# Moist air - does it rise or drop?

• B
• SentinelAeon
In summary, when things are uniform, the relative humidity of air is the same at all levels. However, if the temperature is not uniform, then the air is denser near the temperature of the warmest object.
SentinelAeon
A very simple question that i found some contradicting answers about. It's well known that cold air generaly will drop and hot air will rise. That is why if we open a window during winter time, it will feel almost like a wind is blowing through the window. That is great property that can be used in many fields.

I was wondering is there a similar dynamic between air with higher relative humidity and air with lower relative humidity ? For instance in a room with constant T. Will moist air be near ceiling or at the bottom ? Logic would tell me that because moist air contains more water and is therefor denser/heavier, it will be near the bottom. But i found some answers opposing this.

SentinelAeon said:
Logic would tell me that because moist air contains more water and is therefor denser/heavier, it will be near the bottom.
Nope. When everything else is the same, humid air is less dense than dry air. This is because each H2O molecule displaces a N2 molecule, which is heavier than H2O.

syfry, AlexB23, David Lewis and 4 others
DaveE said:
When everything else is the same, humid air is less dense than dry air.
That is counterintuitive, but true.

Molecular weight of water, H2O = (2*1)+16 = 18.0 g/mole.

Molecular weight of dry air; = N2 + O2 + Ar ;
= (14*2)*0.78 + (16*2)*0.21 + 40*0.01 = 28.96g/mole.

For any fixed gas volume, pressure and temperature, the number of molecules is fixed. Water molecules replace the average air molecules in moist air.
https://en.wikipedia.org/wiki/Molar_volume#Ideal_gases

Lord Jestocost
So if i measure humidity in my apartment, at the ceiling, humidity should generaly be higher than on the floor ?

Also, how fast is this moving of air with different relative humidity ? When there is big difference in temperature (like 20C or more) it can be felt like wind. 10% difference in relative humidity i am guessing is not that fast ? For instance if you have a small box with something inside generating heat and holes on top of the box and sides ... you will literaly have airflow because hot air will exit from top of the box and cold air will come from sides. I am guessing that relative humidity difference is more subtle, not enough to actualy feel the difference with hand ?

SentinelAeon said:
I am guessing that relative humidity difference is more subtle, not enough to actualy feel the difference with hand ?
Saturated air has a relative humidity of 100%. But the mass, or partial pressure, of water in saturated air is highly dependent on temperature.

There are complexities, hot air rises, and hot air can contain more water than cold air. As you heat a parcel of 100% saturated air, the RH falls below 100%.

Search for a psychrometric chart (search that term). It's a chart that shows the density of air as a function of temperature and humidity at a given barometric pressure.

dextercioby and berkeman
SentinelAeon said:
So if i measure humidity in my apartment, at the ceiling, humidity should generaly be higher than on the floor ?
If the temperature is uniform then yes.
One of my favorite teaching /learning tools for air density is a mylar helium-filled balloon weighted to be neutrally buoyant in dry air at room temperature. Too the degree that air (and helium) air are ideal gases at temperature this neutral buoyancy is temperature independent. These balloons will however show you the directly the presence of humidy gradients. Edification from the dolllar store.

256bits
SentinelAeon said:
So if i measure humidity in my apartment, at the ceiling, humidity should generaly be higher than on the floor ?
This is one of the first basic questions in studying the weather. The answer depends on the source of heat in the room (by design or accident) which will almost certainly cause the temperature to be non-uniform. It's easy to confuse cause and effect - starting with the simple case of a room and moving on to the situation of a column of air in the atmosphere going from the warm ground to outer space, via regions which absorb Infra red energy (going up and going down.
If you do cooking in the apartment then warm air may be a result of cooking, which produces water vapour.

You could do some interesting experiments to measure conditions in your rooms and answer your own question. You can get what's needed from Amazon and others for less than a tenner. Heating turned off for a while. But no rushing around the room. You will need to avoid stirring the air up. Also, you could find the effect of your warm body. Thick winter clothing could make a difference.

SentinelAeon said:
... I am guessing that relative humidity difference is more subtle, not enough to actually feel the difference with hand?
https://en.wikipedia.org/wiki/Diffusion#Separation_of_diffusion_from_convection_in_gases

https://www.engineeringtoolbox.com/air-diffusion-coefficient-gas-mixture-temperature-d_2010.html

https://www.engineeringtoolbox.com/density-air-d_680.html

A room can have still air which acts like different stratified layers (regarding density, temperature and absolute humidity).

