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Wet cold vs dry cold

by voko
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voko
#1
Jan18-13, 07:41 AM
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Apologies if this has been addressed earlier.

A common belief is that a cold temperature (say 0 Centigrade) feels colder when relative humidity is higher.

Is there any underlying physical reason for that? As far as I can tell, the differences between dry air's and water vapor's thermal conductivity and specific heat are not very significant. At 0 Centigrade and normal pressure, at 100% relative humidity, air contains 4.8 g water vapor per cubic meter; 1 cubic meter of air is heavier than 1 kg at zero and subzero temperatures. So the variation of vapor content is less than 1% at most, so any effect on net thermal conductivity and specific heat should be negligible.

Is this belief a myth, or am I missing something?
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engnr_arsalan
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Jan18-13, 08:27 AM
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a property names as thermal conductivity is responsible for this phenomenon..which tells how faster can an element transmit or absorb heat..water has higher conductivity than air..consider case of dry air at 0 centigrade it will feel cold but it would be colder if humidity is increased thats is amount of water is increased in air..adding water will increase heat transfer from human body to cold humid air..
Chestermiller
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Jan18-13, 08:44 AM
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Quote Quote by voko View Post
Apologies if this has been addressed earlier.

A common belief is that a cold temperature (say 0 Centigrade) feels colder when relative humidity is higher.

Is there any underlying physical reason for that? As far as I can tell, the differences between dry air's and water vapor's thermal conductivity and specific heat are not very significant. At 0 Centigrade and normal pressure, at 100% relative humidity, air contains 4.8 g water vapor per cubic meter; 1 cubic meter of air is heavier than 1 kg at zero and subzero temperatures. So the variation of vapor content is less than 1% at most, so any effect on net thermal conductivity and specific heat should be negligible.

Is this belief a myth, or am I missing something?
I would think it would be the other way around. Why should cold be different from hot. When it is hot, higher humidity makes it harder for water to evaporate from your skin, and your skin temperature will be a little higher. This effect should also apply at low temperatures, although, at low temperatures, the difference in vapor pressure driving force between your skin and the ambient air for 0% humidity versus 100% humidity will be much less than at higher temperatures.

voko
#4
Jan18-13, 09:13 AM
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Wet cold vs dry cold

Quote Quote by engnr_arsalan View Post
a property names as thermal conductivity is responsible for this phenomenon..which tells how faster can an element transmit or absorb heat..water has higher conductivity than air..consider case of dry air at 0 centigrade it will feel cold but it would be colder if humidity is increased thats is amount of water is increased in air..adding water will increase heat transfer from human body to cold humid air..
Liquid water and water vapor are very different. Thermal conductivity of water vapor is not much different than that of dry air. And there is very little vapor, in absolute terms, at subzero temperatures to begin with.
D H
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Jan18-13, 10:40 AM
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Quote Quote by voko View Post
A common belief is that a cold temperature (say 0 Centigrade) feels colder when relative humidity is higher.
It does. I grew up in Minnesota but now live in Houston. I'd much rather be out in a nice, calm sunny day with low relative humidity and a high of -20C (-4F) than a blustery, humid day hovering just above freezing. That blustery, humid day with the temperature hovering just above freezing is cold.

One reason that that nasty weather feels so nasty is the wind. Low relative humidity most likely means clear skies and light winds. High relative humidity most like means stormy weather: lots of wind. Wind chill is a very real effect. Another reason (at least during the day) is sunshine. Low relative humidity means clear skies, so lots of sunshine to partially offset the cold. Even if it isn't storming outside, it probably is overcast on days with high relative humidity.

