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fudgeit2k
I have a feeling that if I have two separate boxes, one with low humidity and one with high humidity, and I stick in the same heat source - the one with higher humidity will "feel" warmer, faster.

Is it because with more water in the air, the heat source would raise the temperature of the water molecules that can retain heat better than dry air? Thus, the temperature will feel warmer?
Rusty Shackleford
The human body uses evaporative cooling (sweating) to maintain body temperature in hot environments. A "humid heat" feels warmer because the body's cooling system does not function as efficiently as in a low humidity environment.
The_Questioner
Heat transfers best in a solid then in liquid then in a gas. The air with high humidity has more water molecules in it. So heat transfers faster.
mr_homm
@ The_Questioner:

That doesn't really work as an explanation, because once the water molecules are in the air, they are no longer a liquid. They are a vapor, so they aren't any better at transferring heat than any other vapor. Also, the capability to transfer heat faster cannot make any difference in the scenario fudgeit2k was talking about, because the same heat source was used in both boxes, so the rate of heat transfer was forced to be the same.

@fudgeit2k:

I will second Rusty Shackleford's answer. The feeling of higher temperature is subjective. You can test this by inserting a dry thermomenter into both boxes. It should read the same temperature. But if you insert a thermometer with a small bit of damp paper attached to the bulb, it will read a lower temperature in the dry box than in the humid box, because evaporation is cooling the bulb.

As a matter of fact, this is one of the standard methods of measuring humidity: you compare the "wet bulb temperature" with the "dry bulb temperature" to find the humidity. Try searching for "psychrometric chart" on the web. These charts relate temperature (wet and dry bulb) to humidity, dew point, enthalpy of the air, and a couple of other quantities as well.
The_Questioner
QUOTE (mr_homm+Mar 11 2007, 04:23 PM)
@ The_Questioner:

That doesn't really work as an explanation, because once the water molecules are in the air, they are no longer a liquid.  They are a vapor, so they aren't any better at transferring heat than any other vapor.  Also, the capability to transfer heat faster cannot make any difference in the scenario fudgeit2k was talking about, because the same heat source was used in both boxes, so the rate of heat transfer was forced to be the same.

You incorrectly assume vapour and gas as meaning the same thing when they are not.

http://en.wikipedia.org/wiki/Vapor


The matter of the equilibrium between the water (liquid phase) and its vapor
(gas phase), a liquid and vapor phase of the same substance in equilibrium are actually in dynamic equilibrium, in which molecules in the gas phase are combining and condensing to form liquid, and molecules in the liquid phase are evaporating to form gas. The rates of these processes are the same, so the total proportions in the two phases remain constant.

When the water vapour condenses(say on your skin) it gives off kinetic energy.

So my answers is correct. It because there is small liquid droplets in the air. You know 100% humidity is feels very wet although it doesn't have to rain.

MikeMonty
QUOTE (The_Questioner+Mar 11 2007, 04:50 PM)
The matter of the equilibrium between the water (liquid phase) and its vapor
(gas phase), a liquid and vapor phase of the same substance in equilibrium are actually in dynamic equilibrium, in which molecules in the gas phase are combining and condensing to form liquid, and molecules in the liquid phase are evaporating to form gas. The rates of these processes are the same, so the total proportions in the two phases remain constant.

When the water vapour condenses(say on your skin) it gives off kinetic energy.

So my answers is correct. It because there is small liquid droplets in the air. You know 100% humidity is feels very wet although it doesn't have to rain.

I always find the term vapour a bit "nebulous" (Har Har!)

Water is liquid or gas, "vapour" I consider is a water mist, the visible stuff from the kettle spout where the steam (invisible) condenses.

These are the droplets in the air, which might be warm, but are not about to give up latent heat to anything because they don't have any - they gave it up to the air already.

What gives up most of the heat is the "dry steam"

The balance between the gas and liquid phases is termed the dryness fraction and is indeed an equilibrium in a steady state system. There is "dry steam" in the air most of the time in temperate climates. without it you have a humidity of 0%.

Consider this,
You put a cold hand into warm humid air, if it's at 100% humidity you might see vapour (water mist) in the air. If it's less that 100% you won't.
Some of the gaseous water will condense on your skin (that is the dampness in the air) releasing latent heat until the temperature equilibrium is reestablished.
The humidity in the air locally will have dropped a bit as a result of the water condensing.
As a result the "vapour pressure" (misleading term!) will drop, and water molecules will be induced to come in from outside trying to equalise the partial pressure of water in the air.

Back to the original question:
Sorry Fudgeit2K, its kinda badly worded. wink.gif
If I read you right, two boxes, differing humidities, same amount of heat energy applied to each. lower humidity will get warmer because there is not as big a mass of water in the box to absorb heat.
Faster? I'm not sure about the physics of that, you might think that it would get warm faster because there is less mass to heat.
But there's the same volume to heat and if the heat source is convective and the efficiency with which heat is transferred is the same at both humidities then it might take much the same time.
However, I suspect that higher humidity will improve heat transfer, given the higher heat capacity of water than air.
The_Questioner
QUOTE (MikeMonty+Mar 11 2007, 08:31 PM)

These are the droplets in the air, which might be warm, but are not about to give up latent heat to anything because they don't have any - they gave it up to the air already.


