Brunt–Väisälä RDI in a stratified fluid: solid lines show numerically calculated growth rates for differently sized grains, specified by the normalized stopping time ﹩{\bar{t}}_{s}\equiv \langle {t}_{s}\rangle {N}_{\mathrm{BV}}﹩. We set ﹩{{\boldsymbol{w}}}_{s}﹩ to the “natural” settling of grains due to gravity, ﹩{{\boldsymbol{w}}}_{s}={\boldsymbol{g}}\langle {t}_{s}\rangle ﹩ (﹩{\theta }_{w}=0﹩), assume Epstein drag (9) with μ = 0.1, and set k_z = k/2, ﹩{k}_{\perp }=\sqrt{3/4}\,k﹩. The black crosses show the RDI (18) at resonance, k = k_res. Smaller grains excite smaller-scale oscillations because they settle more slowly (﹩{V}_{\mathrm{wave}}\propto {N}_{\mathrm{BV}}/k\propto {w}_{s}﹩), but ﹩\mathrm{Im}(\omega )﹩ is independent of ﹩\langle {t}_{s}\rangle ﹩ when ﹩{w}_{s}\propto \langle {t}_{s}\rangle ﹩. Because grains move through the atmosphere over a timescale ﹩{t}_{\mathrm{settle}}\sim {L}_{\rho }/{w}_{s}﹩, the RDI grows sufficiently fast to clump grains (as observed in Lambrechts et al. 2016) if ﹩\mathrm{Im}(\omega )/{N}_{\mathrm{BV}}\gtrsim {\bar{t}}_{s}=\langle {t}_{s}\rangle {N}_{\mathrm{BV}}﹩.