/ipa/A0001434.html

As I understand it, dry air feels colder because it increases the rate of evaporative cooling on your skin (conversely, in the summer, hot, humid air decreases the rate of evaporative cooling and makes it feel hotter than it actually is). So the kettle on the stove or the bucket on the radiator may decrease performance in terms of actual temperature, but in terms of perceived temps, it may well be an improvement.

I'll leave it to you to figure out the details of the trade-off.

11/30/08, 2:22 PM  
Blogger Turboglacier said...

C.Neal: That is most interesting. But I think it still doesn't come out in Accordion Neighbor's favor. With this morning's conditions, if AN set his thermostat at 70F, his humidity would be 16%, and chart says he would "feel like" it was about 64.5F (interpolating between the data points.)

Meanwhile, even if I (uncharacteristically) let my house go up to 60F, I would have a humidity of 23%, and would "feel like" 56.5F. So, AN's house feels 5.5˚ cooler than it is, but mine feels only 3.5˚ cooler than it is. To make up the two degrees in "feel", he can make the house warmer, or more humid, but either way he's using even more fuel. Chasing the tail, I call it.

[Also: I was annoyed with that chart, because it only goes down to 60˚F, whereas my house is considerably cooler. But then I noticed a trend which leads me to question its validity altogether: as the temperatures go down, the chart says you need more humidity to "feel" like you're at that actual temperature. By 60F, you need 80-90% humidity just to "feel" like it's 60F. If you extrapolate this pattern just a few degrees, at about 57F you'll need 100% humidity to "feel like" 57F. At temperatures below that, "feeling like" the actual temperature would take humidity >100%, which isn't possible. So... I surmise that this chart is good only for a very narrow range of temps-- probably not even down to 60F, and certainly not below that.]

11/30/08, 3:34 PM  
Anonymous girltuesday said...

ok, i'll bite: crackpot.

11/30/08, 10:51 PM  
Blogger Victor said...

Oh, I grew up in Very Smallish town with cold (-20°C) winters and, the chart cannot be - as you suggest - valid for colder temperatures. The amount of water in the air also affects the convection and conduction of heat. In "cold" (< 4°C as a qualified guess) temperatures an increasing relative humidity will be percieved as the air getting colder. This is why +3 and rain feels in october feels like -8°C and -20°C with a clear sky in january feels like -8°C.

This are just opinions. And this was a very nice blog!

12/10/08, 9:57 AM  
Anonymous Anonymous said...

More than you ever wanted to know about apparent temperature (with equations!): http://www.srh.noaa.gov/ffc/html/studies/ta_htindx.PDF

12/10/08, 6:50 PM  

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