How Does Hot Air Rise and Affect Weather Systems?

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Hot air rises due to its lower density compared to cooler air, which is a result of the average speed of air molecules being higher in warmer conditions. This phenomenon creates a displacement effect where cooler, denser air moves in to fill the gap left by rising warm air, contributing to weather dynamics. The discussion emphasizes that air is not a continuous fluid but consists of individual molecules that collide and interact, complicating the mixing process. The concept of buoyancy is highlighted, where the weight of warm air is less than that of an equal volume of cold air, leading to upward movement. Understanding these molecular interactions is crucial for grasping how hot air affects weather systems.
  • #51
Dadface said:
Yes, the hot air spreads in all directions but any obstacles will tend to give the spread a net directionallity.

That doesn't explain why hot air tends to go up, then.
 
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  • #52
Dadface said:
In a given time "hot" molecules will undergo more collisions than "cold" molecules...
Collisions with what? Obviously, the number of times hot molecules collide with cold molecules must be exactly equal to the number of times cold molecules collide with hot molecules!
 
  • #53
I think I finally understand what's happening, thanks to a few pushes in the right direction from people who managed to understand the question I was really asking instead of telling me off for trying to think for myself.

I even think pretty much everyone is going to agree with this post, how about that :wink:

First of all, if the density is the same, hot air tends to expand since the hot molecules with higher kinetic energy can easily push the surrounding cold ones away (while speeding them up and slowing down themselves, aka adiabatic cooling). This is pretty easy to see.

So now let's look at a different situation and let's say a cube of hot air is surrounded by cold air at the same pressure (lower density inside the cube). Suppose the hot air has double the temperature (very hot air indeed) and half the density.

Without gravity, the edges of the cube only tend to change very slowly. An undecided molecule on the boundary will get twice as many pushes from the cold side, but the pushes from the hot side will be twice as energetic. There is no net tendency for the molecules to migrate in a particular direction except for random brownian motion which is a rather slow process, even for molecules traveling faster than the speed of sound. Also, the sharp temperature gradient will smooth out because of the random nature of the collisions but this, too, is a slow process and the important thing is that it does not impose a net tendency on the entire group of molecules. So the "boundary" stays more or less in place.

Now gravity is added to the mix. Apart from the pressures on the sides of the cubes (momentum per unit square of cold molecules flying towards the hot ones and vice versa), a little bit of extra pressure differential is needed to compensate for the weight of the molecules.

If you just look at a cold column of air, a pressure differential will automatically develop so that each individual molecule gets slightly more pushes from below than from above, compensating for its weight. Otherwise all molecules would just tend to fall down and the pressure gradient would automatically develop.

Of course that doesn't mean each molecule stays exactly where it is, only that there's no net tendency for all the molecules to come crashing down.

Away from the cube, the pressure at an altitude just below that of the cube is just enough to support cold air molecules above. In other words, the molecules together are on average giving just enough extra collisions to compensate for the weight of the individual molecules above.

The pressure underneath the cube must obviously be the same because otherwise a horizontal wind would arise (higher energy molecules getting less collisions from the low pressure side and therefore moving that way).

But the hot molecules need less kinetic energy to compensate for their weight, since there aren't as many of them. The kinetic energy due to temperature is already compensated for (the hot molecules are more energetic but there are less of them, so the same pressure is required to keep them in place) but the extra pressure gradient due to gravity exceeds what is needed to counteract their weight. Each hot molecule, on average, is getting slightly more than its fair portion of anti-gravity-pushes, so they all tend to be pushed up.

Now this push is one that has a definite average value (unlike the average value of the pressures on the side which cancel out to zero), so the hot molecules will transmit their extra energy to the molecules above them as well (and meanwhile keep getting more energy from below). This "information" (extra momentum in a definite direction, up) travels at the speed of sound so that very quickly the top of the cube starts moving as well.

At the top of the cube a similar situation occurs: pressure above is lower in exactly the right proportion to roughly keep cold air in place (in fact the gradient works by giving more push from below and less push from above), so the top wants to go up just like the bottom.

So indeed, the whole pocket goes up as a whole! Apart from the diffusion at the edges that always occurs, it does more or less seem to be a collection of molecules going up, unlike the lenticular cloud and the pocket of young people outside Florida. I had posted that latter possibility as such, a possibility, not a theory, and it turned out to be mostly not the case. The actual molecules are going up, not just the distribution of molecules that happen to be hot at anyone time.

I know what you are all going to say now: "see, we were right all along, the cube acts like a single object, the macroscopic laws are correct, I'm glad you saw the light" but the point is that the phenomenon is now explained on a molecular level, which many of you said was impossible (even "nonsensical") to do.
 
