Cloud Nucleation on Insoluble Particles

Abstract

Abstract 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 wettab...