On Hot Air

Although we don’t mean to make generalizations inaccurate, they are not the full story. After all, these statements are, by definition, generalizations. Sometimes people generalize the generalization, but failing to remember that continual generalization moves one further from the truth – thus eventually turning the generalization into misinformation that leads to misunderstandings.

Repeat after me – Hot air does not rise.

Just to make sure, let’s try again – Hot air does not rise.

Regardless of the links you can provide to the contrary, or ever provide a reference for a reputable textbook, hot air does not rise – it never has and I very much doubt ever will. Yet, how many times have we heard or said, Hot air rises? (Yes, I cringe when I hear TV weather forecasters use it.)

Nah baby nah … hot air does not levitate. Hot air does not move upward on its own – It is pushed upward by more dense cold air that displaces it – Just has bathtub water level moves upward on the sides when one takes their seat in a bathtub. However, there is no need for you to run down the street naked like Archimedes yelling, Eureka, eureka, I found it!

Here’s the story. During my science teaching days, we were preparing a lesson for the next day. Because my colleague had not seen this activity, he took one set of the materials to the location of the ice machine – the boys’ locker room.

My colleague returned saying it worked great, but shared an interesting story. One of the physical education (PE) teachers watched without knowing any details, and then said, “This means hot air doesn’t rise.” Bingo! Meanwhile, this gave me an opportunity to have some fun with the good-natured PE teacher.

During the eventually discussion with the students after the activity – and knowing students would quickly deliver my comment – I stated, “If a PE teacher like Mr. X can figure this out on his own, surely this is easy to understand.”

We had fun with it through the years, for as students are so predictable. But the point remains the same – Hot air doesn’t rise because it is pushed upward when it is displaced by more dense, colder air.

By the way, here’s a similar activity – but ours was better. Yo, Starla, show Navar.

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53 thoughts on “On Hot Air

  1. But wait … this proof comes from a CANADIAN source. You know we can’t trust them shifty Canucks. Next thing you know, they’ll be telling us World War 2 DIDN’T start with the Germans bombing Pearl Harbor! 😉
    Now if we could come up with a simple equation for how people mix at a party, or why something as heavy as incompetence always seems to rise to the top….. 😀

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    • John,
      The Canadian video was the best one that I found to demonstrate the point – but my version was better. Heck, with it, a clear boundary was easily seen (as a front) between cold and warm masses. As for people mixing at a party, well … that’s a different topic for a different day. Thanks for visiting.

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      • I’m STILL not convinced. Do you realise them crazy moose-folk actually claim they invaded Europe in 1942? Some dang fool story about Dieppe – not even CLOSE to Normandy. Shoot, they even claim to have fought in the air over Britain in 1940! Everybody KNOWS World War 2 didn’t start ’til 1941! 😉
        And their politicians actually make SENSE. Nope, ya just can’t trust them syrup-suckers…. 😀

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        • Actually, I’m being a VERY devious proponent of Canadians, and especially their services and sacrifices in World War 2. I have dropped hints about both their service in the Royal Air Force during the Battle of Britain, and (without mentioning my beloved Royal Hamilton Light Infantry) have tried to pique US curiosity in just what the heck Dieppe was. If even one of your readers learns about our noble and VERY brave neighbors to the north, I will consider my time VERY well spent!
          See, the idea is to get people to learn, without them thinking their learning. It’s an old teaching trick, so don’t tell anybody! 😉

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  2. This was interesting. But hot air still ends up being up higher than the cold air, even though it technically doesn’t rise, so you know people are still going to go ahead and say it rises. People are still going to cheat!

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    • Weebs,
      Absolutely people are going to cut corners, thus a way misconceptions occur. Just to show my flexible nature, I’m ok with this simplistic explanation: “Hot air rises because colder air pushes it up.” Thanks for commenting and getting it.

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  3. Awesome post. I love this type of stuff. Pretty cool video also. PE teachers always get a bum rap. Pretty funny story. Although I’m waiting to see a comment from a PE teacher who is also a science teacher (if there is such an “animal”). LOL.

