An inherent precipitation mechanism in cumulus clouds

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Abstract
In a fair‐weather cumulus, the largest droplets, which are of 18 microns radius, grow too slowly by differential settling to become precipitation (although the time to reach raindrop size from 50 μ, for instance, would be comparatively short). Even at 18 μ their terminal speed is sufficient for growth, were it not that most of the droplets they encounter are small enough to avoid capture. Using Langmuir's collision efficiencies, the calculated time to grow from 18 to 50 μ radius is 2 hr. This time would be reduced to 12 min if the cloud sample had first ascended until there were 4 g of condensed water per kg of air, as must take place in the updraught of a cumulus congestus.Condensation and coalescence actually occur simultaneously and the first 50 μ droplets appear at about the 6 g/kg level; this is about the level at which the first radar echo should, and does, appear. The apparent descent of the echo can be accounted for by droplet growth at lower levels. The rapid development of precipitation in cumulus can be explained thus without invoking giant nuclei or ice.The intensity required for turbulence to be a potent coalescing agency was found to be greater by a factor ten than the intensity of turbulence deduced from aircraft measurements.

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