[forkandwait]

Isn't density not mass the issue? Methane still floats above air using density from the internets, though I wonder about mixing

0.656 kg/m³ methane

1.98 kg/m3 carbon dioxide

1.225 kg/m3 air.

[teraflop]

Atmospheric gases under typical conditions behave pretty much like ideal gases, which means it doesn't matter whether you talk about density or molecular mass; they're directly proportional to each other. And the molecular mass is independent of temperature and pressure.