They release kinetic energy in the form of heat when they condense. This is elementart stuff. rolleyes.gif
The_Questioner
QUOTE (MikeMonty+Mar 11 2007, 08:31 PM)

Water is liquid or gas, "vapour" I consider is a water mist, the visible stuff from the kettle spout where the steam (invisible) condenses.


It is both. I can also be solid in the form of ice. rolleyes.gif You can see steam it is not invisible. Don't know where you are going with this on ?
MikeMonty
QUOTE (The_Questioner+Mar 11 2007, 08:45 PM)
It is both. I can also be solid in the form of ice. rolleyes.gif You can see steam it is not invisible. Don't know where you are going with this on ?

I was pointing out that its not little globs of water that transfer the heat - the little globs are already close to or at ambient temperature. - its the separate energetic molecules of water that have the energy.

Steam is invisible, take a look at that kettle spout:
for the first few millimetres you cannot see the flow of steam.

After that stage it has expanded slightly, cooled slightly and a proportion has condensed to water vapour, the misty stuff that you CAN see.

Stick you fingers in the vapour (mist) warm, even hot, but not unbearable. Now stick a finger in the invisible portion and just FEEL that latent heat being transferred!

Hint - don't really do this!

When I was but a whippersnapper - apprentice marine engineer - I was told.
If you hear a whistling sound in the engine room be very careful because you might be about to walk through a jet of steam from a burst that will cut you in two and cauterise the edges and you won't even see it!
MikeMonty
QUOTE (The_Questioner+Mar 11 2007, 08:38 PM)
They release kinetic energy in the form of heat when they condense. This is elementart stuff. rolleyes.gif

See above - If you can see it, its a droplet, and if its a droplet - it's already condensed and given up its heat to the air.

This is indeed elementary stuff.
The_Questioner
If it condenses on your hand then its giving its heat to you. wink.gif

You have said you can't see the humidity but you can you can see it in the air.

Have you ever walked around in a Amazon forest in south america? you can see it.
Rusty Shackleford
Body temperature is usually higher than air temperature. How does the water transfer heat to a body that is warmer than the water itself? Your theory doesn't make sense. It is also incorrect, and there is really no reason to theorize as this has long been explained in the manner I have previously described. A quick internet search on "humidity and body temperature" will bring up dozens of links that say the same thing.

QUOTE
Higher relative humidity also makes it feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by preventing the evaporation of perspiration from the skin.


QUOTE (->
QUOTE
Higher relative humidity also makes it feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by preventing the evaporation of perspiration from the skin.


The human body sheds heat by a combination of evaporation of perspiration, conduction to the surrounding air, and thermal radiation. Under conditions of high humidity, the evaporation of sweat from the skin is decreased and the body's efforts to maintain an acceptable body temperature may be significantly impaired. Also, if the atmosphere is as warm as or warmer than the skin during times of high humidity, blood brought to the body surface cannot shed heat by conduction to the air, and a condition called hyperpyrexia results
The_Questioner
QUOTE (MikeMonty+Mar 11 2007, 09:02 PM)
Stick you fingers in the vapour (mist) warm, even hot, but not unbearable. Now stick a finger in the invisible portion and just FEEL that latent heat being transferred!

Hint - don't really do this!

I would not do this. You will burn yourself.
The_Questioner
QUOTE (Rusty Shackleford+Mar 11 2007, 09:20 PM)
Body temperature is usually higher than air temperature. How does the water transfer heat to a body that is warmer than the water itself?

Skin temperature is cooler than blood temperature. fudgeit2k was talking about heated by some souce or heater. So he was talking warm temperatures. Have you been to the Amazon forrest of South America. The humidity condenses on your skin, it pours from you. It very warm there of course. It feels a lot hotter because of it. The temperature in a rain forest is around 36 C with average humidity is between 77 and 88%. You can see it.
MikeMonty
QUOTE (The_Questioner+Mar 11 2007, 09:18 PM)
If it condenses on your hand then its giving its heat to you. wink.gif

You have said you can't see the humidity but you can you can see it in the air.

Have you ever walked around in a Amazon forest in south america? you can see it.

My point was that the water droplets (mist / vapour) are not giving up much heat, it's the water coming out of the gas phase that gives up the heat.

Never been in the Amazon, been to Rio though!

What you are seeing in the air is mist, condensed water, not steam.

The humidity is so high that the air cannot hold any more water at that temperature and some has to come out in the form of airborne condensation - mist.
The_Questioner
QUOTE (MikeMonty+Mar 11 2007, 09:29 PM)
My point was that the water droplets (mist / vapour) are not giving up much heat, it's the water coming out of the gas phase that gives up the heat.