If disturbed, that mass could be moving due to convection, which would reduce the difference of those parameters.

hutchphd and sophiecentaur
Tnx for the answers. Ok, i will give just an example to maybe make things easier to understand. This is just an example, so no bed advices please, this is not about a bed

So for this one example, i picked an average bed i found on google. Its perfect since its basicaly a small wooden box with some holes on the side or gap on the bottom and a heat & humidity source on top (person lying on matress).

Ok, so under the matress the air will be hotter than elsewhere since human body produces heat. And it will also be more moist since human body sweats (probably relative humidity will be higher even though air will be hotter meaning air will hold more water vapor on top than on the bottom). Now the level of this depends on many factors, type of bed, ventilation, person, etc. All that is not important. What i want to know through this case is how air behaves. We know that hot air will rise and cold air will drop. We also know that humid air will rise and less humid air will drop. So since we have both heat and humidity near the top of the box (in our case matress), does that mean that essentially hot and humid air will just float near the top of the box and it will have no "motivation" to actualy leave the box or exchange with the air outside the box ? How would that be different than for instance if we had heat and humidity on the bottom of the box (in this case this would mean matress is on the floor and the poor person is lying under it). Now we could actualy feel the air mass moving from bottom of the box to top of the box right ?

I am sorry if i complicated it to much but with heat alone it is much easier to understand since you can actualy feel it with hand, even just opening a fridge and putting hand under it, you can feel the cold air flowing through your fingers. With humidity it seems a lot more subtle of a movement and its really hard to detect.

#### Attachments

• Zoey_Storage_Bed_Queen_Classic_Walnut_LP.png
51.2 KB · Views: 81
• just an average bed.png
3.9 KB · Views: 80
SentinelAeon said:
Ok, so under the matress the air will be hotter than elsewhere since human body produces heat. And it will also be more moist since human body sweats
These are not good assumptions from which to start.

Mattresses are pretty thick. I doubt much human body heat would be conducted through a six inch mattress. And I'm certain no water vapour would be.

SentinelAeon said:
Ok, so under the matress the air will be hotter than elsewhere since human body produces heat. And it will also be more moist since human body sweats.
I don't think that is true.
More moisture will be lost by breath than by sweating while sleeping. The blanket and mattress of the bed tend to be insulators that reduce metabolism and body cooling to be by breathing, not sweating.

If the air in the bedroom is still, and cool at night, the heat and moisture from the breath of a person in bed, form a column of rising air that flows upwards from their head, to a small area of the ceiling of the bedroom, where it spreads out across the surface, cools, and then falls down the walls to circulate again.

An inner-spring mattress has an open inner-space that allows stagnation of internal air, that prevents heat and moisture moving downwards.
A foam mattress has reduced internal flow, and is thinner under a person, so heat and moisture can soak downwards in the much reduced airflow. That fits better with your statement I quoted above.

Cold and dry air stagnates under the bed, which is why beds in cold climates originally had long legs before houses became well insulated and air-conditioned. In hot climates, a mattress on the floor is cooler than a bed with legs. Never sleep on the floor in a place with cold still air, as that is where the coldest and driest air stagnates, with most accumulated CO2.

Bed details are so varied, that only a specific case can be discussed in a specific climate. There can be generalisations, but not with detailed analysis.

SentinelAeon said:
With humidity it seems a lot more subtle of a movement and its really hard to detect.
I wonder of there are materials that can cheaply and simply indicate humidity such that you cold set up a sensor matrix. Aren't there thigs like rice paper that respond to humidity?

If you have a bed and a person and a normal sized room with closed door and well curtained window then you should try an experiment. You may not be able to reach a full conclusion but you would be able to measure some temperatures and humidities to test your initial assumptions.

As with many practical questions involving human bodies and living spaces, there are too many variables to get a good model but you could get some interesting answers.

DaveC426913 said:
I wonder of there are materials that can cheaply and simply indicate humidity such that you cold set up a sensor matrix. Aren't there thigs like rice paper that respond to humidity?
I can’t imagine that the OP could do better than a budget electronic room thermometer (these days you get humidity thrown in.) you could mount it at different heights and read it with binoculars to avoid moving round the room.

sophiecentaur said:
I can’t imagine that the OP could do better than a budget electronic room thermometer (these days you get humidity thrown in.) you could mount it at different heights and read it with binoculars to avoid moving round the room.
But you could only take one measurement at a time. As soon as you went to move the sensor to a new location, you'd wipe out the effect and - at best - would have to wait quite some time for another reading. Even then, you'd never know from reading to reading if the scenario has duplicated itself.

The ideal experiment would be to take all the measurements at the same time.

