In an environment with gravity, hot air is less dense than cool air, so it rises.

How does hot air interact with cold air in a zero gravity environment, in terms of movement? Does it just stay where it is? Some sort of spherical configuration, maybe with the hotter air on the outside?

For the sake of an example, imagine there's a candle burning in zero-g. I'm aware the flame will be spherical. How will the hot air around the flame act?

I found this Reddit discussion on the topic, but being Reddit there's not really a definitive answer like the kind I expect to see here.

  • $\begingroup$ Heat would expand the gas, sort of like stretching it, so I would expect a spherical expansion of the gas since there are no external acting forces. I am not an expert on thermo though, I'm sure someone else can expand more. $\endgroup$ – M Barbosa Jun 15 '16 at 0:14

In the presence of a gravitational field the hot air raises because it feels a buoyancy force. There is a difference of pressure between the bottom and the top of the hot air portion resulting in an upward force.

If there is no gravity, there is no difference of pressure (in principle) so the hot air does not raise. It just expands uniformly. If you are in an accelerating frame though you get a pressure gradient and the hot air will follow this direction even though there is no gravity.

  • $\begingroup$ Doesn't heating the air change it's density and therefore the pressure? I was never very good with fluid dynamics... $\endgroup$ – DCShannon Jun 15 '16 at 0:35
  • $\begingroup$ @DCShannon The pressure of the neighborhood (cold air) does not change and it remains uniform. So there is no net force acting on the hot air. $\endgroup$ – Diracology Jun 15 '16 at 0:37
  • $\begingroup$ Okay, so I think you're saying that the hot air expands, but since the air all around it is still uniform, and it expands spherically, there's equal pressure all around the "surface" of the sphere, so the air doesn't try to go anywhere in particular. Some cold air would mix in and get heated, but just by chance diffusion. So, unlike on Earth where the air above the flame tends to get more heated than the air on the sides or below, the surrounding air will become more heated in a uniform/random fashion. $\endgroup$ – DCShannon Jun 15 '16 at 0:43
  • $\begingroup$ @DCShannon Exactly what I meant. $\endgroup$ – Diracology Jun 15 '16 at 0:45

In addition to diffusion, you will see thermal conduction. Hot gas simply has more kinetic energy, so they move faster. The hot/fast molecules will travel in various directions away from the heat source and collide with the slower/colder molecules. Upon collision, there will be a transfer of momentum. Eventually everything will mix up to a thermal equilibrium. The following animations illustrate both diffusion and thermal conduction:

  • Video showing hot/cold gas mixing (watch first 15 seconds):


  • Video (45 sec) nicely demonstrates energy transfer of the collisions.


    Note: Molecules in this video are represented from slowest/coolest as blue/yellow to fastest/hottest as orange/red. At first, all molecules are blue/yellow. The right side of the container is a hot plate. As molecules touch the hot plate, they gain kinetic energy. All molecules are eventually converted into higher-energy/faster/hoter as denoted by orange/red.


As you points very well, in the lack of gravity there is no hydrostatical separation. I.e. it doesn't matter that the warmer air has a smaller density, there is no gravity which would separate them.

But, there are many other effects which would still work, because they aren't caused by the gravity. Most important is the diffusion:


Further, in the case of a lighting candle, not only the temperature will differ, but also the material composition around the light and further off. The burning candle converts the oxygene of the air to carbon dioxide and steam. The diffusion will mix them.

The result is that the candle still has enough oxygen to light, but much fewer:

Candle in space

  • $\begingroup$ Thanks for the answer, but I don't see a real conclusion here. Some diffusion occurs. Okay. How does the air move as a result, in relation to the surrounding cooler air? They just passively mix, the same as if the candle weren't still burning? The hot air moves away from the candle and cooler air moves in to get heated? What happens to the air? $\endgroup$ – DCShannon Jun 15 '16 at 0:34
  • $\begingroup$ @DCShannon Yes, they passively mix. This is the diffusion. And there is also a conduction of heat. But these are much slower processes as the hydrostatic separation in the presence of gravity, this is why the candle has much weaker light. $\endgroup$ – user259412 Jun 15 '16 at 0:39

What I can conclude is that in absence of gravity there is no buoyant force so no movement is possible physically but however hot air will start exchanging heat with the cooler air by means of conduction and radiation if they are in contact with each other and only by pure radiation if they are not in contact. ( no convection can takes place because of the absence of gravity) the time required to be in equilibrium can be calculated if mass of gases , volume, and the temperature are known .


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