CaptainPanic

Why does wet cold air feel so much colder than dry cold air?

37 posts in this topic

Here's one that has been bugging me for a while now:

 

In my tiny country (that nice little swamp at the end of the river Rhine) the winter months are crap. Usually above zero degrees (Celcius) and wet. The air humidity is generally at 90% or so.

 

Why does wet air feel so cold?

 

In fact, it feels less cold when it is -1 degrees, because the air is generally a lot dryer... is it me or is that tiny fraction of water really responsible for a major difference in heat transfer?

 

At 5 degrees the vapor pressure of water is about 10 mbar, so the volume (or molar) fraction of water in air is <1%. How can it have an effect that I can really feel?

 

The Cp value of water vapor is twice that of dry air... but that cannot be the difference. Is the heat transfer so much better perhaps? I can hardly imagine that.

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<warning: uninformed speculation follows>

 

I think it has something to do with the density of the air. Wet air is denser, and more molecules come in contact with your skin than less dense, non-humid air, hence the perception of temperature is more intense.

 

I'm thinking of this based on ice baths. You can cool a bottle of wine down by placing it in a pitcher of ice, however, it will cool significantly faster if you put water in that pitcher with the ice... making an ice bath... since so many more molecules of "cold" come in contact with the bottle when the water is present versus when it's just dry air.

 

 

</uninformed speculation>

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It is the heat capacity of the air.

 

wet air has a greater heat capacity than dry air so it can remove more heat from you faster than dry air with less convection.

 

Density has little to do with it but as it increases the number of molecules hitting you every second it also improves conductivity.

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<warning: uninformed speculation follows>...

Wet air is denser, and more molecules come in contact with your skin than less dense, non-humid air, hence the perception of temperature is more intense.

 

Actually, wet air is lighter than dry air. This is because a water molecule weighs less (18 g/mol), than either an N2 (28 g/mol) or O2 (32 g/mol). I found this surprising as well, as I thought of water as "heavy" compared to air, but it is water vapor that counts. The number of molecules in a gas pretty much depends on the conditions (heat, temperature, pressure) rather than on the composition of the gas, so it should be pretty much the same.

 

I'd agree with the heat capacity theory.

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I think it has something to do with the density of the air. Wet air is denser, and more molecules come in contact with your skin than less dense, non-humid air, hence the perception of temperature is more intense.

 

Moist air is less dense that dry air. A molecule of water has an atomic weight of 18. Nitrogen and oxygen molecules, with atomic weights of 28 and 32, make up the bulk of the atmosphere. PV=nRT (approximately), so two gases at the same pressure and temperature will have the same molecular density n/V but if they have different atomic weights, they will have different mass densities. Think of it this way: helium balloons rise.

 

Cold moist air feels colder than dry moist air (and hot moist air feels hotter than hot dry air) because moist air has a higher specific heat than does dry air. Diatomic nitrogen and oxygen molecules form the bulk of the atmosphere. Water is a polyatomic molecule that can rotate and translate in 3 dimensions. Moreover, the hydrogen atoms are not rigidly connected to the oxygen atom in a water molecule; the intermolecular bonds are a bit "springy". Diatomic oxygen and nitrogen molecules have a lot fewer degrees of freedom, and hence have a lower specific heat than does water vapor.

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Thanks for the corrections, guys. I think it was mostly a poor word choice, and until this thread, I'd not previously heard the term "specific heat" or "heat capacity." I think I was on the right track with the idea that a water bath cools something faster than just ice, but lacked the terminology to convey that idea successfully.

 

Cheers. :)

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DH and insane alien are absolutely right.

 

I think of it as energy transfer. Our skin has receptors that measure the temperature of the skin. The skin is warmed by our blood, and cooled by outside air. If the cooling is rapid, then skin temperature will drop, and the skin receptors will report on the lower skin temperature.

 

"Skin receptor calling brain. Local report. It is bloody cold down here."

 

Since water is able to carry more heat energy than air (air is a good thermal insulator) then wet air can carry more heat energy away from the skin than dry air. More energy lost means temperature drop and the skin reports it is cold.

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The vapor pressure of water at low temperatures means that the water concentration in air is <1%...

 

I agree that the specific heat capacity (Cp) and heat transfer are the factors that seem to be of importance here. The heat capacity of water vapor is about twice that of air (weighted average of N2 and O2). But that means that the overall difference of the Cp of dry air and wet air is about 1%, given its low vapor pressure...

