Why Does Hot Air Rise Above Cooler Air?

[somy]

Just think about a "parcel" of air.
make the energy balance around it. The parcel which doesn't get any turbulency in it.
It will help you!

 

[zoobyshoe]

Just think about a "parcel" of air.
make the energy balance around it. The parcel which doesn't get any turbulency in it.
It will help you!

A hot air balloon.

 

[zoobyshoe]

Why don't the cold air molecules just fall through the space between the hot air molecules and simply make a mix?

The hot air molecules which are zipping around much more rapidly would knock the slower, cold air molecules out of the warm air zone before they could fall very far into it.

Any cold air molecule that beat the odds and got very far into the hot ones would soon be hit so many times it would become one of the hot ones. Any hot one that hit it would become a little cooler.

Heat travels to cold, so I think the majority of the interactions would involve cold molecules being knocked away from the faster hot zone. Kinetic energy would slowly be transferred from the higher energy molecules to the lower energy ones.

Since the flow of thermal energy is really only one way, from hot to cold, and since that is a relatively slow process compared to the buoyant force, buoyancy acts first.

 

[zoobyshoe]

Perhaps it is not so much the hot air rising but the cold air around it sinking, its pressure pushing the warm air up?

I think this is correct. It seems to be what is happening in boyancy.

Buoyancy
Address:http://theory.uwinnipeg.ca/mod_tech/node67.html

 

[Janitor]

Thanks for the link, zoobyshoe.

One can look at floatation of a solid object in liquid as due to "the difference between the pressure at the top of the object, which pushes it downward, and the pressure at the bottom, which pushes it upward," as the link page says. I think of that as a "local viewpoint." But it seems to me that there is a global viewpoint too: one could also look at it in terms of the virtual work theorem. Releasing the solid from a point just above the water, the work gravity has done in pulling the solid down into the liquid to the equilibrium depth must balance the work the liquid does in moving its surface up against gravity. When we are talking drinking glass or bath tub, I can visualize either of these explanations as being valid. But when we are talking about a helicopter releasing a little block of wood into the Pacific Ocean, the global view has an unrealistic feel to it. How high does sea level rise in order to float the three-ounce piece of wood to make the energy balance?

 

[zoobyshoe]

How high does sea level rise in order to float the three-ounce piece of wood to make the energy balance?

By precisely the amount it takes to float the wood. Obviously not measurable in bodies of water over a certain size in comparison to the block of wood, but real none-the-less.

In the ocean that displacement would propagate away from the dropped piece of wood as a wave and probably spend its entire existence tied up in wave form.

 

[jammieg]

So the less dense hot unit of air is pushed from the bottom by the more dense air because the less dense unit is less affected by gravity? Heat is the cause of the relative difference in density from a hot unit of air to a cool unit which is really just the constant extra pushing of random motion, do I have this right? Why wouldn't gravity only affect the individual molecules alone, why the whole unit, it's as if it was working on both scales, it's as if the unit of hot air was connected in a way with each individual atom linked to the unit otherwise wouldn't each molecule sink as any other cooler molecule, I mean with gravity affecting them equally? Could the gravitational affect be operating on multiple scales, micro and macro, or is this an illusion of a simple mechanism for defying gravity that I still haven't quite grasped?
It's strange how a waft a smoke sticks together for so long...supposing all matter had some sticky property to neighboring matter, what would cause this?

 

[zoobyshoe]

Why wouldn't gravity only affect the individual molecules alone, why the whole unit...

A dam good question, one that points to the Booda/DubYa/Maxwell's entity as the real mechanism. Something like this: in a gravitational field higher energy molecules are able to do more work against gravity than lower energy ones.

I would think that this could be tested by containing some air in an insulated container with sensitive thermometers at vertical intervals. If this mechanism is the correct explanation, then an initial uniform temperature from top to bottom should change to a hotter reading from the top and a cooler reading from the bottom as the molecules with higher kinetic energy pass that energy to the ones on top from the ones lower down.