• Nylon ("is a particularly hygroscopic material, which has the property of expanding in the presence of moisture. Nylon ribbons humidistats use this property by measuring the elongation of a strip of nylon as a function of relative humidity.")
• Cellulose
• Titanium Dioxide
• Polyaniline
• Graphene Oxide

Perfect. Now all the OP need do is jerry rig about a thousand of these devices and he can get the humidity reading of a 10x10x10 room with precision down to the foot. Easy peasey!

sophiecentaur
Old fashioned methods would be fun but require an old fashioned experimenter. A couple of instruments suspended on a fishing rod could be moved up and down with little disturbance of air.
You could sit in the room with your phone and spend a night quietly reading PF threads and doing 20 minute pairs of measurements from the suspended meters.

jrmichler
Horse hair is available in the form as replacement violin bow hair for less than a tenner. However, all the diagrams of hair hygrometers show how much trouble is involved in measuring the expansion of hair due to humidity. I found this Scientific American article which gives an idea of just how much trouble is involved. (But no pictures unfortunately). Nuff said, imo.

It would be easy enough to do pairs of spot measurements of temperature and humidity in a room. Just two cheap instruments would probably answer all the OP's questions and no one would ned to be in the room for hours - just dash in and take the readings.

Two birds with one stone: buy two cheap home weather stations and get hours of fun. PS my internal humidity measurements don't seem to change much when I'm cooking, except right next to the cooker and I've never tried it in the bedroom.

sophiecentaur said:
It would be easy enough to do pairs of spot measurements of temperature and humidity in a room. Just two cheap instruments would probably answer all the OP's questions and no one would ned to be in the room for hours - just dash in and take the readings.
But again, the observer observing changes the observations.

His movements through the room will roil and mix the air.

DaveC426913 said:
But again, the observer observing changes the observations.

His movements through the room will roil and mix the air.
Those meters take many seconds to change reading so there would be plenty of time. If there's a problem then use the binoculars from a partly opened door.

There's always a solution when you really want a result. In this case, the extra 'cost' is a long time to do the complete experiment.

PS "roil's" a new word for me. I like it.

DaveC426913 said:
But again, the observer observing changes the observations.
A robot operated fishing rod, that supports the sensors, could make the measurements with a minimum of disturbance while the generator of heat and moisture sleeps.

sophiecentaur said:
Those meters take many seconds to change reading so there would be plenty of time.
Sure. For the first reading. What about the second? How long does he have to wait for the room to re-stabilize? And does it restabilize the same way?

DaveC426913 said:
It all depends on the accuracy you are after. If it has to be done quickly, you would need many meters with simultaneous readings. But it could establish some basic trends with just two meters. I'd find it interesting to find if the air over a sleeper in bed were significantly warmer than the air under the bed and that's just one set of measurements (repeated over several nights).

Push the boat out and buy a thermal imaging camera and you could get a lot with just one image. But that's more than £1k, which is the price of a half decent bed.
EDIT: Prices appear to have reduced lately but I don't know if the eBay models are good enough.

Last edited:
sophiecentaur said:
Push the boat out
TIL a new (English) phrase.

Is this anything like smoking a cigarette and watching smoke rise and roil, until it stabilizes ( in altitude ) somewhere above.
Heat rises the smoke at the beginning and then it spreads out.

Perhaps the guy could have a smoke box set above his chest(at the chest temperature) , release some smoke and see what happens. An instant visual.

Some effects as in post 9 from @Lnewqban might come into play.

It's not about bed, remember. Matress can be a random heat and moisture source for this discussion. Though, just to add, i actualy did measure temperature and humidity under a matress some time ago when i had wierd sweating "attacks" during sleep. One time i actualy recorded 99% humidity under a bed, a visible moisture stain on wood and temperature was also higher than average in the room. Humidity in room at that time was as usual, about 50%. Obviously that high humidity is due to back design of that bed - very poor ventilation.

But like i said, the bed thing was just an example since i am trying to understand the physics of it here. So we can just say on top of the box we have an element that produces heat and humidity. And the question i was asking myself was, would that actualy stop the airflow since moist and hot air will stick near top of the box. For instance, lets say sides of the box had holes near the top and temperature under that top would be more than room temperature. Would hot air therefor want to leave the box and go higher (or colder air from outside the box would simply push it out)

SentinelAeon said:
moist and hot air will stick near top of the box.
Hot air will generally rise to the ceiling.
Moisture will generally diffuse throughout the room.

256bits said:
Is this anything like smoking a cigarette and watching smoke rise and roil, until it stabilizes ( in altitude ) somewhere above.
Heat rises the smoke at the beginning and then it spreads out.

Perhaps the guy could have a smoke box set above his chest(at the chest temperature) , release some smoke and see what happens. An instant visual.

