I assume dry and moist air are different in density. Is that difference large enough to make a balloon float?

I mean moist air in terms of relative humidity, as is used when talking about weather.

That depends on the material and geometry of the balloon, but is it even worth trying?

I think a simple balloon made of rubber as used as toy for children may create pressure, which is probably not helpful.

Is it worth trying to make a balloon containing completely dry air float in completely wet air, or the opposite of it?


2 Answers 2


Well, a balloon filled with wet air could float in dry air. This is because wet air is made of water vapor which has a lower density than dry air. However, in order for there to be a significant enough density different for the balloon to float the temperature would have to be extremely high. You can take a look at the image in this link to see.


  • $\begingroup$ Oh, so my basic assumption was wrong! Would you mind if I remove it? Or should I turn the question around to moist air in dry air? That would invalidate your answer, unfortunately. $\endgroup$ Nov 25, 2019 at 1:18
  • $\begingroup$ It's your choice really $\endgroup$ Nov 25, 2019 at 1:21
  • $\begingroup$ I made an edit, your answer stays valid. I am really surprised that vapor is less dense than air. I mean maximally moist air in terms of relative humidity, not sure that makes a difference for your answer, what is used when talking about weather. $\endgroup$ Nov 25, 2019 at 1:30
  • $\begingroup$ It's fine, I fixed it anyways. $\endgroup$ Nov 25, 2019 at 1:37
  • $\begingroup$ These are good answers, I just want to point out that a few droplets of water from condensation would quickly increase weight, thus the high temps Vincent mentions. $\endgroup$ Nov 25, 2019 at 3:24

The dry air is denser than the air with water vapor (water molecules in the gaseous state).

Dry air is composed primarily of oxygen $O_2$ gas, molecular wt 32, and nitrogen gas, $N_2$, molecular wt 28. Water vapor is $H_{2}O$ gas, molecular wt 18. Therefore when lighter water vapor molecules replace heavier oxygen or nitrogen gas molecules, the density decreases.

Hope this helps.


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