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    • Mobius,
      Sure there are teachers who double in PE and science, but yes, not common. And to me, the PE teacher made the story. Meanwhile, glad you also found it interesting. Thanks for stopping by.

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  4. As you know, my apartment in a brownstone building lacks A/C. I live on the third floor in a hovel with a northern exposure. I’m friends with a tenant on the fifth floor that’s in the same A/C-less bind as me. Her garret has a southern exposure and it feels infinitely hotter than mine. If heat rising is a misconception, do you think her place is hotter due to the sun being more like a blast furnace from hell through her top floor window than mine in the back — which is just facing a rather large tree (personal note to you: no bags are in that tree, but a long time ago it had a towel hanging off a branch for at least a year)?

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    • Lame,
      Good one! Yes, her apartment is hotter for several reasons. 1) Warmer air being pushed upward. 2) The facing is a huge factor. For instance the AC in buildings facing the mid-afternoon sun are working like crazy! 3) You mentioned the building is made of stone – which commonly retain heat. Add the three factors together = YUK for your friend. Hope this makes sense … and suggest limoncello to your friend. Thanks for commenting.

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  5. I don’t have much of a science brain I’m afraid. I just know that heaters high up on a wall are useless because the heat rises (somehow) to the top of the ceiling leaving me cold around the ankles. xx

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    • Spiced,
      I remember you saying that you are science challenged, but you are closer than you think. 🙂 The hot air from the heaters above is less dense, thus it doesn’t sink to the floor (it stays high). Thanks for visiting.

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  6. Interesting. I learned something new. We’re in the middle of a heat wave, and I’m upstairs. I guess it’s hot up here because the cold air displaced it? Whatever the explanation, I need an air conditioner. 🙂

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    • Spinny,
      That a way because learning something new each day is important. And yes, whatever the reason doesn’t hide the need for AC. On a side note, we have a two-story home. During the summer, we set our thermostat lower by 2-3 degrees before going to bed. Hmmmmmmmmm …. Thanks for commenting.

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  7. Could we see the term ” hot air” in different contexts? A perception not supported by facts is also hot air. Does this hot air rise? Not necessarily. Even if more thought out or gounded”cold air were to try and push it up….

    Shakti

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    • Shakti,
      First of all, welcome first-time commenter … and all the way from Muscat, Oman! I posted a beautiful promotion video about the Tour of Oman bike race … stunning scenery!

      Because of the way you switched to another context, you seem to have a good sense of humor. Unfortunately, the inverse may be true because the those who don’t know seem to move toward those who don’t know, thus making it difficult for the grounded to make progress. Thanks for visiting and commenting — and I hope you return again.

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  8. Does this theory hold good for politics? I’m thinking that if might when you consider who has risen to the top positions and how much hot air they seem to be full of.

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    • Fasab,
      Interesting way to look at it. I was thinking about the smoother effect of political hot air – which must be very high density, but instead of extinguishing fires, it serves as an accelerant. Thanks for visiting.

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  9. Good Morning Frank :+) I will indeed show this to Navar! Thanks! Always fun here, nice video as well. Navar is starting to gear up for back to school. Another thing I’ve learned this week we had a propane leak from our stove while the guy was here with his beeping gas leak meeter. He said propane is heaver than air so it has a tendency to stay low to the ground. Any way hot air doesn’t rise and propane is heavy and stays low to the ground. Thanks for the shout out always appreciated.

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    • Starla,
      I see it’s been density week for you, so consider yourself lucky! 😉 One bit of a misconception to correct though. Propane is more dense than air, not heavier. For example. Will 5 pounds of oil float on 1 pound of water? Yes, because oil is less dense than water. Which of the two is heavier? Obviously 5 pounds of oil is heavier than one pound of water. Now, the next time your propane guy comes out, set the record straight. 🙂 Thanks for visiting.