Try that with your kettle. You soon find they are transfering the heat to you skin. Better not eh. unsure.gif
MikeMonty
QUOTE (Rusty Shackleford+Mar 11 2007, 09:20 PM)
Body temperature is usually higher than air temperature. How does the water transfer heat to a body that is warmer than the water itself? Your theory doesn't make sense. It is also incorrect, and there is really no reason to theorize as this has long been explained in the manner I have previously described. A quick internet search on "humidity and body temperature" will bring up dozens of links that say the same thing.




http://en.wikipedia.org/wiki/Humidity

Sorry Rusty,
I'm not sure if this meant for me?

If so, my statement is that it is not the water phase that transfers its heat but the gas phase, and my example was a cold hand in a warm atmosphere.

I think we are going awry because of the original question which was a bit "wooly"

We could be talking physiological or thermodynamics - I took the thermodynamics route.
Rusty Shackleford
QUOTE
Have you been to the Amazon forest of South America. The humidity condenses on your skin, it pours from you. It very warm there of course. It feels a lot hotter because of it. The temperature in a rain forest is around 36 C with average humidity is between 77 and 88%. You can see it.


Yes, I have been to many humid environments. I in fact come from a region with higher average temperatures and humidity than the Amazon. I have also been to regions with a "dry heat" or high temperature with low humidity. It is much more comfortable in a dry heat, even with higher air temps. because you body's evaporative cooling system can work more efficiently. In a humid heat, sweat does not evaporate off the skin as easily, so the sweat just pours down you. You get wet because your sweat isn't evaporating, not from the water vapor in the air condensing on you.
The_Questioner

The question concerns two boxes with different humidity and, which would feel hotter. Therefore we can rule out the sweat theory because if you put your hand in it will feel hotter then the low humidity box. That what he says anyway.
MikeMonty
QUOTE (The_Questioner+Mar 11 2007, 09:29 PM)
Skin temperature is cooler than blood temperature. fudgeit2k was talking about heated by some souce or heater. So he was talking warm temperatures. Have you been to the Amazon forrest of South America. The humidity condenses on your skin, it pours from you. It very warm there of course. It feels a lot hotter because of it. The temperature in a rain forest is around 36 C with average humidity is between 77 and 88%. You can see it.

It would be very difficult to say what was sweat and what was condensation.

However as a mammal, I expect you to be warmer than your surroundings and so unlikely to be a target for condensation.

What will happen, as has been said before is that at high ambient humidities there is little scope for evaporative cooling because the sweat cannot evaporate, the air is saturated with gaseous water and can't take any more.

So you feel hotter.
Rusty Shackleford
QUOTE
What about people that don't sweat much.


When your body overheats, it sweats, unless you have a medical disorder. People who don't sweat much are people who don't overheat much. Get them moving and cause the body to heat up, they will start sweating.

QUOTE (->
QUOTE
What about people that don't sweat much.


When your body overheats, it sweats, unless you have a medical disorder. People who don't sweat much are people who don't overheat much. Get them moving and cause the body to heat up, they will start sweating.

Not a bit of both then?


As MikeMonty said: However as a mammal, I expect you to be warmer than your surroundings and so unlikely to be a target for condensation.
MikeMonty
QUOTE (The_Questioner+Mar 11 2007, 09:33 PM)
Try that with your kettle. You soon find they are transfering the heat to you skin. Better not eh. unsure.gif

Not sure you got my point, not sure I'm getting yours. smile.gif
The_Questioner
Ultimately it it boils down to the person whether they are feeling hot or not. I've heard the temperature when you begin to feel uncomfortably hot is 91C air temperature.

http://www.wonderquest.com/skin-temperature.htm
fudgeit2k
Thank you for the answers/thoughts thus far. I worded my original post in a weak attempt to remove all other confounding factors ... but, here's the whole situation.

I have a thermometer in my room with a hygrometer built in. I also have a space heater, and a humidifier (cold air through wet wick). Though this could be a very biased observation, whenever I have my humidifier on to where the relative humidity is approaching 50%, I feel as if my space heater heats up the room much faster.

It *feels* warmer, probably due to the fact that my body isn't evaporating as much water due a more humid environment; however, the thermometer seems to record a rise in temperature faster compared to when the relative humidity was at around 30%. Whether this is true or not, I've no idea !

If I read what some of you said correctly, the rise in the actual temperature shouldn't be affected by the rise in humidity. ...

But what about the distribution of heat?

From what I remember, heat rises up. But what about humidity in the air? If the humidity in the air is high, does it mean the heat will be more distributed within a room?
The_Questioner
I would say the water vapour in the air contributes to the faster temperature increase. Not only will the high humidity box feel hotter, it will be hotter.

I know this has not a lot to do with sweat. I know also that I feel hotter when I sweat and not when I don't sweat. Two people can be in a room, one can feel hot the other can feel cold, this has little to do with feeling temperature either as we all know that can vary between person to person.

Another point to note is 'The hotter the air is, the more water it can contain'

Only if the air is at 100-percent relative humidity, sweat will not evaporate into the air because it is saturated.
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