  • #54
michelcolman said:
The average speed of the individual molecules inside a gas is actually around 1.5 times the speed of sound, I looked it up. Of course they don't move very far at that speed, but that's just what I meant when I said that.
It really almost sounds like you are being purposely argumentative here. What we are discussing is the average velocity of the molecules, which is the bulk fluid velocity. The average speed affects only the diffusion rate.
Just like they impart momentum sideways on the side boundary, yet there you say it is happening incredibly slowly. And the bottom boundary should be going down too, then. In other words, you just described why hot air expands, not why it goes up. But at least we're finally talking about molecules.
The reasons why it expands and goes up are the same. They are both a function/product of buoyancy.
People keep saying that the speed of the boundary is so slow that it can be treated like the surface of a ballon, much less than the speed of the pocket as a whole. That's what I meant with "faster than the boundary". I meant the speed of the boundary relative to the pocket.
No. Obviously the boundary isn't moving faster than the bulk velocity. You're misunderstanding what people are saying. In terms of deviation from that average, the boundary isn't moving, it is just getting less defined. It is spreading out (and in).
Suppose you make a model about the location of old people and young people over several centuries. For example, there are large "pockets" of old people in Florida. Would you then say that young people by definition stay inside the boundary around the young pockets? Since the boundary of the pocket is a clear word with a clear meaning? Or might you consider the fact that young people get old while new young people are born, so there's a constant migraton to florida even though the pockets stay at the same place?
That's gibberish. It has nothing whatsoever to do with physics.
My point exactly. People would argue that the cloud is obviously stationary...
No, people wouldn't - or perhaps more correctly, no, you wouldn't. Not even you would, except that you are trying to argue against reality.
 
  • #55
Cyrus said:
Q: Is the term buoyancy physically meaningful at the molecular level.

Answer this question and you can finish this thread off once and for all (The answer is no).

Fluid dynamics discussed here is for a continuum. Pop open any fluids book and it will tell you this in the first chapter.
Cyrus, I'm an engineer too, but I've been in enough arguments with physicists here to know that they often prefer statistical molecular models to bulk fluid transfer models for such things.
 
  • #56
Borek said:
That doesn't explain why hot air tends to go up, then.
It does if the heat source is close to the earth.It is easy to demonstrate,for example,that a flame close to the Earth's surface gives rise to a convection current where there is a bulk upward movement.If the same flame was placed at a higher altitude then the upward movement would be reduced.
russ_watters said:
Collisions with what? Obviously, the number of times hot molecules collide with cold molecules must be exactly equal to the number of times cold molecules collide with hot molecules!
Yes collisions with other molecules.Consider a hot and a cold molecule within a gas containing numerous other molecules.The mean free path of each molecule would be the same at any instant but the hotter molecule will be moving faster and in a given time will undergo a greater number of collisions.By random walk the displacement of the hotter molecule will be greater.
 
  • #57
russ_watters said:
It really almost sounds like you are being purposely argumentative here. What we are discussing is the average velocity of the molecules, which is the bulk fluid velocity.
I'm not sure whether I used "speed" or "velocity" in my earlier posts, but never mind.
No, people wouldn't - or perhaps more correctly, no, you wouldn't. Not even you would, except that you are trying to argue against reality.
I really do have to explain everything, don't I?

While I was considering possibilities where the microscopic behavior of the molecules was less intuitive than the common macroscopic description, people were telling me off for not just "seeing" that the air goes up. After all, you can see condensation in air going up etc.

So I replied that you can also "see" a stationary lenticular cloud (condensation, too), so the fact that you "see" something does not mean it is so.

Obviously I don't expect you to think the molecules in a lenticular cloud is stationary, i was COMPARING the discussion about pockets of warm air with a hypothetical discussion about lenticular clouds where, using the exact same arguments ("you can see it") you would be wrong.

This was merely to show that the argument itself was wrong, which only meant I was looking for a more convincing argument.

Now let it rest, it's not important.
 
  • #58
russ_watters said:
That's gibberish. It has nothing whatsoever to do with physics.
No, only with science in general.

You said the boundary around hot molecules by definition means that all the molecules stay inside. I gave an example of a boundary in a different context (the boundary around young or old people) that ends up containing different people after a while.

I was just pondering the possibility that the "pocket" was just the collection of molecules that happened to be hot at any particular time and, as the "pocket" went up, this merely meant molecules at the bottom had cooled down while molecules at the top had warmed up.

This turned out not te be the case, but it could have been a possibility and you shot it down by saying the molecules can't get out by definition.

So I gave the example of pockets of young and old people. It was not gibberish. But I don't know why I'm wasting time explaining the concept of metaphors or any other linguistic feature instead of doing something useful.
 
  • #59
russ_watters said:
Cyrus, I'm an engineer too, but I've been in enough arguments with physicists here to know that they often prefer statistical molecular models to bulk fluid transfer models for such things.
EDIT: Oops, I misread that one, he was actually making my point rather than the opposite, so disregard what I wrote below.

