Why does hot air really go up?


by michelcolman
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michelcolman
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#1
Aug18-09, 04:49 PM
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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?

You often see exactly the same explanation, hot air is less dense so it goes up, but this does not make any sense if you consider the fact that air is just a bunch of molecules flying around freely and bouncing into each other a lot, and temperature is a measure of their average momentum. There's no such thing as a "pocket" of hot air that somehow pushes other air away while going up. All you can say is that in a certain area the average speed of the air molecules is higher.

So why would this cause the air to rise, to such an extent that it even draws surface winds that fill the gap? I mean, the effect is real, gliders use it all the time, but what is really going on on a molecular level? The faster molecules should surely be pushing in all directions, not just up? I would expect them to transfer their excess energy to nearby molecules through collisions until an equilibrium is reached, but can't imagine why a whole "pocket" of air would tend to rise and leave a low pressure underneath, even drawing in surrounding air instead of pushing it away.
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Borek
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Aug18-09, 04:55 PM
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Quote Quote by michelcolman View Post
There's no such thing as a "pocket" of hot air that somehow pushes other air away while going up.
Yes there is - air doesn't mix fast enough.
turbo
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Aug18-09, 04:55 PM
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Cool air is dense and tends to fall in the presence of warmer air, while warmer air is less dense, and it is displaced upward by the cooler air. None of this happens in isolation, of course, and the dynamics of air movement due to heating and cooling effects make predicting weather tough because modeling it is complex.

A good example of the dynamics is the cooling provided by onshore breezes along the coast. Today, it was over 90 deg F inland in Maine, while it was in the 70s in Rockland, on the coast. The reason? Solar heating over land made the air less dense, and the cooler, denser air from over the Atlantic moved in to displace it.

Gear300
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Aug18-09, 04:57 PM
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Why does hot air really go up?


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).
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Aug18-09, 04:57 PM
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Quote Quote by Borek View Post
Yes there is - air don't mix fast enough.
And it can't equilibriate through conduction.
Feldoh
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Aug18-09, 04:58 PM
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See: http://www.physicsforums.com/showthread.php?t=32331
turbo
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Aug18-09, 05:04 PM
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Quote Quote by Gear300 View Post
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).
You are not wrong. Density of air equates to "weight" of air.
michelcolman
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Aug18-09, 05:09 PM
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Quote Quote by Borek View Post
Yes there is - air don't mix fast enough.
You are considering air to be a continuous fluid, but it's not. It's a bunch of molecules flying around randomly and bouncing into each other. So, once again, there's no such thing as a "pocket" of hot air. There are no two different kinds of air, hot and cold, that would have to mix somehow. The hot molecules don't have any reason to stick together, each of them just flies ahead through free space until it hits some other molecule.

That was the whole point of my question, but you don't seem to have taken the time to really understand what I was asking.
russ_watters
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Aug18-09, 05:09 PM
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Quote Quote by Borek View Post
Yes there is - air don't mix fast enough.
And even if it did, what you would see (if you could see air) would be big bubbles breaking apart into smaller, less well defined bubbles. And they'd still rise.
russ_watters
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Aug18-09, 05:11 PM
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Quote Quote by Gear300 View Post
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).
Yes, that density thing referred to in the OP is a reflection of buoyancy. The weight (ie, gravitational force) of a volume of warm air is lower than the weight of an equal volume of cold air.
michelcolman
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Aug18-09, 05:11 PM
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Quote Quote by turbo-1 View Post
You are not wrong. Density of air equates to "weight" of air.
Guys, forget about density for a minute. Just think of little billiard balls flying around and bouncing into each other. That's what air really is.

It is NOT a continuous fluid, even though that coincidentally happens to be a pretty good approximation for a lot of problems.

