Droplet freezing by surface nucleation has been observed during the course of studies of ice nucleation in our laboratories. A constant rate of cooling apparatus and a specially constructed cold chamber were used for this study. Silver iodide particles sprinkled on supercooled, millimeter-size water drops are effective in freezing the drops at −5C. Particles of naturally occurring silicates were found to be effective in the range −7 to −10C. Thus, the ice nucleating ability of such dry particles is much greater by the surface contact mechanism than when particles are embedded in the drops.
The importance of the surface nucleation mechanism in forming solid hydrometeors in cumulus clouds is assessed.