That doesn't mean they don't want to understand the microscopic behavior at all. It's just a matter of convenience.

Didn't you derive the ideal gas law from molecular motions in one of your physics courses? I know I did, and I found it quite interesting.

Also, macroscopic models often have to introduce strange and underexplained concepts (boundary layer, etc...) to explain certain phenomena which later turn out to actually be easier to explain at a microscopic level.

There was even a post on one of my earlier threads, about how a wing works, which explained pretty much the whole thing (boundary layer, Coanda effect, etc...) by considering the air to be billiard balls. All of the observed phenomena (yes, before you start typing away again, the boundary layer does exist) were explained through the random collisions of molecules. (lots of molecules, lots of collisions).

Computers are not powerful enough for this kind of simulations (and won't be for a long time) but this would actually be the most accurate model for aerodynamics.
 
  • #60
Dadface said:
Yes collisions with other molecules.Consider a hot and a cold molecule within a gas containing numerous other molecules.The mean free path of each molecule would be the same at any instant but the hotter molecule will be moving faster and in a given time will undergo a greater number of collisions.By random walk the displacement of the hotter molecule will be greater.

Good one. Since the hotter molecules undergo more collisions with the surroundings they lose more heat to the surroundings which intuitively means it expands and cold air comes in.I'm not sure of the mean free path being same everywhere at all instants.Lesser dense air has more free path of its molecules.And remember the hotter air does not remain hot so you cannot say that about the displacement of a single molecule.
 
  • #61
vin300 said:
Good one. Since the hotter molecules undergo more collisions with the surroundings they lose more heat to the surroundings which intuitively means it expands and cold air comes in.I'm not sure of the mean free path being same everywhere at all instants.Lesser dense air has more free path of its molecules.And remember the hotter air does not remain hot so you cannot say that about the displacement of a single molecule.

Also,don't forget that since the collisions are elastic any kinetic energy lost by one molecule during a collision is gained by the other molecule.
 
  • #62
Dadface said:
Also,don't forget that since the collisions are elastic any kinetic energy lost by one molecule during a collision is gained by the other molecule.
Yes the hotter air transfers energy to the colder and the latter becomes hot, that is what is meant by expansion of hot air and replacement by colder air.
 
  • #63
Dadface said:
Borek said:
That doesn't explain why hot air tends to go up, then.
It does if the heat source is close to the earth.

Trick is, hot air goes up even if it is far from the surface - so you will need different explanation for the same process depending on the distance from the surface.

It is easy to demonstrate,for example,that a flame close to the Earth's surface gives rise to a convection current where there is a bulk upward movement.If the same flame was placed at a higher altitude then the upward movement would be reduced.

Any proof for the latter statement?
 
  • #64
michelcolman said:
the point is that the phenomenon is now explained on a molecular level

I admit I have not read through your explanation, but I suspect that if it is correct, it will work also for the baloon with rigid surface. At the scale we are talking about characteristic of the boundary is less important than its existence. That's why these pockets do exist.
 
  • #65
Borek said:
Trick is, hot air goes up even if it is far from the surface - so you will need different explanation for the same process depending on the distance from the surface.



Any proof for the latter statement?

Is it agreed that when air is heated the molecules move faster and the air tries to expand?If so do a thought experiment where a heat source is placed in a closed box which is suspended from strings.If a single hole is made in the top of the box the expanding air will move out of that hole.If the hole is at the bottom the air will move out of that hole.The same reasoning applies no matter where the hole is made.It can be concluded that the movement of the hot air is not determined only by gravity and the other factors but is also affected by the structure and geometry of the surroundings.Close to the ground the Earth itself forms a barrier which restricts the downward movement of the hot air but at a higher altitude the downward moving hot air has greater space in which to move.The upward moving air transfers kinetic energy by collision to other atmospheric molecules, as does the downward moving air, but the latter also transfers some energy to the Earth when it arrives there.If the Earth heats up as a result then energy can be transferred back to atmospheric molecules which can only move up or sideways but not down.
 
  • #66
Dadface said:
If so do a thought experiment where a heat source is placed in a closed box which is suspended from strings.If a single hole is made in the top of the box the expanding air will move out of that hole..If the hole is at the bottom the air will move out of that hole.

If your box has negligible mass, then the entire box would also go up as the air comes out from the bottom hole. Which means your hot air is going up AND down =) I don't know if you can use this thought experiment.
 
  • #67
Borek said:
Trick is, hot air goes up even if it is far from the surface - so you will need different explanation for the same process depending on the distance from the surface.



Any proof for the latter statement?

