 Thermal and compositional interfacial buoyancy criterion

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.