It would be fascinating to discover that gravity has been Maxwell's entity all along.

 

[ArmoSkater87]

PV = nRT
That is the ideal gas law, P=pressure, V=volume, n=moles, R=gas constant, T=temp.
u can minipulate the equation:
(MM)PV = (MM)nRT [MM= molar mass]
(MM)n = mass
(MM)n/V = D = density
P(MM) = DRT
D = P(MM)/RT

As u can see, when T goes up, D goes down
If the air has a lower density, it will rise because of boyant forces

 

[omin]

Gravity.

Take two bouncy balls. Exert less force on one than the other when you spring them off the ground and see which one rises from the ground up under gravity farthest. The one you launch with greater speed.

The warmer molecules push against gravity harder because they are moving faster, therefore they rise.

I'd say a bubble rising in a glass of beer is mass for mass much warmer than the beer.

 

[Rampantbaboon]

well, because the weight of the atmosphere causes continous(sp.) pressure increasing as hight above sea level decreases, the pressure at any point directly below another point is higher. Because pressure is defined as a force per unit area there is more force on the point below causing a net force in the upward direction. This is true for all things in air, except the buyant(sp) force is usually negligible. Armo proved that the air is less dense and when and object is less dense than the fluid around it, the net force upward is greater than the weight.

 

[JohnDubYa]

So the entire atmosphere should accelerate upwards, leaving the planet with no atmosphere?

 

[brewnog]

Yes, but we have gravity.

 

[BobG]

Loren Booda's explanation would seem to make sense when you're talking about gases. But how would that explain a cork floating on the water?

The difference in density between two gases is due to the extra kinetic energy in the warm gas. Because of the molecules moving faster, one of two things has to happen. They push harder on their 'container' than slow moving molecules or the 'container' expands until the pressure from the fast moving molecules is the same as the pressure from the slow moving molecules (in other words, the warm gas becomes less dense).

The cool air, being more dense, having more mass per unit of volume, experiences greater gravitational force than the warm, less dense air. Relative to each other, the cool air has a net downward force and the warm air has a net upward force. Relative to the surface of the Earth, both have a net downward force (none the less, the Earth does lose a few molecules into space).

 

[kishtik]

I can't imagine the buoyancy acting to a single molecule (of hot air for example). For a balloon, yes, the pressure is higher at bottom so a net force is exerted to it upwards. Or for a wooden block sunken at sea, when we release it, it goes up because of buoyancy and the difference of pressure (between top and bottom) causes buoyancy. A group of hot air molecules isn't an "object" to feel buoyancy. And the same for alcohol rising through water I think. So where am I wrong? Thinking of "alcohol", yes its density is lower etc. but I couldn't imagine the density difference logic for individual molecules.

I made an experiment in Working Model for the subject (not sure if it represents the truth really). I created a box (which represented a tube), placed small balls in it (for air molecules) at the same height (so they would have same KE when the reach the bottom of the box). Gave full elasticity to the molecules and the box (and because there wouldn't be any force between the balls, I had an ideal gas I thought).

When I clicked the Run button, the "molecules" fell down as I figured, but after a second or so, I realized there were great KE differences between them, and the faster ones were higher. The other thing I realized was the molecules very lover moved VERY slowly, and they seemed like liquid! I had both gas and liquid! Now, was that a reliable simulation? It seemed to me, being liquid or gas and having vapor equilibrium was directly caused by elastic collision laws and gravity (for ideal gases of course)! The higher parts of the box had less number of balls/unit area ratio (like mass/unit volume (so lower density of course)).

It was entertaining so I continued to play with that, and "saw" diffusion etc.
At a point of development of the simulation, I had a mechanism which gave the red balls higher KE and the default yellow ones lower, it was completely a fuse! I noticed that, again the molecules which had higher KE were at higher altitude, but they weren't all red of course.

Love!

 

[zoobyshoe]

I can see your point. Why should a more energetic molecule weigh less than a less energetic one? What's the difference? The more energetic molecule requires more "territory" so to speak, per unit mass, but at the level of two individual molecules why should that make it weigh less than the other?