The density of air goes on decreasing with altitude, so does the specific heat.You need more heat to raise the temperature of the same volume of air so the temperature difference is less and the convection is less as implied by the formula that follows
The difference of pressure is thus lesser(BtW,g is also a bit lesser)
Air goes up because like static pressure in liquids the difference pressure is due to its depth in the atmosphere
Copied from wiki:
Convection is predicted using the rayleigh number
Ra=ρ0gαΔTL^3/κ μ
Ra=ΔρgL^3/ κ μ
Δρ is the difference in density between the two parcels of material that are mixing
ΔT is the temperature difference across the medium
ρ0 is the average density of the medium
α is the coefficient of volume expansion
κ is the thermal diffusivity
L is the characteristic length-scale of convection
μ is the dynamic viscosity.

The value of L is uncertain
 
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  • #68
vin300 said:
The density of air goes on decreasing with altitude, so does the specific heat.You need more heat to raise the temperature of the same volume of air

But we are talking about the difference that have its source in the presence of the near surface, not in the differences that depend on the pressure/density. At least that's where the discussion started and I have not seen anyone stating "we are discussing different case and phenomena now".
 
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  • #69
Borek said:
I admit I have not read through your explanation, but I suspect that if it is correct, it will work also for the baloon with rigid surface. At the scale we are talking about characteristic of the boundary is less important than its existence. That's why these pockets do exist.
I am still refining my theory, and now realize that the pocket of hot air is slightly different from a balloon after all (although the behavior looks the same macroscopically).

For the balloon, the momentum is transferred at the surface of the balloon. More pressure (collisions of surrounding air molecules with the tissue of the balloon) below than above, gives boyancy to lift the balloon. Nobody cares what happens inside the balloon (or any other lighter than air object).

In the pocket of hot air, the transfer of momentum is NOT just happening at the boundary! (I only just realized this).

The surrounding area has a pressure gradient that is just enough to keep the cold air stable. A cold air molecule will, on average, receive slightly more collisions from below than from above and this will be just enough to compensate for its weight. That doesn't mean none of them come down: some will still come down, while others will go up, in random brownian motion, but there is no net average tendency. So you won't feel any up- or downdraft.

Inside the pocket of hot air (not just at the top and bottom), the same pressure gradient exists because pressure can even out sideways with the surrounding cold air. So each hot air molecule, too, is receiving slightly more push from below than from above. But this total extra push, summed over all the molecules at a certain altitude, is divided over a smaller number of hot air molecules in the layer above that altitude as compared with the number that would be in a similar slice of cold air. This means the hot ones get more than their fair share of upward gravity-canceling momentum and therefore, on average, all the hot air molecules will tend to go up.

Unlike the rigid balloon, the effect is NOT caused at the boundary but happens everywhere inside the volume of hot air.

Of course you are free to make a statistical consideration about the total momentum being transferred to the volume of hot air from all sides, and come to the correct conclusion that the hot air must be pushed up because otherwise it doesn't add up, but I think my analysis is more detailed.

(At least that's my theory, feel free to correct me if I'm wrong)
 
  • #70
jachyra said:
If your box has negligible mass, then the entire box would also go up as the air comes out from the bottom hole. Which means your hot air is going up AND down =) I don't know if you can use this thought experiment.

Exactly, the hot air goes up and down,it travels in all directions outwards from the source.The point I am making is that the surroundings have an effect on the net movement of air.
 
  • #71
Dadface said:
If so do a thought experiment where a heat source is placed in a closed box which is suspended from strings.If a single hole is made in the top of the box the expanding air will move out of that hole.If the hole is at the bottom the air will move out of that hole.

It is understood that the box itself must be too heavy to rise due to buoyancy. If the hot air is initially under pressure when you drill the hole, air will escape at the top or bottom simply due to the excess pressure.

Consider what happens if you drill two small holes, one at the top and one at the bottom. If there is excess pressure, then at first hot air will escape from both holes due to pressure. But after the pressure has equalised, yes, more hot air will escape from the top, but at the bottom cold air will rush into the box. How does your theory explain that?
 
  • #72
Dadface said:
It is easy to demonstrate,for example,that a flame close to the Earth's surface gives rise to a convection current where there is a bulk upward movement.If the same flame was placed at a higher altitude then the upward movement would be reduced.
Then why does my barbecue have an air hole at the bottom? If I open that hole, the coals start burning a lot more intensely and you can actually feel the air current through the hole. So I don't think a flame close to the surface would give more convection, on the contrary. Air below the flame seems to give a significantly faster convection.

For a different example, it is a well-known phenomenon that hot air will tend to build up in a large bubble at the surface and then suddenly let go. This, too, seems to indicate boyancy is lower if the pocket is touching the ground.

(although I can't quite come up with an explanation for that yet)
 
  • #73
DrGreg said:
It is understood that the box itself must be too heavy to rise due to buoyancy. If the hot air is initially under pressure when you drill the hole, air will escape at the top or bottom simply due to the excess pressure.

