Why Does Hot Air Rise Above Cooler Air?

[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.

 

[DaveC426913]

With the exception of a couple of recent blips, this thread is 8 years old.

 

[JakusLarkus]

I'm aware that this question has been very successfully answered, but allow me to post a more equation-based contribution;
In accordance with P=m/V, if we lower the density of a subsystem (here, the hot air), then providing that its mass does not change, the volume of the subsystem increases. This causes the mass of the hot air to become lower in proportion to the volume.This, due to the weight law W=mg causes a lower overall gravitational effect on the subsystem. Hence, the effect of the Earth on the hot air (lowered in density by an increase in Ek) becomes less significant and so it rises above the cooler air (which is more greatly affected by gravitation).
Hope this helps ^.^

 

[JakusLarkus]

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

Mmh... not technically true, but a very interesting principle. As previously stated, in accordance with General Relativity, a particle's mass increases with its energy. Unfortunately this has no effect on the overall charge of a particle or indeed an atom as the relative electrical energy inside the atom remains proportional. In fact, there is no effect.
For example, the mass of an electron is 9.11x10^-31kg and its overall charge is deemed '-1'. A particle experiencing a change in Ek (due to heating) would be subject to a change in mass, but not a change in overall charge. Were this incorrect, particles would behave entirely differently in relation to modern observations; they would have a stronger repulsive effect on some particles and a stronger attractive effect on others (possibly infinite when traveling at c).
There's also no reason why only cooler molecules would be repelled, nor why they would be necessarily repelled towards a gravitational field (i.e. downwards).
I hope I've provided a valid insight =)

 

[Philip Wood]

Just think about an imaginary bubble enclosing the gas to be heated. V is proportional to T at constant p, so the gas in the bubble expands, becomes less dense and rises (if it is surrounded by cooler, denser gas).

 

[dougy]

I have an answer that I think hasn't been given yet.

Consider a pocket of hot air surrounded by colder air. The cold air is slightly less dense above the pocket than it is below it, because there is less weight pushing down the air above than the air below (and so the air molecules above the pocket are less compressed together than the air molecules below the pocket). By definition the hot air molecules have higher velocity than the cold air molecules, but since the cold air molecules above are less packed together than the cold air molecules below, the hot air molecules going upwards will be able to go further than the hot air molecules going downwards. Since statistically there are in the pocket about as many hot air molecules going upwards than going downwards, on average the hot air molecules will rise.

 

[klimatos]

By definition the hot air molecules have higher velocity than the cold air molecules, but since the cold air molecules above are less packed together than the cold air molecules below, the hot air molecules going upwards will be able to go further than the hot air molecules going downwards. Since statistically there are in the pocket about as many hot air molecules going upwards than going downwards, on average the hot air molecules will rise.

It doesn't work that way. When a mass of air moves, there will be more molecules moving in the direction of movement than in the opposite direction, but the relative proportions are still pretty equal. The differences in mean molecular speeds in each direction are negligible at normal wind speeds. An air mass will move from A to B only if the molecular flow rates (not speeds) are greater from A to B than from B to A. As I showed in an earlier post (#60) in this thread, molecular flow rates increase with a decrease in temperatures under isobaric conditions. Therefore, winds blow from cooler to warmer areas under those conditions.

 

[mfb]

This thread is from 2004 and the answer has been given long ago, it is time to let the thread rest.