Humidity does play a role, but it's not the heat conductivity of air. It's how the moisture affects your clothing. The number one rule to staying warm in the cold is to dress properly. Closely behind that is to stay dry. Let your clothing get even slightly damp and you'll lose heat quickly. The insulating quality of clothing diminishes rapidly if ones clothing gets even slightly damp. It's fairly easy to stay dry when the relative humidity is low. It's very hard to do so when the humidity is high, particularly if it's raining.
Integral
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Jan18-13, 10:44 AM
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Quote Quote by D H View Post
It does. I grew up in Minnesota but now live in Houston. I'd much rather be out in a nice, calm sunny day with low relative humidity and a high of -20C (-4F) than a blustery, humid day hovering just above freezing. That blustery, humid day with the temperature hovering just above freezing is cold.

One reason that that nasty weather feels so nasty is the wind. Low relative humidity most likely means clear skies and light winds. High relative humidity most like means stormy weather: lots of wind. Wind chill is a very real effect. Another reason (at least during the day) is sunshine. Low relative humidity means clear skies, so lots of sunshine to partially offset the cold. Even if it isn't storming outside, it probably is overcast on days with high relative humidity.

Humidity does play a role, but it's not the heat conductivity of air. It's how the moisture affects your clothing. The number one rule to staying warm in the cold is to dress properly. Closely behind that is to stay dry. Let your clothing get even slightly damp and you'll lose heat quickly. The insulating quality of clothing diminishes rapidly if ones clothing gets even slightly damp. It's fairly easy to stay dry when the relative humidity is low. It's very hard to do so when the humidity is high, particularly if it's raining.
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voko
#7
Jan18-13, 10:45 AM
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So the bottom line is not the humidity per se, but what, shall I say, comes bundled with it?
sophiecentaur
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Jan18-13, 11:30 AM
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russ_watters
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Jan18-13, 12:26 PM
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You can have high humidity without wind though. In fact, there is a positive corellation between dry air and wind, so that would even tend to make the introduction of wind to the issue wrong.

I agree with Chestermiller.
D H
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Jan18-13, 04:07 PM
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Quote Quote by Chestermiller View Post
I would think it would be the other way around. Why should cold be different from hot.
Because you aren't sweating much when it's cold.

Relative humidity has little effect on how cold you feel if run outside in the cold buck naked1 or scantily dressed2. Dry or humid, you are going to feel cold pretty quickly if you aren't dressed properly. Sans the human guinea pigs in the cited experiments, Finlanders doing their ritual naked post-sauna roll in the snow, and silly scientists vying for 300 Club membership, people generally don't go running around outside buck nekkid in the winter. Sane people wear outdoor clothes when they go outside in the winter. We perceive damp cold as being cold because of how that wetness decreases the insulative quality of that clothing3.

1A.C. Burton, R.A. Synder, and W.G. Leach. Damp cold vs. dry cold; specific effects of humidity on heat exchange of unclothed man. J Appl Physiol. 1955 Nov;8(3):269-78.

2P. F. Iampietro1 and E. R. Buskirk. Effect of high and low humidity on heat exchanges of lightly clothed men. J Appl Physiol. 1960 Mar; 15(2):212-14.

3R.M. Crow, Why cold-wet makes one feel chilled: A literature review. Defence Research Establishment Ottawa Technical Note 88-22
Integral
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Jan18-13, 04:18 PM
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Quote Quote by russ_watters View Post
You can have high humidity without wind though. In fact, there is a positive corellation between dry air and wind, so that would even tend to make the introduction of wind to the issue wrong.

I agree with Chestermiller.
About a month ago I was out walking my dogs, it was about 33F and windy as all get out. I was dressed in my normal layers, including tee shirt, sweat shirt, and wind breaker. I about froze to death, the wind was getting underneath my wind breaker, making it worthless. Yes it was a moist cold wind. Not sure about your dry air and wind correlation, perhaps in hot dry areas? Here in the PNW we get plenty of cold moist wind.

I have walked my dogs in 20F no wind conditions with my normal 3 layers and been comfortable, the wind was the major factor in my discomfort.

I have now added a fleece lined Levi jacket to the layers, that keeps the wind out!


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