Consider what happens if you drill two small holes, one at the top and one at the bottom. If there is excess pressure, then at first hot air will escape from both holes due to pressure. But after the pressure has equalised, yes, more hot air will escape from the top, but at the bottom cold air will rush into the box. How does your theory explain that?
Precisely ,hot air rises and cooler air comes into take its place.I am not disputing that but I am stating that the surroundings have an effect on the net movement of hot air which is upwards.The hot air which initially comes out of the lower hole can diffuse sideways beyond the extremities of the base of the box and then this also can rise.Suppose the box was in an atmosphere which for the purposes of this analysis can be considered as being totally remote from the Earth's surface.Can you explain in what direction the net movement will now be?
michelcolman said:
Then why does my barbecue have an air hole at the bottom? If I open that hole, the coals start burning a lot more intensely and you can actually feel the air current through the hole. So I don't think a flame close to the surface would give more convection, on the contrary. Air below the flame seems to give a significantly faster convection.

For a different example, it is a well-known phenomenon that hot air will tend to build up in a large bubble at the surface and then suddenly let go. This, too, seems to indicate boyancy is lower if the pocket is touching the ground.

(although I can't quite come up with an explanation for that yet)
To answer your question we need to refer to the theory of combustion and perhaps this is irrelevant to the main point being discussed.
 
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  • #74
michelcolman said:
Unlike the rigid balloon, the effect is NOT caused at the boundary but happens everywhere inside the volume of hot air.

If it happens inside of the volume, there is no need for boundary - nor need for the cold air around...
 
  • #75
Borek said:
If it happens inside of the volume, there is no need for boundary - nor need for the cold air around...
Not so fast. The cold air around creates the pressure gradient. Away from the pocket of hot air, this pressure gradient is exactly sufficient to keep the cold air stable. A hot air pocket, inserted anywhere in this cold air, will necessarily take the same pressure gradient because the pressure can even out horizontally. This pressure gradient, everywhere inside the volume, makes the hot air particles go up like I explained.
 
  • #76
Dadface said:
To answer your question we need to refer to the theory of combustion and perhaps this is irrelevant to the main point being discussed.
No need to explain, just clarify that you believe a flame will cause more updraft if it is burning just above the surface of the Earth instead of being higher up with plenty of fresh air underneath? Maybe I misunderstood, but that's what I thought you said. And it seems contrary to my experience, although I could be wrong.
 
  • #77
michelcolman said:
No need to explain, just clarify that you believe a flame will cause more updraft if it is burning just above the surface of the Earth instead of being higher up with plenty of fresh air underneath? Maybe I misunderstood, but that's what I thought you said. And it seems contrary to my experience, although I could be wrong.

For reasons I tried to explain in my earlier posts I think that the higher up the flame the greater the directionality of the hot air i.e less will go up and more travel in other directions.
 
  • #78
Gear300 said:
Remove the Earth and leave just the hot and cold air behind - which way is up and which way is down? From this, you could probably say it has something to do with gravity (I might be wrong).

The hotter air would still push the colder air out of it's place because of faster expansion.
 
  • #79
One assumption being made here is that it's the same original heated air molecules that travel upwards. As mentioned heat is trasnferred from the hotter molecules to the colder molcules, increasing the density of the previously hotter molecules, and decreasing the density of the colder molecules.

What is is traveling upwards is a bubble of heat and a somewhat changing set of molecules, not the original set of molecules that initiated a thermal at near ground level. Rather than dissappating in all directions, a heat bubble (thermal) travels upwards and in a circular motion (like a tornado). Cold air tends to flow downwards around the perimeter of the thermal, while hot air rises up internally. At the bottom part of a thermal, the surrounding air is drawn inwards into the thermal, and the thermal expands in diameter with altitude.

http://www.thermikwolke.de/thermals/pdf/page25.pdf

http://www.thermikwolke.de/thermals/pdf/page30.pdf

http://www.rcsoaring.com/docs/thermals_2006.pdf


wiki link:
http://en.wikipedia.org/wiki/Thermal
 
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  • #80
For reasons I tried to explain in my earlier posts I think that the higher up the flame the greater the directionality of the hot air i.e less will go up and more travel in other directions.

I understand what you are trying to say. Thanks for you earlier post. It helped me look at the phenomenon in a different perspective.
 
  • #81
Jeff Reid,

Great links! Really helps to show what Borek and the others have been trying to explain since the beginning.
 
  • #82
Borek said:
But we are talking about the difference that have its source in the presence of the near surface, not in the differences that depend on the pressure/density. At least that's where the discussion started and I have not seen anyone stating "we are discussing different case and phenomena now".
Good.Now bring to light your theory of convection without pressure difference :zzz:
 
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  • #83
Jeff Reid said:
One assumption being made here is that it's the same original heated air molecules that travel upwards. As mentioned heat is trasnferred from the hotter molecules to the colder molcules, increasing the density of the previously hotter molecules, and decreasing the density of the colder molecules.

