Why does hot air really go up?

[michelcolman]

That said, speaking in terms of "billiard balls", where each ball represents one molecule, the hotter molecules are balls of the same mass, yet a larger diameter than the cooler ones.

That is not correct. This may be a useful way of looking at solids (vibrating molecules approximated by larger balls) but for gasses, all that changes is the velocity of the molecules. They are all the same size. Hot air expands because if you put a volume of hot air next to a volume of cold air at the same density, the hot (fast, energetic) molecules will push the cold ones away until the pressure reaches an equilibrium with the hot air less dense than the cold air (less molecules times more speed makes the same pressure).

 

[michelcolman]

This notion is WRONG. Warm air is not being "pushed up". It rises because masses of warm air are more buoyant because it is less dense. Do you have some alternative physics in which an envelope of hydrogen gas is being "pushed up"? Please don't mislead young people who might come here to learn.

And how do you explain buouancy then? O, are you one of those people that thinks objects become lighter because of buoyancy? I'm afraid I'm going to have to disappoint you, the objects keep the same weight, they are pushed up by the differential pressure, a higher pressure below and a lower pressure above.. For example in water, the higher pressure below the object is just enough to support a similar volume of water (otherwise the water itself would not be stable). If the object is lighter than that, the pressure differential will exceed its weight and... guess what... it will be PUSHED UP.

Please don't mislead young people by suggesting buoyancy makes things lighter. They are pushed up by the different pressures above and below.

Of course when we start talking about gasses, at a microscopic level, we are no longer talking about large objects but rather about statistical distributions of speeds over quadrillions of molecules. But it turns out there's still a lot of pushing going on, just a lot of small pushes instead of one big one. The end result is coincidentally very similar.

 

[Dadface]

As I stated in a previous post ,the geometry and structure of the surroundings have a large effect on the net movement of the air.Consider an imaginary, gravitational free environment where there is a source of heat and where the surrounding are so positioned that their effects are negligible.The heat will spread from that source, by the mechanisms outlined above, but the spread will be multidirectional.If the same source is now close to the Earth's surface the downward spread is restricted by the solid/liquid parts of the Earth (and any other obstacles that may be around).The Earth will heat up as a result and if some of this heat is transferred back to the atmosphere the net movement will be upwards, any downward flow heating up the deeper layers of the earth.If the same heat source is taken to a higher altitude there will be less restriction on the downward flow.A fuller analysis will bring gravity and other factors into account.

 

[Dark Horse]

There are some really nice pics of what a candle looks like burning in Zero-G at
http://science.nasa.gov/headlines/y2000/ast12may_1.htm
http://chemistry.about.com/od/chemistryfaqs/f/firegravity.htm

A microgravity flame forms a sphere surrounding the wick. Diffusion feeds the flame with oxygen and allows carbon dioxide to move away from the point of combustion, so the rate of burning is slowed. The flame of a candle burned in microgravity is an almost invisible blue color (video cameras on Mir could not detect the blue color). Experiments on Skylab and Mir indicate the temperature of the flame is too low for the yellow color seen on Earth.

Smoke and soot production is different for candles and other forms of fire in space or zero gravity compared to candles on earth. Unless air flow is available, the slower gas exchange from diffusion can produce a soot-free flame. However, when burning stops at the tip of the flame, soot production begins. Soot and smoke production depends on the fuel flow rate.

 

[Dark Horse]

Firstly classic thermodynamics, hot air expands which takes up a greater volume. The given mass for that volume is less than the surrounding air hence it rises.

At the molecular level this makes no sense why a hot molecule which weighs the same as a cold molecule should rise.

Anyway my theory is you can't consider it as single molecules because the millions of random collisions the individual hot molecules are coupled with all those molecules around them so rather acting as individuals they are really acting as a macroscopic entity and do have as an entity less mass and hence rises.
(its probably something really subtle like the rings and spaces of saturn, I wonder if you could simulate it, the hot air that is not the rings)

 

[Cleonis]

Warm air is not being "pushed up". It rises because masses of warm air are more buoyant because it is less dense. Do you have some alternative physics in which an envelope of hydrogen gas is being "pushed up"?

