Köhler activation radius ($r_c=\sqrt{3B/A},\ln S_c=2A/(3r_c)$)

In the dilute radius form $\ln S_{\mathrm{eq}}=A/r-B/r^3$, the curvature coefficient A has length units and B has volume units. Setting its derivative to zero gives the displayed maximum. Ambient supersaturation above it permits activation into a growing cloud droplet. Below it, a sufficiently small wet equilibrium can be stable, while the larger-radius intersection is unstable. The approximation should not be extrapolated into the dry core.