What is is traveling upwards is a bubble of heat and a somewhat changing set of molecules, not the original set of molecules that initiated a thermal at near ground level. Rather than dissappating in all directions, a heat bubble (thermal) travels upwards and in a circular motion (like a tornado). Cold air tends to flow downwards around the perimeter of the thermal, while hot air rises up internally. At the bottom part of a thermal, the surrounding air is drawn inwards into the thermal, and the thermal expands in diameter with altitude.

http://www.thermikwolke.de/thermals/pdf/page25.pdf

http://www.thermikwolke.de/thermals/pdf/page30.pdf

http://www.rcsoaring.com/docs/thermals_2006.pdf


wiki link:
http://en.wikipedia.org/wiki/Thermal
Too bad the links only contain the word "molecule" once (in the rcsoaring one), and even there it's in a different context. So all the links are macroscopic, while the point of the discussion is what happens at a microscopic level.

You did say something interesting in your own text, though. You say that individual molecules are not traveling up, but rather their momentum is transferred up and the bubble ends up containing different molecules?

I can see how this is happening at the boundaries (momentum is always being exchanged there, and molecules move through the boundary in different directions all the time) but for the vast majority of molecules, I thought they should simply move up with the bubble. Am I wrong there? Where did you find this information?
 
  • #84
jachyra said:
Jeff Reid,

Great links! Really helps to show what Borek and the others have been trying to explain since the beginning.
The links are interesting, but a bit pointless in this discussion (unless I missed some interesting part of one of them).

Yes, hot air rises, and you can describe the whole thing by looking at densities and pressures, but that's all an approximation, a very useful approximation, but an approximation nonetheless.

I wanted to know what was happening at a microscopic level, with individual molecules being pushed around. Just like I learned how to derive the ideal gas law that way, I now wanted to see convection explained that way too.

Since few people seemed to even understand what the question was, I finally came up with a theory of my own, which nobody has really commented on yet (except by saying "I admit I haven't read it yet")

Too bad, because it's really not far away from the macroscopic view! If the other posters would just take the time to read it, they would find it corresponds pretty much to the macroscopic theories they love so much, only in slightly more detail.

Executive summary of my theory: a pressure gradient has built up in the atmosphere (cold air) which is just right to keep cold air stable in the field of gravity. Each molecule gets slightly more collisions from below which, for a cold molecule, is just enough to support its weight (on average! they are all still moving around randomly, some get more pushes than others, they just don't get a net tendency in any particular direction). Inside the hot bubble, the same pressure gradient exists but the total extra momentum transfers from below are now divided over fewer hot molecules. This means every individual hot molecule (on average) gets slightly more than its fair share of gravity-canceling momentum. The sum of all pushes exceeds its weight, so it goes up. Of course this is only a general tendency, superimposed on their own brownian motion zigzagging faster than the speed of sound, but since it has the same average sign everywhere, it does tend to move the whole bubble up as a whole.

There, was it really that complicated/impossible/"nonsencical"?!
 
  • #85
Hi,
I had the same thought, 'why does hot air rise', or to put it another way 'why does a more energetic gas molecule rise when it essentially has the same mass as a cold molecule'
This is what I postulated

Air gets less dense as you increase in altitude.
If you heat up a bit of air the average velocity of the individual gas molecules increases, so the pressure of that patch of air increases and in an attempt to equalize with the surrounding air expands. Since the pressure above this parcel of hot air is ever so slightly less than the pressure below this parcel of air there are less molecular collisions from above so the net effect is the pocket of hot air moves upwards; you can feel the draft caused by the mass of air moving.

Watching food dye circulate in a glass of water would sort of confirm that it is the same molecules which are moving rather than just the energy being transferred to another group of adjacent molecules or some form of diffusion is dominant.

It would be an interesting experiment to see how convection currents flow in a centrifuged medium.


cheers
Martin
 
  • #86
michelcolman said:
Inside the hot bubble, the same pressure gradient exists but the total extra momentum transfers from below are now divided over fewer hot molecules.
Except the hot bubble doesn't rise if the surrounding air is at the same temperature. The hot bubble needs the surrounding air to be cooler (and denser) in order for the hot bubble to rise.
 
  • #87
Jeff Reid said:
Except the hot bubble doesn't rise if the surrounding air is at the same temperature. The hot bubble needs the surrounding air to be cooler (and denser) in order for the hot bubble to rise.
The cold air created the pressure gradient. Without the cold air there would be no excessive pressure gradient so the hot air would not rise.
 
  • #88
Here's my hypothesis. You know that toy with the five stainless steel balls in a row that have nearly elastic collisions with each other - I don't know what it's called - Sprott's book 'Physics Demonstrations' just calls it the colliding balls toy ... I think the group of molecules in the hot air do that with each other for a while, exchanging momentum among themselves while they stay segregated in a group to some extent. This allows them as a group to be subjected to the larger pressure below them and the lesser pressure above them.
 