In earlier replies to you Jeff Reid and michelcolman have pointed out the physics of buoyancy:
- https://www.physicsforums.com/showpost.php?p=2332228&postcount=103
- https://www.physicsforums.com/showpost.php?p=2332327&postcount=108

Obviously I concur with them, and for good measure I give my version here.

For buoyancy effects the atmosphere behaves like a fluid, so I will use the example of buoyancy in the case of a fluid. If you submerge a cube of ice it will move to the surface again, rising up above the surface until the point of neutral buoyancy is reached. Let's say the ice-cube 1 meter on each side; a volume of 1 cubic meter, and that initially it is pushed down to a depth of 2 meters. The top of the cube is supporting a watercolumn of 1 meter high, the waterlevel below the cube is supporting a watercolumn of 2 meters. Hence the force exerted on the bottom of the ice-cube is stronger than the force that is exerted on the top of the ice-cube. the net upwards force that the ice-cube is subject to is the amount of force that is required for neutral buoyancy of a cubic meter of water. The ice-cube is less dense than the surrounding water so it does not have enough weight to keep the water below it from pushing it up.


Cleonis

 

[Gerenuk]

Let me start with quote devoted to all pseudo-scientists that are unwilling to break out of their textbook scope and who do not understand the initial question:
"A man cannot learn what he thinks he has already learned"

I admittedly haven't read the thread - too much spam. Here is my simple proposal for the answer:
Obviously gravity is the key factor as without gravity air would just equilibrate temperature.
In an environment of some density all molecules will collide after some mean path. As faster molecules spent less time for this path, they acquire less of the gravitational drag downwards while traveling the same path. Therefore they are able to reach higher elevations "before falling back".
Maybe that can explain it.

I hope Michel can update us with his own thoughts.

 

[joelarc]

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.[/Gravity makes any fluid medium have more pressure at the bottom layers. This pressure is greater at the bottom of the balloon than on top as you can easily calculate. If the mass within the bubble is less than the force this pressure difference generates, the balloon rises. It is the same effect which underlies Archemdes principle.]

 

[A.T.]

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.

Try to think the other way around. The cold surrounding air is pulled down by gravity under the hot air and pushes it up.

 

[joelarc]

Consider a pool of water. At any depth there is a pressure which depends on the depth. Now consider a flat slab of material or arbitrary density and immerse it to a any depth. Assume it horizontally orientated and calculate the difference in pressure on the top and bottom. Now calculate the force on the slab useng Newton. You'll see that if the density of the material is different than that of water, it will rise or fall and it it's the same, it will stay immobile. Now imagine that water is replaced by air of a given density and go through exactly the same calculation and see that you get a similar result, it the density is more or less than air density the obfect either falls or rises. Heating air makes it less dense. The atmospheric case is a little more complicated than a pool of water but the end effect is the same: the atmosphere is pushed down by gravity causing pressure. It is this pressure that caused hot air to rise.

 

[joelarc]

Ok, now that we know it's pressure pushing the hot air up, what happens after that? The balloon starts going up and the place it left is immediately replaced by the ambient air because as the balloon vacates it's place, the only thing that can replace the small volume it left per unit time vacated is the ambient air. If the "balloon" of hot air is hot enough and a large enough volume of air, it goes up very quickly. The ambient air is forced to resupply air at a very high rate. The demand for air is so high that an "engine" is formed which cycles air from the stratosphere to supply it in an up-down fashion. . It is this effect that creates thunderstorms. If the area being heated is larger still and the heating source can supply enough heat to keep it going, it turns into a a hurricane because the thing is so massive that the Coriolis force has enough time to act on it and turn it into something quite different that a thunderstorm.

As you can see, hot air rising casts a very, very large shadow. And we haven't even discussed the same kind of pressure difference being responsible for the Archimedes principle which applies to another fluid, namely water, well, usually.