= Köhler activation radius
{c}
{title2=$r_c=\sqrt{3B/A},\quad\ln S_c=2A/(3r_c)$}
In the dilute radius form $\ln S_{\rm eq}=A/r-B/r^3$, 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.
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