Overview

The particles constituting a cloud, once they have grown large enough, often collide with other particles. Initial growth from the vapor causes cloud particles to acquire mass and increasingly come under the influence of the Earth's gravitational attraction. With new mass, the particles fall ever faster relative to the air in their immediate environments. At some point, as faster particles overtake slower particles, the inertia suffices to cause them to collide and possibly stick together. The growth of one particle by the collection of others gives that particle favored status (by virtue of its new mass and increased fallspeed) as a collector of still other particles. Growth by gravitational collection is an important class of mechanisms responsible for the development of rain and snow.

Cloud particles can be either liquid or solid, and they can be small or large. We therefore find it useful to devise a set of categories by which particles grow, as shown in Fig. 9.1. Particles growing from the vapor do so as individual members of the cloud, but particles growing by collection always involve pairs. When both members of an interacting pair are liquid, the process is called collision-coalescence. The larger (“collector”) drop of a liquid–liquid pair collects one or more smaller drops, because of the difference in fallspeeds, and leads to the growth of the collector at the expense of the collected drops.