2 Helpful?

"What must we say?"

I stumbled on this ancient video recently and heard a sort-of contrarian statement:
"don't EVER say that hot air rises..because that AIN'T so"
(just watch the first 1:30 minutes)

"What happens is that the less dense air is pushed up by the colder air"

I had never heard it put that way ...hmmm.... very interesting

So yesterday Allison Bailes Posts a Blog on the same subject.

A good blog..by the way

And this "Heretic" (Bud Poll) comes along saying almost the same thing as Professor Sumner Miller
and he posts a link to his "worksheet"


I like Bud's words and illustrations ... they are helping me to finally undserstand Stack Effect

Thank You ......Bud Poll

Asked by John Brooks
Posted Jan 31, 2012 11:11 AM ET


61 Answers

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Hi James
Actually reading my question again I agree with you. I did not word that very well.

Answered by terry grube
Posted Feb 5, 2012 4:53 PM ET


It's not hat I can't get past that people heat and cool their homes.
It's more a concern that by jumping "right past it" something important is being taken for granted.

Here's a few quotes from the comments in Allison's blog post:

Warm air moving up is indeed what we see, but warm air has no internal energy that makes it move up or down.

I'm not sure what "internal energy" means exactly, but there is definately energy embodied in the warm air... otherwise it would not be warm.

The objective here is to dispel the notion that warm air is leading the way in the process of convection, when in fact it is the cold air that initiates the process.

It is the application of energy that initiates the process. Otherwise, the interior would remain in thermal quasi-equilibrium with the exterior and there would be nothing interesting happening.

It is obvious that both must move, but the warm air cannot rise by itself and therefore cannot be credited with pulling the cold air in behind it.

No, air can't rise all by itself, it needs to be heated - using some form of energy.
Whether its pulling cold air or being pushed by cold air...
You can't have one without the other.
Gravity does some pushing while the energy it took to expand the warm air does some pulling.
Hence my earlier toungue-in-cheek comment about yin yang.

People heat or cool their homes for comfort but we maybe don't always appreciate the fact that by doing so we set in motion a chain of events and "unintended consequences".
"Stack effect" is just an "unintended consequence" of heating for comfort.
Likewise, "reverse stack effect" is an unintended consequence of cooling for comfort.

And I also agree that "discussion is good".

Answered by Lucas Durand - 7A
Posted Feb 5, 2012 5:38 PM ET
Edited Feb 5, 2012 9:14 PM ET.


I'm falling behind, but I will catch up. A couple of quick comments, it's half time.
If a high density gas and a low density gas are side by side and being pulled down by gravity, It is my perspective that the higher density gas will initiate the movement as the resulting direction is that of the heavier gas. The lower density gas was also pushing down, but ended up moving up. The consequences of this denser gas initiating the downward motion is that it does indeed create a pulling effect behind it, where as the motion of the less dense gas moving up is a result of an increase in pressure at the boundary between the two gasses, that pushing force.

I have tried to shift this statement to two gasses of different densities to separate the warm air, cold air issue from the fluid dynamics. With stack effect, heat is a contributing element, but only in expanding the cold air, not in determining the direction of motion. I realize that needs more clarification as the less dense gas is part of this motion, but the expanding process doesn't create the push that creates the motion.

I have more diagrams coming.


Answered by Bud Poll
Posted Feb 5, 2012 9:53 PM ET


Are we still talking about the same OP? 'Hot air rises' is simply an observation, and as such it is clearly neutral as to causative influence. It does not pretend to offer an explanation of the observed phenomenon, and explorations of what 'initiates' the motion, whether it's 'pulling' or 'pushing' that's at work, while interesting and possibly entertaining, do nothing to invalidate it. Just saying.

Answered by James Morgan
Posted Feb 5, 2012 11:26 PM ET


'Hot air rises' is simply an observation, and as such it is clearly neutral as to causative influence.

I agree.
I also think I am probably being pedantic.

As to whether we are on-topic for the OP...
I believe so.
John opened the door pretty wide - which I don't think is a bad thing.

Answered by Lucas Durand - 7A
Posted Feb 6, 2012 9:25 AM ET


The Thread Title and the "OQ" Original Quote were from Julius Sumner Miller.
He was a very passionate Student and Teacher.
I would rather watch his demonstrations than attend KahnAcademy.
He was a Student and longtime friend of Albert Einstein.
He spoke to all ages....
and he was very passionate about "words".
In my mind he is VERY credible......
There must be a reason for him to make such an outrageous statement???

Do we just "know better now"..... 50 years later?
Or was JSM "right" about most things and a little half-baked with some of his stuff?

Maybe he (JSM) is quoting his friend and teacher "Albert"... who knows??

Answered by John Brooks
Posted Feb 6, 2012 10:45 AM ET


Don't get so stuck in either-or thinking. There's always more than one way to describe a phenomenon.

I think a "word police" attempt to ban the phrase "hot air rises" is the silliest proposal to come down the pike in some time...

Answered by Martin Holladay
Posted Feb 6, 2012 10:50 AM ET


Bud, I am looking forward to seeing your new illustrations.
I don't think that you or Dr. Sumner Miller are "Silly"

John Straube's Illustrations are good... but I think yours are better

Answered by John Brooks
Posted Feb 7, 2012 10:32 AM ET


Hi John and thank you, I'm just a guest here so I'm trying to skip over those little jabs..
You said "There must be a reason for him to make such an outrageous statement???". In regards to JSM's, I have an opinion (a guess) as to why he might have said that. As a professor, a teacher, he would want to convey the "why" as well as the "what" is happening. There is nothing wrong with what "hot air rises" says, it is what it doesn't say that has left the public to fill in the blank. As I sit here watching the smoke rise from a chimney across the street, who could possible guess, beyond the well informed, that it is the cold air infiltrating the basement that is pushing that smoke up and out the chimney. Certainly not an obvious conclusion, but lacking even an occasional explanation, a foot note or some different wording, the general public has filled in that blank, but with the wrong information. The suggestion that attempting to correct this misunderstanding would only create confusion, I would suggest that the confusion is already in place and that an occasional foot note or change in wording would only add clarity to this process, a correction that is long overdue. If I can take the liberty to modify the JSM quote, the title to this thread, "What Else Must We Say?

John, I am making rather major changes to my web site with the diagrams on stack effect to try to be more complete. I will include a contact path to where I can be reached should I not make it back here.


Answered by Bud Poll
Posted Feb 8, 2012 11:05 AM ET


Bud & Others,
I am curious if you have found any references for the origin of the term "Stack Effect"?
Is it named after Chimney Stack or is Chimney Stack named after Stack Effect?

Answered by John Brooks
Posted Feb 8, 2012 12:24 PM ET
Edited Feb 8, 2012 12:24 PM ET.


Hi John, I have watched as I have searched for related information and I have seen "stack" referred to in old structures where each floor would have its own fireplace, thus one stacked above the other. I have also seen references to the layers of our atmosphere stacked up as clouds form. But nowhere have I seen either take credit as the origin of the term.
But I will keep watching :).


Answered by Bud Poll
Posted Feb 9, 2012 4:35 PM ET

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