I want to know what really happens if you consider air molecules to be billiard balls.
negitron
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Aug18-09, 05:13 PM
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Quote Quote by michelcolman View Post
That was the whole point of my question, but you don't seem to have taken the time to really understand what I was asking.
Yes, he did. He understood it perfectly and answered it correctly.
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Aug18-09, 05:15 PM
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Quote Quote by michelcolman View Post
That was the whole point of my question, but you don't seem to have taken the time to really understand what I was asking.
Quite the opposite - I have pointed to a mistake in your understanding of the situation. There is no sharp border of the pocket, but the pocket exists. Given enough time, air will mix and the pocket will disappear, but this process is relatively slow. Slow enough that the pocket has time to rise.
russ_watters
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Aug18-09, 05:15 PM
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Quote Quote by michelcolman View Post
You are considering air to be a continuous fluid, but it's not. It's a bunch of molecules flying around randomly and bouncing into each other.
What happens when a bunch of warm molecules are bouncing around near the ground and a bunch of cooler ones are bouncing around above them? How do they mix evenly? The warmer molecules near the boundary can just about mix on a one-to-one basis, but the path length of the moving molecules is very small - fractions of a milimeter (bolded, because this is the critical flaw in your understanding). So there can't be much rapid mixing beyond a few centimeters. At the same time, the layer of warm air could be 100m high.
So, once again, there's no such thing as a "pocket" of hot air. There are no two different kinds of air, hot and cold, that would have to mix somehow. The hot molecules don't have any reason to stick together, each of them just flies ahead through free space until it hits some other molecule.

That was the whole point of my question, but you don't seem to have taken the time to really understand what I was asking.
I'm sorry, but you're just plain arguing against reality here. You are the one who doesn't understand that your current understanding (and thus the direction of your question) is simply wrong. We know the way we are explaining it is correct because you can actually see this warm air when it condenses into clouds or fly around in it in a sailplane, using it without seeing it. We (the scientific community) know for certain (as certain as scientist can be) that this is a reality.
russ_watters
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Aug18-09, 05:19 PM
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Quote Quote by michelcolman View Post
Guys, forget about density for a minute. Just think of little billiard balls flying around and bouncing into each other. That's what air really is.

It is NOT a continuous fluid, even though that coincidentally happens to be a pretty good approximation for a lot of problems.

I want to know what really happens if you consider air molecules to be billiard balls.
Air molecules are a lot of billiards balls and if you have an area where they are moving quickly next to an area where they are moving slowly, the quick moving ones will crash into the slow moving ones and over time they will equalize their energy. But, as I started the explanation, there are a lot of billiards balls and the regions of high and low energy billards balls are much larger than the region in which they interface.
turbo
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Aug18-09, 05:20 PM
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Quote Quote by michelcolman View Post
Guys, forget about density for a minute. Just think of little billiard balls flying around and bouncing into each other. That's what air really is.

It is NOT a continuous fluid, even though that coincidentally happens to be a pretty good approximation for a lot of problems.

I want to know what really happens if you consider air molecules to be billiard balls.
You have been given some pretty concise replies, and even a simple example of the dynamics involved. Gas molecules that form air are not little billiard balls acting in a vacuum. The molecules act collectively, and that collective behavior can be devastating. Ever hear of a hurricane, typhoon, tornado?
russ_watters
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Aug18-09, 05:24 PM
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Also, look into the concept of diffusion:
Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material.[emphasis added]
http://en.wikipedia.org/wiki/Molecular_diffusion

The word "gradual" means it takes some time. How much time? Well it turns out that the rate of diffusion depends on the starting conditions, but the size of the masses of air are not part of those conditions. Ie, the boundary will diffuse at a certain rate (in cm per minute or some similar metric) if the two volumes are 1 cubic meter or 1 cubic kilometer.
negitron
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Aug18-09, 05:33 PM
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Quote Quote by russ_watters View Post
the path length of the moving molecules is very small - fractions of a milimeter
A very, very, very small fraction. The mean free path of an air molecule at STP is less than 70 nm; that's 1/10th the wavelength of a photon at the far red end of the spectrum.


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