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  10. Personally, I love the clarification! I have more fun with my not-yet-five year old granddaughter who has great curiosity about science. I’m not the best at teaching it, but when I can call her attention to something observable I do. I think I need to hang out close to videos like this one. We’re still working on water as liquid, ice and steam, but in time we’ll tackle density! I loved this! Debra

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  11. Having just gotten my upstairs a/c back after 26 days, I can say honestly I agree Hot Air does not rise it simply stays exactly where it is oppressing and weighted with drops of sweat.

    But, glad I learned something new!

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    • Val,
      Wow … concerning your situation, this post was perfect timing. I can’t imagine almost an entire month without AC – let alone in Texas! Nonetheless, great that it’s back! Thanks for visiting.

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    • Navar,
      I don’t know whose comment I mentioned in, but the way I used it, students could also see a defined frontal boundary between the two masses. I hope it works well for you. Thanks for stopping by.

      Like

  12. I appreciate the science, though in a colloquial sense I don’t know that most people generally are concerned with the actual mechanics. Much like no one cares that an ice cube doesn’t radiate cold, but absorbs heat.
    Pity, and an excellent explanation.

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    • Guapo,
      “Hot air rises” is a classic misconception that is perpetuated by simplifying a generalization. Simply put, the more one generalizes, the further away from the truth one gets … and possibly, ending up incorrect. This is one of my peeves, put working in the PE teacher added a twist. Thanks for catching up! 🙂

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  14. I’m not really enlightened (I disliked science very much) but you did remind me of an essay I wrote on Archimedes. The only reason the teacher (grade 4) did not give me an ‘A’ was because she thought my handwriting (no computer back then) was messy, thus I received a ‘B’.

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  15. A fine explanation of why hot air doesn’t rise, but rather is displaced. I submit that a similar misunderstanding is how a low pressure or a vacuum doesn’t suck, but rather is beset by external pressure. If you keep this stuff up, Frank, you may actually engage Archimedes’ lever (credit: AlwaysARedhead) against the mountain of science ignorance. 🙂

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  16. So, what did the PE teacher see?. I don’t see any video(s), just large white spaces; wish I could.

    hot air balloons don’t rise…hmmm. I guess birds and airplanes can’t “fly” either. They really produce an action of air moving downward behind the craft, due to the oft quoted difference in pressure of the curved wing surfaces, which has a corresponding upward reaction of the craft [a.k.a. action and opposite reaction].
    This is playing just a bit with semantics (obviously for effect) as I see it, and is simply a complete explanation for a common phrase used as a memory aid for those less educated in science. i.e.Hot air rises vs. hot air rises because…
    Hot air “going up” for whatever reason is still hot air “rising”.
    Those who want to understand more completely can find out why to a deeper level if they so choose and some have come here to do so.

    Don’t get me wrong, I’m all for better understanding of science, less bad science and seeing more YouTube junk videos gone.
    Regards, science advisor, Steve

    P.S. “Density week”… I like that.

    P.P.S.
    Propane being heaver that air is predicated on the understanding that a standard volume, temp and pressure is assumed by those well educated.

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    • Steve,
      Welcome first-time commenter. I just checked and the video is still there. Too bad you can’t see it.

      Generalizations and misconceptions lead to misunderstandings. Someone says hot air rises because it is pushed upward by colder air … I accept that because it shows a level of understanding. Similar to your propane comment, it’s not about the cold air’s weight, but it’s density. Therefore, we disagree because I don’t see it as semantics.

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  17. Okay, teach–so where is the cold air coming from that pushes/displaces the dang hot air up to my apartment in summer, where it remains?? My first floor neighbor doesn’t have AC, nor is he pointing fans toward his ceiling, my floor. I’m not arguing with you–I’d just like a fuller explanation for my significant discomfort (and I’d LOVE a super smart factual explanation so I can deflate this particular nabe–who truly thinks he knows everything, including that heat rises…).

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    • Morrow,
      Even in within your unit, the colder air is along the floor, thus the warmer at the ceiling … and the same for your first floor neighbor … but your temps are not the same as the neighbor.Given the warm/hot summer air outside, you are stuck with discomfort. … On the plus side, it can work to one’s advantage during cold times.

      Glad the explanation helps. Bottom line is that hot air doesn’t go up on its own.

      Like

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