The recent paper by Johnson [1993] draws attention to the two stages of rain formation through coalescence of water drops. The first stage involves the production of embryos large enough to start the collision-coalescence process, while the second stage considers the environmental factors necessary for growth to precipitation size.
Traditionally, cloud physicists have focussed on the former problem, as outlined in the recent overview article by Beard and Ochs [1993]. As Johnson [1993] points out, the existence of embryos large enough to initiate coalescence does not necessarily mean that a cloud will develop precipitation. In the following we consider recent developments in both these areas.