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An internet search will find many people claiming that Humid Air (water vapor) rises. However, I am skeptical because if lighter molecules rose then, it follows that the air would be stratified by molecular weight.

Facts: Water vapor has molecular weight = 18 Nitrogen has molecular weight = 28 Oxygen has molecular weight = 32

So, Water Vapor has the least mass.

However, If lighter gases rose then (using the above data) Nitrogen would rise Oxygen would fall and we would be breathing 99% Oxygen (neglecting the 1% of other gasses) this is not true So I believe that gasses do not behave this way as illustrated by the fact that we breathe 78% nitrogen and 21% oxygen Therefore, it seems to me that (in a closed environment) gasses mix randomly, with perhaps a very small gravitational contribution.

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You have to look at Boltzmann factors:

$$ e^{-\frac{E}{kT}} = e^{-\frac{mgh}{kT}}$$

for $m=18u$ and $m=28u$. Roughly at 300K and uniform $g$. For 1 amu, the scale height is:

$$ h \approx \frac{kT}{mg} = \frac{4\times 10^{-21}J}{1.6\times10^{-26}J/m}=250,000m$$

Divide that by 18 or 28 your get a scale height on the order of 10-14km, which is the scale height of the atmosphere. Hence, you don't see any stratification locally.

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you are right- at room temperature, the different constituents of air are well-mixed and tend to stay that way. However, gases like butane and carbon dioxide are dense enough relative to air that they will indeed settle toward the floor of a room. regarding humid air, it does indeed have less density than dry air at the same temperature. Airplane pilots know that humid air produces slightly less lift than dry air for that reason.

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  • $\begingroup$ Even for butane (mass 58 daltons) or heavier gases like sulfur hexafluoride, the Boltzmann scale height is several kilometers. Room-level vertical striations are not long-term stable and don't occur spontaneously. $\endgroup$ – rob Nov 22 '18 at 16:46
  • $\begingroup$ Rob, out here in oregon we have marijuana labs that process THC out of plant matter using butane as a solvent. these illicit operations blow up with alarming regularity due to gas enrichment at low spots in the lab space, like crawlspaces and sumps. Anything at floor level that produces a spark will set one of these off which means that the operators have to install floor-level vents and fans to exhaust the leakage. $\endgroup$ – niels nielsen Nov 22 '18 at 18:44
  • $\begingroup$ +1, but I think the reason there are variations in the concentration of water vapor, butane, or carbon dioxide and not in oxygen and nitrogen have to do with the sources of these gasses. $\endgroup$ – Ben51 Nov 22 '18 at 18:57
  • $\begingroup$ @ben, that is a testable hypothesis. I propose buying 10 singlewide mobile homes with ineptly-enclosed carports, setting up THC/butane labs in the carports, hiring the requisite number of enterprising but stoned hippies to run them, and take data... $\endgroup$ – niels nielsen Nov 22 '18 at 19:02
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There are no sources or sinks of oxygen or nitrogen to speak of, so there are no fluctuations in the ratios of these gasses to speak of. Water vapor, on the other hand, is constantly being added and removed--air can be dried by cooling it to condense out the water, and then reheated, for example. It is quite possible to have humid air interacting with dry air of the same temperature. In that case, until the two air masses mix, the humid one will tend to rise above the dry one.

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