What really causes a warmer air mass to rise?

[jbedow]

I know we all learn that when we heat a certain volume of air, as in a balloon, the heated volume will be less dense than the surrounding volume of air, so it rises or floats. But I would like to see a layman's explanation of what is really happening at the molecular level. The energized molecules inside the balloon do not move in unison, they are just moving faster. So why do they "float" above the slower, outside molecules? Am I correct that it is a matter of probability, i.e., that an energized molecule would tend to have more interactions (be stopped) moving down towards denser area of slower molecules than it would moving up towards less dense area of molecules, so in the course of multiple random movements, the energized molecules will have more successes moving up away from denser areas? I appreciate any effort you may spend in explaining this. Thanks

[psynapse]

Well hot air is less dense. This is because the molecules have more kinetic energy and zoom around faster and colliding with their container more causing greater pressure and in the case of a balloon it expands. the colder air around the balloon which is more dense is pulled down due to gravity with a stronger force and displaces the hot air balloon up. Likewise if you were in a car and the driver swerves sharply you will lean to one side of the car, but a helium balloon will move to the opposite side you feel the force. The balloon feels the force but the more dense air around it will be pulled more and push the balloon to the other side.

[psynapse]

If you are talking a mass of air then you might be looking for convection as the sun heats up the ground and the column of air above this ground it turns into a lower pressure system and cold air will rush in causing wind. the cold front comes in like a wedge forcing the hot air up.

[Martin]

I know we all learn that when we heat a certain volume of air, as in a balloon, the heated volume will be less dense than the surrounding volume of air, so it rises or floats. But I would like to see a layman's explanation of what is really happening at the molecular level. ...

 

a balloon full of hot air needs fewer molecules of air to fill it

 

because those molecules are buzzing around more actively, hitting the wall of the balloon more often, and harder, than cold molecules would

 

so they are better at occupying space and generating pressure on the balloon wall (so it doesnt collapse because of outside pressure0

 

they are more efficient at their job so it takes fewer of them to do it

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That is one way to tell the story to a layman who wants to imagine what is happening at molecular level.

 

A physicist will have the kneejerk reflex response to tell you the LAW

 

perfect gas law:

PV = NkT

 

N is number of molecules, k is Boltzmann const, T is temperature. The bigger T is, the bigger PV (pressure x volume) is.

 

the bigger T is, the more volume the crew of molecules can occupy and the more pressure they can exert on the walls, because they are more active.

 

With a balloon the pressure P is fixed-----it has to equal the surrounding air pressure. So then raising T makes it possible to reduce N, the use fewer molecules.

 

fewer molecules means less weight. so the balloon rises.

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synapse already answered adequately, didn't see his answer

[[Tycho?]]

Ha, I just glanced at your post, wrote up a big explanation, but realized I couldn't back it up, and deleted it. Turns out its exactly what you thought it was, with the density idea. It does seem to make sense, but it may not be the best way of thinking about it.

 

After reading and disliking my first explanation, I thought it may be better to think of it in terms of buoyancy. So think of a balloon, and keep in mind your thoughts on density of the air. At the bottom of the balloon the air would be more dense than at the top of the balloon. This means that there will be more pressure exerted on the bottom of the balloon than the top. And since there is an unbalanced force, you get an acceleration, which is the balloon floating upwards.

 

Now, the problem with this is obviously that a balloon has a surface, while hot air is simply a region of hotter air, and so I can't be totally sure that the hot air thing can be explained solely with buoyancy. This would be part of it though.