The effect of turbulence within a cloud on the distribution of the sizes of droplets growing by condensation has been examined in some detail with the aid of a scheme for simulating a fluctuating updraught numerically. It is found that, contrary to the conclusions previously reached by some Russian and Chinese workers, the spread in the radii of droplets at any given level which this mechanism produces is very small (generally < 0.2 γm). This result agrees with the conclusion of a similar analysis carried out by Warner.

Other factors which may account for the observed distributions of droplet sizes have been considered and the most important appear to be mixing between a cloud and its environment and the variable composition of condensation nuclei. It also appears that the absorption of organic surface active substances on to some condensation nuclei probably accounts for the existence of small droplets in the interiors of well-developed clouds.