= Fingering convection
<Fingering convection> is a form of <double-diffusive convection> in which a rapidly diffusing stabilizing contribution to <buoyancy> competes with a more slowly diffusing destabilizing contribution. The combined <mass density> stratification can resist an ordinary vertical displacement, yet unequal <diffusion> permits narrow rising and sinking structures to grow.
A displaced parcel initially carries both contributions. If the restoring anomaly diffuses away faster than the destabilizing anomaly, the remaining <buoyancy> reinforces its motion. In the common thermal-compositional case, <thermal diffusivity> exceeds <solutal diffusivity>; the initial stability of the combined <mass density> profile is therefore insufficient to decide stability once <diffusion> acts.<Double-diffusive processes in stellar astrophysics by Pascale Garaud>[Section 2.2.1: fingering regime, density ratio and diffusivity contrast]{ref}
In seawater, <salt fingering> realizes this competition with <temperature> and <salinity>: the thermal contribution stabilizes while salt-rich material above fresher material supplies the destabilizing contribution. Its laboratory examples make the different transport rates visible.<The Salt-Fountain and Thermohaline Convection by Melvin E. Stern>[Warm saline water above cooler fresh water and unequal thermal/salt transport]{ref}
In a star, <chemical composition> can play the slower role. A larger <mean molecular weight> tends to increase <mass density> at fixed pressure and temperature, so a molecular-weight inversion can oppose the thermal restoring effect. Rapid thermal transport can then permit fingering even when the total local stratification is stable.<Double-diffusive processes in stellar astrophysics by Pascale Garaud>[Sections 1–2: inverse composition gradients and fingering instability]{ref}
The <density ratio (double-diffusive convection)> measures the competition, and the <salt-finger instability criterion> shows why there is also an upper limit to the stabilizing contribution: if it is too strong, unequal <diffusion> no longer produces growing modes. The <elevator mode (double-diffusive convection)> gives a particularly simple local pattern with vertical motion and no height dependence.
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