From the examination of a large number of droplet samples in cumulus clouds it is found that many droplet-size distributions have neither positive skewness nor a single mode. Bimodal distributions increase in frequency with height above cloud base and with decreasing stability in the cloud environment, and it is suggested that they are produced by mixing between the cloud and its environment. The relative uniformity observed in the shape of the spectrum at a constant level across the cloud, and the fact that bimodal distributions are not confined to the cloud edges, lead to the belief that entrainment inward from the cloud edges is not dominant; rather, it is the mixing at the growing cloud top as it builds which determines the shape of the droplet spectrum.

This content is only available as a PDF.