= Density ratio
{disambiguate=double-diffusive convection}
In <double-diffusive convection>, a <density ratio (double-diffusive convection)> compares the buoyancy contributions from two stratifying quantities. A convention must be stated. For vertical coordinate $z$ increasing upward and warm salty water above cooler fresher water, take $R_\rho=\alpha T_z/(\beta S_z)$, with positive thermal expansion coefficient $\alpha$ and <haline contraction coefficient> $\beta$.
The linear equation of state gives $\rho_z=\rho_0(-\alpha T_z+\beta S_z)$. Thus $R_\rho>1$ makes the combined density stratification stable, even though the salt contribution alone is destabilizing. Faster heat diffusion can still enable <salt fingering>. The reciprocal ratio is also used in the literature, so a numerical density ratio is meaningless without its convention.<The Salt-Fountain and Thermohaline Convection by Melvin E. Stern>[Warm saline water above cooler fresh water and differential heat/salt transport]{ref}
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