# How does hot air act in zero gravity?

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.

• 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. Commented Jun 15, 2016 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.

• Doesn't heating the air change it's density and therefore the pressure? I was never very good with fluid dynamics... Commented Jun 15, 2016 at 0:35
• @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. Commented Jun 15, 2016 at 0:37
• 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. Commented Jun 15, 2016 at 0:43
• @DCShannon Exactly what I meant. Commented Jun 15, 2016 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:

• 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? Commented Jun 15, 2016 at 0:34
• @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. Commented Jun 15, 2016 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 .

Im not sure i agree entirely. Where there is air and temperature differences there is movement. Im not sure what an air current does in absence of gravity What shape it takes or how that shape changes with movement snd direction but i think the equal sphere of heat or pressure could only happen in s motionless lifeless controlled situation. I think the currents and motion of warm and cold wiuld cause places in the sphere to spill outwardly or get pushed away in streams or collapse or cool and cause a build up or back up which will puddle until it reaches s temperature or current change so it would move more like s lava lamp whatever it does without gravity but a tiny current could cause all kinds of reactions. Where warm and cold meet there are swirling eddies not just expansion. What is the sperical equivalent to a collum of rising hot air? A rolling mushroom or a torus like rolling a rubber band or o-ring along a cylinder?

• Fire causes its own weather system flamenados and powerful winds what would that look like with no gravity. Some effects of current movement thermo transfer etc would not change snd some would i also think some processes may not exist and other may only exist in that environment. A small match flame is spherical but what about a giant flame beyond that suface tention of a bubble flame. Think about proportions like with fluids Commented Dec 18, 2020 at 4:26