Overview

At this point in our step-by-step study of cloud formation, we assume that a set of cloud particles exists via aerosol activation or nucleation. Liquid cloud droplets likely formed by condensation of water vapor onto cloud condensation nuclei, and ice crystals may be present if the temperature were sufficiently low and ice nuclei were active. We ask next how rapidly and by what mechanism the particles grow under any given set of conditions (i.e., temperature, pressure, supersaturation).

In the most general sense, growth takes place in one of two ways: a particle may grow from the vapor phase, meaning that it increases in size molecule by molecule; or, a particle may grow by collecting other particles, in which case the particle grows in size particle by particle. Both categories of growth may operate simultaneously, but we treat them separately to facilitate presentation of the underlying physics. Growth by collection is discussed in the next chapter.

A net amount of water vapor deposits onto a surface whenever the partial pressure of vapor (e) exceeds the equilibrium value (eeq) of the surface. How rapidly the vapor deposits depends on the partial pressure excess (e - eeq), the temperature and nature of the surface, and the total air pressure. If the surface is liquid, the equilibrium vapor pressure is that of a solution droplet having the given temperature, solute concentration, and size.