Coalescence of raindrops and cloud droplets has been studied in the laboratory by observing the growth of water drops when falling through a three-meter column of cloud. The increase in mass of the drop is expressed in terms of a collection efficiency, defined as the fraction of the cloud water in the path swept out by the drop, which is actually picked up. Collection efficiencies for a drop of 3-mm diameter falling through three clouds of different size-distributions are found to be in agreement with the aerodynamic collision efficiencies calculated according to a theory by Langmuir (1948). The theory and his assumption that collision always results in coalescence are thus substantiated.
The effect on coalescence of charge on the drops, comparable to that observed on raindrops in nature, is found to be small.