 

To increase the mystery, I'll give some numbers:

The heat transfer coefficient of water vapor is 16 W/(m K)

The heat transfer coefficient of air is 24 W/(m K)

 

I looked up some equations for heat transfer... and I conclude that one factor that I didn't mention myself, and that I haven't seen here yet is the diffusion coefficient of gases. But, that doesn't seem to be the one either: the diffusion coefficient of water in air at 25 deg C is 25.6E-6 m2/s, while that of oxygen is 20.6E-6 m2/s. The difference is not very big...

 

Perhaps it's just psychology, not physics that makes that air feel colder?

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It has to do with evaporation. The cold air has a partial pressure. When the moisture comes in contact with your skin it heats up and able to evaporate due to your warm body able to increase the vapor pressure. The evaporation will absorb heat taking more heat away from the body.

 

Another way to look at it, is the cold air can hold x water. Near the micro environment of you body it is warmer and can hold X+. There is a migration of water to you to balance out the chemical potential. There is also a heat conduction due to water being such a good heat absrober

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If evaporation would have anything to do with it, I'd imagine that dry air feels colder: with less water in that air, your sweat will evaporate faster, therefore cooling you better. That's also why it feels so hot in a sauna when you throw some water on the hot stones (thereby increasing the moisture in the air: you reduce your sweating).

 

In wet air, the water already has evaporated. We're talking about water-gas, not droplets (fog and rain are crap, and you will feel miserable and cold, there's little science needed to explain why cold liquid water feels colder than air).

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If evaporation would have anything to do with it, I'd imagine that dry air feels colder: with less water in that air, your sweat will evaporate faster, therefore cooling you better. That's also why it feels so hot in a sauna when you throw some water on the hot stones (thereby increasing the moisture in the air: you reduce your sweating).

 

yep, thats why hot muggy days feel like hot muggy days.

 

In wet air, the water already has evaporated. We're talking about water-gas, not droplets (fog and rain are crap, and you will feel miserable and cold, there's little science needed to explain why cold liquid water feels colder than air).

 

says it all.

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Captain, it may well be that psychology is more important than physics here! I cannot speak from experience because here in Durban, South Africa, we simply do not have the cold weather you speak of. But if I had to look for a physical reason I would look more to the human body than the air for the cause.

 

In summer it is the inability of the body's sweat to evaporate and cool the person that makes muggy hot weather feel worse than dry hot weather. Perhaps in winter if the air is dry it will cause a thin layer of dry skin to form on the body, making a thin insulating layer? In wet weather the skin will stay moist and be a better conductor of the body's heat out to the air, making it feel colder. This would make it the effect of the air's humidity on the body that impacts on how cold we feel, rather than the heat transfer properties of the air itself. But this is just conjecture.

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it is not psychology as a warm object in cold wet air will cool faster than a warm object in bone dry air (I have done experiments which show this even though we were looking at a different parameter).

 

humans would percieve this as 'feeling colder'.

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insane_alien,

 

At what conditions did you do those measurements (temperature, and air humidity differences)? And how must faster was the object cooling down?

 

 

Somehow I think that katmar's reply makes sense too... the cold air will prevent sweat from evaporating properly. This can make the tiny hairs on your skin sticky, thus preventing them from standing up. That could reduce the thickness of the thin stagnant layer of air that insulates your body.

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between 95% relative humidity and 20%. the temperature was 120*C (relatively it was very cold). pressure was 5 Bar.

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At 95%RH, 120 deg C and 5 bar the water constitutes about 38% of the gas. This is surely enough for the specific heat difference to become significant. I don't think this is comparable with CaptainPanic's conditions.

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Water vapour does have a higher heat capacity than air; but that's not the whole story. It also has a higher conductivity.

Imersed in wet air (a better conductor) your surface attains a temperature nearer that of its surroundings than it would surrounded by dry air (a poorer conductor). If, as in the OP those surroundigs are colder than you, you will feel colder in wet air.

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At 95%RH, 120 deg C and 5 bar the water constitutes about 38% of the gas. This is surely enough for the specific heat difference to become significant. I don't think this is comparable with CaptainPanic's conditions.

 

it is comparable. the effect is proportionally greater.

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can i know if this has anything to with with heat transfering?

i.e. moist air transfers heat better than dry air??

sounds a contradiction, but from what you are saying, it has to be so.

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Why would it be a contradiction?

It's true and it doesn't sound like a contradiction to me.

 

What still puzzles me however is how 1% water in air can have such an influence.

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someone might be able to help me out with lab experiments u've already done.

 

it is not psychology as a warm object in cold wet air will cool faster than a warm object in bone dry air (I have done experiments which show this even though we were looking at a different parameter).