 

[HallsofIvy]

It's not true that an energetic molecule weighs more (we're not talking relativistic speeds here!). What is true is that an energetic molecule has more energy (of course!) and so can go to a greater height before it loses all of its kinetic energy (total mechanical energy is conserved). Over a "long" period of time (and for molecules, this might be a fraction of a second), the more energetic molecules, the ones that CAN get higher, will be higher than less energetic molecules.

 

[HungryChemist]

I don't get why mechanical energy has to do with it rising...can somebody elaborate a bit on that?
Anyway, what I think(which may be wrong) is that since both the hot air and cold air are at the same pressure(atmospheric pressure), by the ideal gas laws, the same mass of hot air would occupy more space than the cold air. That means that the mass per unit volume, which is density, is smaller. By fluid mechanics, less dense objects float on top of denser objects, and so the hot air floats.
Anything wrong with that?

Yes, Cuz water is not ideal gas.

 

[BobG]

It's not true that an energetic molecule weighs more (we're not talking relativistic speeds here!). What is true is that an energetic molecule has more energy (of course!) and so can go to a greater height before it loses all of its kinetic energy (total mechanical energy is conserved). Over a "long" period of time (and for molecules, this might be a fraction of a second), the more energetic molecules, the ones that CAN get higher, will be higher than less energetic molecules.

How are fluid dynamics going to come into play in this (if at all)?

Assuming we're talking about a certain amount of hot air, you have several molecules at approximately the same temperature. The molecules in the center of the hot air mass can't lose their heat to the cooler air very rapidly. In other words the 'hot' energetic molecules(1) in the middle collide with the molecules(2) a little further out, adding back in some of the kinetic energy that molecule(2) might have lost, etc. There's a gradual gradient from the center of the hot air mass to the cooler hot air.

So why wouldn't the hot air mass, as a whole, be considered as an entity that was less dense than its surroundings?

Especially since 'warm' molecules are moving in random directions - not only up, but down?

 

[HallsofIvy]

How are fluid dynamics going to come into play in this (if at all)?

Assuming we're talking about a certain amount of hot air, you have several molecules at approximately the same temperature. The molecules in the center of the hot air mass can't lose their heat to the cooler air very rapidly. In other words the 'hot' energetic molecules(1) in the middle collide with the molecules(2) a little further out, adding back in some of the kinetic energy that molecule(2) might have lost, etc. There's a gradual gradient from the center of the hot air mass to the cooler hot air.

So why wouldn't the hot air mass, as a whole, be considered as an entity that was less dense than its surroundings?

Especially since 'warm' molecules are moving in random directions - not only up, but down?

Yes, but the "cool" molecules are also moving down. The only difference is that "warm" molecules can go higher that "cool" molecules

"So why wouldn't the hot air mass, as a whole, be considered as an entity that was less dense than its surroundings?"

They can- but I thought the question was asking WHY that was true.

 

[BobG]

Yes, but the "cool" molecules are also moving down. The only difference is that "warm" molecules can go higher that "cool" molecules

Good point.

 

[HungryChemist]

Yes...But

Good point.

Yes...But the Hot molecules could go down also and it will go down deeper than the cold molecules could go.

 

[falseVacuum]

Although it looks like the question has been answered sufficiently, here's a bit of additional reading on hot vs. cold air

 

[Loren Booda]

Take a mole of molecular oxygen (more dense molecules) and a mole of molecular nitrogen (less dense molecules) and combine them in a vessel so that they coalesce randomly at one atmosphere pressure. What is the time evolution of this system while undisturbed?

 

[dedaNoe]

Chroot is correct, hot air rises because it occupies a greater volume per unit mass of air than cooler air. It is simply an application of Archimedes principle.
Incidentally, if the volume remained constant as the temperature rose, then according to GR/SR since energy is a mass equivalent the hotter body would sink rather than rise.

I'm glad you mentioned Archimedes.
I had the same problem covered in the topic "heat & weight".
I don't know if it's still around.

