On the phenomenon of nuclei enhancement during the evaporative stage of a cloud1

The number of nuclei observed just after a cloud evaporates is classically equal to the sum of the number of interstitial nuclei plus the droplet number density. There are occasional reports of more nuclei produced in the vicinity of evaporating cloud droplets than the classical amount. The purpose of our paper is to provide an overview of the phenomenon of nonclassical nuclei production, with the ultimate purpose of laying the foundation for future efforts designed to formalize the mechanism that explains this phenomenon. The available literature is discussed in conjunction with field and laboratory measurements and suggests that a nonclassical generation of particles depends on the (i) relative humidity of the air surrounding the droplets, RH, and (ii) the physical and chemical characteristics of the evaporating clouds. Implicit within the available literature is the importance of the temporal and spatial resolution of sampling during the final stage of a cloud cycle and shortly thereafter. A possible explanatory mechanism is presented and is `naturallyʹ preliminary.