The theory of heterogeneous nucleation on partially-wettable insoluble spherical particles as developed by Fletcher is used to obtain relations readily applicable in the meteorologically important limit of low supersaturation ratio S. Taking 1.03 as the probable absolute maximum value of S in natural clouds, it is found that activation of a particle demands that the contact angle θ be less than 12°. Data from surface chemistry indicate that this θ-limit separates airborne particulates into a small class of active nucleating substances and a remaining broad class of nonactive cases. However, roughness effects (Wenzel's rule) may shift some from the latter to the former class, a trend possibly offset under special conditions (e.g., over industrial areas) by adsorption of non-wetting layers. A simple physical basis for understanding nucleation enhancement in terms of reduced size of the critical embryo is suggested, and a rule for predicting the approximate size-limits for activation of fully wettable particles of irregular shape is deduced. The silicate dusts comprise one important class of wettable atmospheric particulates, but it is concluded that typical clouds characterized by average values of maximum S (≐1.001) can activate only silicates of size larger than a few microns. The most favorable conditions (maritime cumuli, strong updrafts) may lead to activation of dusts with maximum diameters just under 0.1 μ, contributing to the global precipitation-scavenging of the silicate dust component of the atmospheric aerosol, a contribution especially important for those in the gap between the particle sizes scavenged by Brownian diffusion on cloud droplets and sizes scavenged by aerodynamic impaction by raindrops.