Experiments on the Generation of Raindrop-Size Distributions by Drop Breakup

An artificial rain column was generated by letting a continuous stream of water fall under zero pressure from an open hose. Within a meter of fall the stream broke into large drops. These large drops further subdivided, and interaction between the drops was noted during the remaining 60 m of fall. At the bottom of the rain column drop-size distributions were measured at intensities that ranged from 190–1900 mm hr−1. Over this range the largest drops were about 9 mm in diameter, and the number of these drops per cubic meter was constant. For smaller drops, however, the concentration increased with intensity. Drops ≳5 mm are unstable. It is felt, had the rain column fallen several hundred meters more, that these drops would have broken up, causing an increase in the slope of the drop distribution. Calculations suggest that an exponential type distribution, similar to that found in natural rain, would have been produced. This lends credence to the hypothesis that in heavy rain the shape of the drop-size distribution is determined by drop collisions and breakup.