 

[Romeo Ponce]

Can someone tell me what this is about

 

[DaveC426913]

Can someone tell me what this is about

Certainly. This is a Physics Forum on the World Wide Web where people of all types and backgrounds come to discuss various aspects of science.

 

[RidleyPrime]

Jammieg has posed an excellent question.

There are quick answers to it, but then there must follow deeper and more detailed answers IMO. Why doesn't the less dense hotter air simply diffuse into the denser cooler air across the given boundary layer? Why doesn't the system just slowly move to thermal uniformity without material exchange?

There is something called a temperature gradient that is supposed to tell much of the story. I haven't found any really good explanatory text online about this subject (free convection).

Jammieg has posed an excellent question.

The heated air moves faster and therefore takes up more space. The reason the air doesn't diffuse is because the amount of the material is about the same. The heat takes time to transfer because the air has specific heat of around .24. Specific heat is the measure of the heat energy required to increase the temperature of something. In comparison, iron has a specific heat of .11

 

[mighty oliver]

quartodeciman...Dear friend, actually when ever i ask "why hot air rises up" people just tell same old answers but just like you , i honestly believe there is something more waiting to be explained.

Dear I will try a bit to explain :

when air gets heated it starts rising because on heating it's molecules gets charged in a way that they repell cooler molecules around them and gets repelled from the Earth as well,(hence don't get chance to distribute heat energy) till they reaches a point where the force of gravity (attraction) and force of repulsion because of Earth's charge gets balanced and a equilibrium forms.This charge has been observed by NASA as well,this charge helps Earth to repel charge coming from the sun.

Its my humble attempt ... people please let me know if I am right or wrong..

 

[klimatos]

Why does hot air rise?
What I'm really getting at is why should the speed of kinetic motion of the individual atoms of heated air rise compared to it's slow moving neighbor, is it merely because it bounces around more often and so all air competes for dominance upward such that the fastest moving air must go up? I mean when I watch smoke rise I think well these must be some heavy particles in that smoke and so they should go down but instead they go up because it's warmer than the surrounding air...seems odd to me, but then my thermodynamics knowledge is basic maybe that's why or maybe I'm too philosophically trained to accept this answer and be done with it.

1. Hot air does not "rise". It is pushed up by denser (cooler and/or drier) air underneath it. Stop the pushing and the air stops rising. Nothing moves against the pull of the force of gravity unless pushed by a stronger force.

2. Since we are dealing with moving air, we are dealing with non-equilibrium conditions. None of the classical equations that require conditions of equilibrium can be easily applied. The discussion is best approached through the physical disciplines of statistical thermodynamics and non-equilibrium kinetic gas theory. These disciplines describe macroscopic air movement in terms of statistical functions on the molecular level.

3. Molecular flux is the number of molecules passing through one square meter of an imaginary plane in one second. Under conditions of equilibrium, the molecular flux is the same along either arm of any axis of movement. That is, there are as many molecules with an eastward flux as there are with a westward flux. And, there are as many molecules with an upward flux as there are with a downward flux. This number is one-half the mean molecular number density (n/2) in each case.

4. When a parcel of air is moving, this equivalence no longer holds. When a parcel of air is being pushed upward, the upward flux exceeds the downward flux. That is, more air molecules will have an upward component of movement than will have a downward component of movement.

5. Flux rates are affected by both molecular density and molecular speeds. Under the conditions that are normally found in our atmosphere, density seems to be the more important of the two. Cool air “pushes” against warm air more strongly than warm air pushes against cool air. Hence, the hot-air balloon is pushed up and the cool air spills down off of the Greenland ice-cap.

6. At 1000 hPa, the molecular flux rate (x 10²⁷ molecules m² sec^-1) is:

2.73 at 50°C
2.84 at 25°C
2.97 at 0°C
3.11 at -25°C

7. Hence, cool air flows toward warm air. We describe this by saying that “warm air rises”, but it is actually being pushed up by the cooler air.