Some effects as in post 9 from @Lnewqban might come into play.
That's a familiar phenomenon that you can see with bonfire smoke and, indeed it's similar with cumulus clouds over warm ground. I'm not too sure how it relates to the humidity / temperature distribution, though. It tells you when the extra thermal energy of the smoke runs out as it gains gpe. Would the smoke stop at the interface between dry and damp air?

I think the OP's question is actually a big one and needs to be 'trimmed' if we want an actual answer. Is the air near the ceiling always warmer and / or is it always damper? In a black metal vertical column (equal temperature all the way up), could we measure a humidity difference as we go up?

The walls would maintain a 'zero' temperature gradient so there would be a steady change in the mixture of molecules on the way up, depending on density.
DaveC426913 said:
Hot air will generally rise to the ceiling.
Moisture will generally diffuse throughout the room.
remember that most of our experiences involve a heat source somewhere and a heat sink. How much and where are very important factors.
Diffusion will be affected by molecular mass, too.

SentinelAeon said:
... So we can just say on top of the box we have an element that produces heat and humidity. And the question i was asking myself was, would that actualy stop the airflow since moist and hot air will stick near top of the box.
That element acts like a source that constantly adds thermal energy to the immediately surrounding air.
At first, it lingers close to the surface of that element, reducing the heat transfer (acting as a powerful insulation film or stagnant air layer).

That increases the temperature of the element and the heat transferred into the air layer, part of which will reach enough temperature and "lightness" to move up as a convective stream.

Part of that stream gets diffused into surrounding cooler air, and most reaches cooler horizontal surfaces, over which some cooling and microscopic condensation occurs.

Then, it starts moving horizontally to give room to new ascending mass of hotter air.
If the room is perfectly sealed to exfiltration and infiltration of exterior ambient air, our mass of air will eventually be forced down to occupy the place of the continuous mass of air heated and humidified by our element.

Not exactly the same cases, but please see the visualizations shown in the link below of air being heated by kitchen equipment and induced to move towards and inside kitchen hoods.

https://www.captiveaire.com/sitedocs/videos/hoods/schlieren/svideo.asp?EXH=1&PSP=1

Last edited:
One problem here is that a thermal plume forms above the sleeper's head, where they breathe out warm saturated air. That rises and then moves outwards, across the ceiling, then circulates down the walls as it cools.

There is a limit to how much water can be dissolved in the air within a room. Once that limit is reached, if respiration continues to heat air, without sufficient external ventilation exchange, condensation must occur somewhere in the room.

Hot air has a higher capacity to dissolve water, so the RH will actually increase as the air sinks, without the addition of any more moisture, to settle on the floor and under the bed, with 100% RH. The cool underside of the mattress provides the preferred surface for condensation.

That is probably a partial explanation for why the bottom of a mattress is wet. It suggests that better ventilation is required, or that air conditioning should dry the air sufficiently to prevent condensation from 100% RH cool air, near the floor.

sophiecentaur, DaveC426913 and Lnewqban
Baluncore said:
There is a limit to how much water can be dissolved in the air within a room.
Air is just a mixture of gas molecules and the proportions depend on the partial pressure of water vapour. It will depend on the temperature of any water surface. I think the term "dissolved" may not be appropriate here because it suggests something different about the water compared with the other air molecules. Air is not really a 'sponge'. It's just a matter of luck that water has a vapour pressure about the same as AP in habitable rooms. Just above the surface of boiling water, the proportion of H2O will be nearly 100%. In a freezer, the proportion will be very low.

This whole thread is really dealing with a dynamic situation - warm, wet surfaces and other warm and cold surfaces. Hard to predict what will happen as water is constantly being transported from place to place

sophiecentaur said:
I think the term "dissolved" may not be appropriate here because it suggests something different about the water compared with the other air molecules.
But there is something different about water compared to the mixture of nitrogen, oxygen and argon, we call air. Those gasses do not condense, to precipitate out, at the temperatures and concentrations found in a bedroom.

Water has a temperature dependent "solubility product" in air. Air saturated with water vapour has a RH = 100%.

If you fear the complexity of RH, the psychrometric chart, or the steam tables, you should opt out of the discussion, rather than try to confuse the situation with extreme examples that do not occur in a bedroom.

• Other Physics Topics
Replies
8
Views
2K
• Other Physics Topics
Replies
34
Views
3K
• Classical Physics
Replies
8
Views
2K
• General Engineering
Replies
5
Views
3K
• Thermodynamics
Replies
11
Views
4K
• Other Physics Topics
Replies
12
Views
32K
• Introductory Physics Homework Help
Replies
5
Views
2K
• Thermodynamics
Replies
8
Views
3K
• Mechanical Engineering
Replies
28
Views
2K
• Thermodynamics
Replies
32
Views
2K