Why Does Hot Air Rise? Exploring the Phenomenon of Rising Hot Air

[jammieg]

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.

 

[chroot]

Hot air is less dense and experiences a buoyant force, just like a bubble of air in water.

- Warren

 

[Loren Booda]

According to statistical mechanics, where air has greater kinetic energy it has a greater probability to occupy a higher gravitational potential than less energenic air. That is, the system tends toward maximal entropy.

 

[Ian]

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.

 

[quartodeciman]

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.

 

[chroot]

There are, in general, two forms of heat transport in play here: radiative transport, and convective transport. When the temperature gradient is low, radiative transport is most efficient, and heat "diffuses" without any mass moving around. When the temperature gradient exceeds a threshold, however, convection becomes more efficient and heat is exchanged by the movement of mass.

- Warren

 

[krab]

A related question, also very interesting, is Why do cold and hot water regions not self-mix? We've all experienced these when swimming in a quiet lake in the summer. In particular, one can dive to the bottom of a deep lake and discover many layers of different temperature. Why are there layers instead of a more or less constant gradient? and why does the warmer layer not warm the colder one? I've never seen a good explanation of this.

 

[RAD4921]

Hot air rising

I agree with chroot and the others. Cold air is denser and heavier than hot air. I also agree that hot air has more "kinetic energy" which in turn makes the atoms take up more space and therefore makes it less dense and lighter.

 

[napoleonmax]

Hmmmmm... I'm not sure it has to do with buoyancy (though this would appear to make sense). I remember reading somewhere the fire system used on the International Space Station and it said the flames do not rise in space but rather moves in all directions. Therefore, I think Loren Booda's explanation makes the most sense.

 

[napoleonmax]

A related question, also very interesting, is Why do cold and hot water regions not self-mix? We've all experienced these when swimming in a quiet lake in the summer. In particular, one can dive to the bottom of a deep lake and discover many layers of different temperature. Why are there layers instead of a more or less constant gradient? and why does the warmer layer not warm the colder one? I've never seen a good explanation of this.

I think that layers of cold and hot DO mix, just not necessarliy at a very fast rate. The laws of thermodynamics state that heat moves form more dense to less dense, so hotter regions must get colder and vice verse. However, in a lake, the top is getting much more sunight than the bottom and therefore absorbs more heat than it can transfer, causing layers of hot and cold in water.

 

[Janitor]

After one experiences the sight of the snowy top of a mountain ridge while sweating in the heat of the desert floor below, one might naively assume that heat falls rather than rises. This strikes me as one of those situations like when someone finishes explaining why an airplane wing generates lift because of the shape of the airfoil, and then some other guy pipes in with, "Yeah, but I was at an airshow last Saturday, and doggone it if there weren't some airplanes flying upside down!"

 

[Moe]

You can't fly upside-down forever, just for short periods of time.
The temperature profile of our atmosphere is quite jaggy, at first it drops with altitude, then it starts to rise again. The reason for that are different layers that absorb, reflect and radiate heat. At first it gets colder because you are moving away from the earth, which stores heat fairly well, that is why you can have snow on a mountain overlooking a desert. The fact that the air gets thinner (=less dense, less total heat capacity) might play its part as well.

 

[JohnDubYa]

I think Loren's explanation is the most accurate.

Systems want to minimize potential energy, which is why rocks fall when released. In order for a particle to increase its potential energy, it must have sufficient kinetic energy that it can transform into potential energy during the process.

The hotter the air, the more likely an individual molecule will have sufficient kinetic energy that it can transform into potential energy (which means raising its height).

So it is a statistical property.

I try not to use the concept of buoyant force in such explanations, because it tends to beg the question. Instead, think in terms of the inherent desire of systems to minimize potential energy. Helium balloons rise because the potential energy of the room is less after it has risen than before. (Light volume of gas moves up, heavier volume of gas moves down to replace it.)

 

[kuenmao]

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?

 

[chroot]

You can't fly upside-down forever, just for short periods of time.

Wrong. In a proper aircraft, you can fly upside-down all you like, forever. The problem with some small aircraft is that they make use of gravity in their fuel systems, and the engines will be starved for fuel after a long period of inverted flight. In a properly design aerobatic airplane, however, you can fly inverted forever.

- Warren

 

[krab]

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?

That's the best explanation so far.

 

[chroot]

Isn't that what I said in the first response to the thread?!

- Warren

 

[kuenmao]

Yeah chroot, you were right, but you could've elaborated a bit...kinda confusing to just say that it's less dense.

 

[somy]

hi.
in fact rising of air depends on the tempreture profile of the atmosphere and also the tempreture of the smoke. sometimes whene the atmosphere is in the "inversion condision" there is no rising in the air!
it happens a lot in Los Angles. where there is a very hot blast of air in higher altitudes, that makes the inversion condision.
other condisions are :
adiabatic
subadiabatic
superadiabatic
the last one is the best for rising the polutants!
also see the "air polution control " references.