  • #89
I am going to have to revise my answer to 'I don't know'

Pressure = number of molecular collisions per area * average momentum exchange per collision.

so in a hot body of air each molecule has a statistically higher energy so exerts more force on the surrounding molecules and expands until the number of collisions has decreased to the point where the forces are in equilibrium. Sounds ok so far, agrees with observation that hot gasses expand and cold gasses contract.
Previously I said the hot air rose because there were statistically more collisions from below than from above due to minute changes in are density due to gravity but that is true for cold air as well as hot air and I don't observe cold air rising hence I must revise my answer to 'I don't know'
 
  • #90
I'll take a shot at this. The higher temperature in a thermal decreases it's density compared to the surrounding cooler air. Then it's a buoyancy effect. Ignoring dynamic pressure effects, the total pressure = static pressure + ρ g h, where ρ is density, g is the force of gravity, and h is height. The pressure decreases with altitude, and for any given volume of air, the weight of that volume of air is opposed by an equal and upwards force on that volume of air (otherwise the air would be accelerating upwards or downwards). If that volume of cooler air is replaced by hotter air with lower density, than the upwards force is more than the weight of the volume of the hotter air, and that hotter air is pushed upwards.

http://en.wikipedia.org/wiki/Buoyancy
 
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  • #91
I'm a noob. Just want to ask. Air is made up of different gases - water vapor, nitrogen, carbon dioxide, etc. So why not look into the effects of each separate molecule, and the characteristics of those elements?
Could it be, that the different temperature effects of each element while being gaseous, caused them to separate into their own elements, which gives the appearance that it is hot and cold air?

For example:
Oxygen density is 1.429 g/L, while nitrogen density is 1.251 g/L.
Oxygen heat capacity is 29.378 J·mol−1·K−1, nitrogen is 29.124 J·mol−1·K−1.
Therefore, with the same amount of energy, nitrogen's temperature will be higher than that of oxygen. Since nitrogen has a lower density, it will rise and be on-top of oxygen. Since it will be warmer than oxygen - due to its specific heat capacity - it will give the effect that warmer air is on top of colder air.

Note that I'm a noob, but to me this is logic.
 
  • #92
the_awesome said:
Could it be, that the different temperature effects of each element while being gaseous, caused them to separate into their own elements, which gives the appearance that it is hot and cold air?

This can be easily check experimentally - and separation is not observed.

Similar idea was planned to be used for separation of isotopes, see cryogenic distillation.
 
  • #93
Borek said:
This can be easily check experimentally - and separation is not observed.

Similar idea was planned to be used for separation of isotopes, see cryogenic distillation.
I don't really understand the site. So yeah, whatever lol.
Another question:
Methane and carbon dioxide has a higher density than most gases. And yet, when it is being produced at coal plants, etc, you see it given off - and it rises deep into the atmosphere. So therefore, it is because heat as lowered the density - making it lighter than normal air molecules such a oxygen and nitrogen.
 
  • #94
The classical reason for why hot air rises is
A volume of hot air expands there for it has less density that the surrounding air and therefor rises. The question here is how does this work on a molecular level since hot molecules weigh the same as cold molecules (ignoring E=MC2 stuff)

(which then poses another question, how does a molecule of say N2 store heat energy?, How does He store heat energy? Please don't digress, open another forum if you want to answer this one)
 
  • #95
Dark Horse said:
which then poses another question, how does a molecule of say N2 store heat energy?, How does He store heat energy?

The temperature of a substance is a measure of the average kinetic energy of the individual molecules, which have a statistical distribution. Heating a gas means to give many of its molecules greater speeds.
 
  • #96
Since we imagine gas particles as moving in a straight line (until collisions
happen - in which case momentum is conserved), then we have to assume that at
any given time, there will be gas particles moving "down" instead of "up". If
there are no other forces acting on the gas, then a gas would simply disperse in
all directions independent of its relative temperature.

However, if we factor in the effect of gravity on these particles, then we have to
imagine that each particle not only has its kinetic energy (a vector containing
speed and direction) (KE) but is also affected by gravitational force (a vector
that is always pointing towards the center of the Earth) (g). If we follow this
reasoning, then a particle would have a higher "downward" (KE + g) vector than an
"upward" vector (KE - g).

As such, only particles with high KE will tend to be moving up. Even at collisions,
the resultant motion will favor the downward motion since this is the direction of
g. But since, on average, hotter particles will have higher KE's, then those tend to
move up.