 

humans would percieve this as 'feeling colder'.

 

take for example a furnace.

you have moist air (superheated), then the heat transfer to the object you are trying to heat is better than having dry air.

if the fuel is firewood for example, you will have moist air in the furnace.

 

two scenarios-

1.moist firewod.

this requires energy to superheat the moisture, BUT heat transfer to the object is better.

2.dry firewod.

this saves the energy that is used to vapourize the water in firewood, BUT reduces the heat that is being transfered to the object.

 

How do i know which case saves more energy or is better?

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I'm afraid we're going a bit off topic now...

 

I think that combustion is not the right process to study heat transfer differences of gases, because all kinds of other things are also happening, and they all interfere with what we're interested in (at least, in this topic we discuss only heat transfer of wet air compared to dry air... not energy conservation from wood combustion).

 

The water that will evaporate from the wet firewood will perhaps block out oxygen, which makes it harder to introduce enough oxygen. But more importantly, the combustion characteristics are totally different! The temperature of the fire will be different, the rate at which it burns is different, the char production and the exhaust gas composition and aerosol composition will be different. So, not only the concentration of water changes, but everything else as well! This makes it a little hard to look at just the heat transfer from the gas to the solid object.

 

But we can say that dry wood will produce a hotter flame... and probably will give a more complete combustion.

 

Again, I'm afraid it's irrelevant to this discussion... but feel free to open a new topic, and ask more questions about wood combustion. :D

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oh! ok.

Practically, it is observed that performance is not as much as we would expect it to be when dry wood is used as opposed to wet wood.

So what are the parameters we need to be looking at?

Started a new thread "Firewood Furnaces"

Thx. :)

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Environment Canada has studied this alleged effect. They have not been able to find an appreciable link between cooling rates (for humans exposed to the air) of relatively humid cold air and relatively dry cold air. There have been many supposed factors put forward in this forum that Environment Canada has ruled out as contributing to the phenomenon. First: evaporation is not a factor. Water vapour is already a gas; it doesn't need more energy to evaporate. Second: condensation is not a factor. The human skin surface is warmer than winter air, therefor water vapour will not condense on the skin. As for heat capacity, thermal conductivity etc., these are atmospheric variables that do change in different conditions. However, these variable have not been proven to account for the significant "wet cold" phenomenon many people claim to exist. So what causes it? So far, nothing has been proven, but there are a few good guesses that have been put forward.

 

 

1) In many areas during the winter, high humidity conditions often co-exist with some kind of precipitation. Any kind of liquid water (mist, fog whatever) coming in contact with the skin, in conjunction with cold temps is obviously going to feel cold. This is a no-brainer, but it is important to stress the different effects of water vapour and liquid water.

 

2) Probably most important: in the winter, high humidity almost always occurs during overcast conditions. Under heavy cloud cover, the earth's surface receives only diffuse sunlight. In these conditions, a person is not likely feel the radiant heat of the sun. Even in the winter and at high latitudes, direct sunlight can warm surfaces on which it shines. Even on the Canadian prairie, during a January cold snap, where temps can drop below -30 Celcius, finding a sunny spot out of the wind makes a big difference if you're trying to stay warm.

 

Also, keep in mind that there is no scientific measurement for the "wet cold" phenomenon. There is a measurement for low temps and high wind: the wind-chill effect. This is a predictable, observable phenomenon that both people and instruments can detect. There is also a measurement for high temps and high humidity: the humidex. Again, this is predictable and observable.

 

Canadians are weather fanatics, and to match their fervor, they have a government service (Environment Canada) that provides them with almost every possible weather fact observable. Canadians have been clamoring for a "wet cold" measurement, but Environment Canada can't find anything to give.

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Thanks Shoneswa for resurrecting this thread, i also live in an area where we get very moist but cold air in the winter. I have to admit that cold moist air does feel colder than cold dry air but I think it has to do with water droplets in the air. I used to love to fish on the local fishing piers in the winter at night, very few things feel colder than being out over cold water with cold moist air blowing over your body. It seems to creep into clothing in a way that cold dry air cannot, impossibly it almost feels like the cold is radiating into you like heat does when you stand in front of a heater. But one thing that cold wet air seems to have that dry air doesn't is mist, when you have very high humidity and very cold air the water will condense out of the air column onto your skin and clothing, something about air being very close to 100% humidity and moving over a surface that makes it condense out of the air column. All surfaces get moist and the cold feels worse than dry air. I'm not sure it is worse but it certainly feels that way. Even if you can't see a mist there often is one when water comes close to it's freezing point and close to 100% humidity.

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