 

[quartodeciman]

Lower density vs. higher density and higher temperature vs. lower temperature explain why the upper body of air gets heated by the lower body and the lower body gets cooled by the upper body. But this doesn't by itself explain why the material and its energy don't simply diffuse across the given common boundary between them, leading ultimately to equalization of both density and temperature. Instead, new and closed boundaries are formed and the material and energy move bodily in units (upward and downward). I suspect that this fact requires more description of the process.

 

[mee]

Perhaps

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

 

[quartodeciman]

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

 

[franznietzsche]

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

Because they don't feel like it? Simply, fluid dynamics doesn't work that way...why it doesn't...well...anyone?

 

[zoobyshoe]

Interesting discussion.

There are a lot of things going on when hot air rises. If you watch the smoke from a cigarette you can see that there is an initial column where the chimney effect dominates. I would say boyancy wins out over the other forces here. Then the smoke gets to a point where it suddenly becomes very turbulent. My guess about this is that the initial flow has created a Bernoulli effect where the rising smoke represents a low pressure system by virtue of its fluid speed. It starts to entrain the colder surrounding air into the flow from all horizontal directions. If you look at this region you see the flow often exhibiting a twisting motion.

There seems to be a chaotic, non-repeating, interplay between the buoyancy and this Bernoulli effect that shreds the smoke column more and more the higher up you get from the heat source. There is actually quite a bit of mixing going on and two feet above the cigarette the twists and curls of smoke are very much more spread out than they were in the initial filament-like column.

 

[JohnDubYa]

Take a glass box that is partitioned into two chambers, one on top of the other. In the top chamber are oxygen molecules moving at an average speed of 1000 miles per hour. In the bottom chamber oxygen molecules are moving at an average speed of 2000 miles per hour.

Now remove the partition. What happens?

The slower moving molecules will migrate downwards, because they do not have sufficient kinetic energy to maintain their elevation. On the other hand, the faster moving molecules will migrate upwards, because those that had sufficient kinetic energy to hit the top of the partition are now free to travel farther upward now that the partition has been removed. They stay towards the top because they are launched back upwards when elastically colliding with neighboring molecules.

So hot air rises.

 

[kurious]

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

Kurious:

Because the cold air molecules have less kinetic energy and are closer together than hot air molecules, and stick more to each other using electric forces ( like van der waal's and dipole forces if water is present to any large extent).This means that cold air molecules move in bigger aggregations and so may be too big to get in the gap between hot air molecules.Also, some cold air molecules will mix with hot air molecules, but it takes a long time for significant numbers to mix.There is also the possibility that as one body of air passes another friction between them creates some static electricity which could prevent molecules at different temperatures from mixing.This is probably significant for large volumes of air such as those in the Earth's atmosphere.If a hot volume of air rotates it might expel cold air molecules
like a washing machine drum kicking out water as it spin dries clothes.

 

[salamander]

WATER
This disscussion seem to have died out but I just thought somone might be interested...
H2O is at it's densest at approx. 4 deg centigrade and hence (regardless of the underlying principe...) the water closest to the bottom of a lake will have this temperature all-year round, assuming that the lake is not very shallow (I.E. the lake is not a puddle). Closest to the surface (0.5 to a couple of metres, depending on water opacity, air temperature, exposure to sunlight etc...) there will be a narrow zone of temperated water where the temperature wary with the seasons (cold in the winter, hot in the summer, this is the place where we like to swim). Between the two layers, there is a layer of water that keeps a quite even temperature through out the year. In the spring and in the fall the two top layers are mixed by winds (at least in the temperate areas where I live [Sweden]) and the layers are exchanging heat as well as the water is dissolving oxygen from the air. The middle layer here acts as a thermostate, effectively preserving the temperature it was given by the top layer in the last change of season and giving it back the next time season changes.
It is my understanding that the main reason for this is the low thermal conductivity of water and the energies required to stir around the bottom of a 20-30 m deep glass of water by blowing on its surface.

It is also important to note that colder water can dissolve more oxygen and that combined whit this layered structure of sea water widely influences what lives on a certain depth at certain times of the year. This is the key to great fishermanship...

as for aeroplanes
Modern aerobatic planes have symmetrical or almost symmetrical wing profiles that generate good lift regardless of which way happen to be up at the moment. The only problem I can see with inverted flight in such aircraft is the color and state of the pilots' head after a couple of hours... :surprise:

cheers fellows

 

[zoobyshoe]

Now remove the partition. What happens?

John, have you ever performed this demonstration, or seen it performed? If so, how could you see what was happening?

Is there any signifigance to chosing oxygen, rather than air?

 

[JohnDubYa]

My question was posed as a thought experiment. I have not done the experiment.

The choice of gas is irrelevant. My bad.

 

[zoobyshoe]

I think it would be worth actually doing, just using air for convenience, and having smoke in one of the layers so that what happens could be seen.

I have never seen anyone do this so it would be interesting to find out exactly what does happen in the situation. It is a different situation than most where hot air rises. Usually there is a specific, local heat source: a candle, a fire.
------------
It seems to me that if hot air rises by specific virtue of its greater kinetic energy then you should be able to take any quantity of gas, contain it, and find later that the higher energy molecules have separated out and taken up position in the upper part of the container and that the lower energy molecules will be found at the bottom. You will have found Maxwell's Entity and his name is gravity.

 

[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?