As air (or any other medium) is heated, its molecules vibrate more rapidly, and move
over a greater distance. The net result is that the average spacing between molecules
will increase, and therefore the density of the material will decrease (same mass in a
greater volume = less density)
Why does the hot air rise? Technically it does not.
It is passively displaced upwards as the cooler and DENSER surrounding air moves
downwards, propelled by gravity. Hot air can only rise if there is cooler air to
push it upwards.
The motion and collisions of individual molecules has very little to do with the
process, as temperature is a product of the AVERAGE motion of the molecules in a
sample of air.

You will not find an explanation in terms of individual molecules because
there is not one. Convection is a collective phenomenon. The masses of
fluid that move convectively are VERY LARGE compared to the sizes of
the individual molecules and to the mean free path of the molecules.

At the level of individual molecules, energy is transferred in
collisions between molecules and in movement of molecules from one place
to another. This energy transfer is conduction, of course. In real
fluids, molecules do not go very far before running into each other. So
molecular transport through the bulk fluid (diffusion) is slow over long
distances. Consequently, masses of hot or cold fluid really do tend to
travel as groups, and the model of contained masses is appropriate.

If molecules did not interact, if they truly acted as ideal gas point
masses, hot fluid would not spontaneously rise. One mass of gas would
not exert a pressure force on another. Fast-moving (hot) molecules
would simply spread through the available space very quickly, and that
would be that. The speeds (related to temperatures) of gas molecules at
high altitudes would be lower than at low altitudes, because of their
gravitational potential energy (projectiles slow down as gravity pulls
against them). So you would still observe a "lapse rate," (temperature
decreasing as altitude increases) but hot air (or other fluid) would not
spontaneously rise unless it were contained.

In real fluids, that containment is provided by the molecules getting
physically in the way of each other.

http://www.Newton.dep.anl.gov/askasci/chem07/chem07217.htm"


I have a theory that contrary to popular belief hot air does not rise, (at least of its own accord) but that it only appears to do so because it is forced out of the way by colder, denser air. Does anyone agree?
You may be right. The force that they respond to is gravity. Cold air, being denser, will, in a fluid medium, be drawn closer to the earth, displacing warmer, less dense air.
http://uk.answers.yahoo.com/question/index?qid=20061110052203AAyQKx9"
 
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  • #97
---------quote-----------
"However, if we factor in the effect of gravity on these particles, then we have to
imagine that each particle not only has its kinetic energy (a vector containing
speed and direction) (KE) but is also affected by gravitational force (a vector
that is always pointing towards the center of the Earth) (g). If we follow this
reasoning, then a particle would have a higher "downward" (KE + g) vector than an
"upward" vector (KE - g).

As such, only particles with high KE will tend to be moving up. Even at collisions,
the resultant motion will favor the downward motion since this is the direction of
g. But since, on average, hotter particles will have higher KE's, then those tend to
move up."
---------end quote---------

Isn't what you said also true of cold molecules?
Why would the hotter particles will have higher KE's, tend to move up?
Are you implying its some sort of escape velocity issue and the slower molecules just can't make it to the higher altitudes and the million or so collisions made every second is just noise in the macroscopic molecules trajectory?
 
  • #98
michelcolman said:
I know a hot air balloon goes up because the density of the hot air inside is lower. But what about free air, not trapped in a balloon?

[...] why would this cause the air to rise, to such an extent that it even draws surface winds that fill the gap?

(Admittedly I have not read up on the thread, so I may be duplicating here an explanation already given.)

Let me consider the simplest setup I can think of where the considered phenomenon occurs. In an enclosed room a volume of air is present. On one side of the room there is a heater. The output of the heater is conductive; the air that is in contact with the heater is heated up. Outside the room the temperature is lower so that heat is conducted out of the room.

Then a circulation will commence. The heated air does not have enough weight to counterbalance the denser air on the unheated side of the room. As a consequence the air mass in the room will barrel roll. Since heat is conducted away from the room the heat unbalance persists, so the flow pattern persists.


I believe this translates to large scale motion in the atmosphere.
In different areas air mass is heated differently. The expanded air in warmed areas does not have enough weight to counterbalance the denser air of colder areas, and a pattern of convection flow will arise.

When air mass rises it is always in the process of being _pushed_ up. Moving up is always going against gravity. Surface winds are not drawn in; inflow of surface winds is the cause of less dense air mass being pushed up.


Cleonis
 
  • #99
Cleonis said:
When air mass rises it is always in the process of being _pushed_ up. Moving up is always going against gravity.
Cleonis
This notion is WRONG. Warm air is not being "pushed up". It rises because masses of warm air are more buoyant because it is less dense. Do you have some alternative physics in which an envelope of hydrogen gas is being "pushed up"? Please don't mislead young people who might come here to learn.
 
  • #100
In a macroscopic context, where you talk about air pressures, densities, volumes convection is pretty easy to understand; air heats up, it expands, is less dense than its surroundings and hence rises.
But the question is how does this work at the molecular level?
Hot molecules weigh the same as cold molecules hence why does a hot molecule rise?

cheers
 
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