The Growth of Cloud Drops by Condensation. I. General Characteristics

Abstract

Condensation in 'cloud is studied to demonstrate certain general characteristics, without special assumptions about the condensation nuclei or the vertical velocity. The equation of growth of a droplet is constructed, taking account of ventilation due to its falling velocity. Lifting is assumed to occur adiabatically. An approximate value is found for the supersaturation of cloud air, which under certain conditions can be computed from observable quantities-the cloud droplet spectrum and the vertical velocity. Growth of drops at small sizes is very rapid : at constant supersaturation, in a period of a few seconds, a drop in general either grows beyond r= 1μ, or else comes so close to equilibrium at some smaller size that growth practically ceases. When the supersaturation is moderate or large, a threshold effect appears : the size of the condensation nucleus determines whether or not the drop shall grow, but, if it does, has little influence on its final size. The definition of the size of a small condensation nucleus is reviewed and a more precise interpretation is suggested.