= Thermal and compositional interfacial buoyancy criterion
{title2=$T_-/T_+>M_-/M_+$}
For two <ideal gas> layers at a common interfacial <pressure>, the lower layer has smaller <density> precisely when $M_-/T_-<M_+/T_+$, where $M$ is mean <molar mass>. That ordering is unstable to an infinitesimal interchange. A heavy condensable constituent can therefore inhibit <thermal convection> in a light background gas even when the lower layer is hotter. This local criterion assumes no phase change during the initial displacement.
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