Do gas molecules align with gravity?

[Loren Booda]

In a gas, do the molecular centers of gravity align significantly toward the dominant source of gravity? E. g., do atmospheric water vapor molecules tend to have their oxygen atoms facing Earth's center?

 

[Mapes]

It's interesting to consider the energy penalty of a water molecule being "upside down"; that is, with the oxygen atom on top. When I multiply the mass of an oxygen molecule by the force of gravity by the difference in height between the two configurations, I get about 10^-35 J. I'll leave it to you to compare that with the average rotational energy of a water molecule to determine if gravity has any considerable influence on orientation.

 

[Loren Booda]

My initial guess is that such a molecule would have the average rotational energy of kT/2=10^-35 J. So T=10^-12 K, about the current limit of cryogenic technology. The influence is negligible.