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Precipitation prediction using weather radars requires detailed knowledge of the shape parameters of raindrops falling ar their terminal velocities in air. Because the raindrops undergo oscillation, the most important shape parameters from the radar prediction point of view are the equilibrium drop shape, the time-averaged axis ratio, and the oscillation frequency. These parameters for individual water drops with equivalent diameter from 2.5 to 7.5 mm were investigated in a vertical wind tunnel using high-speed video imaging. A very good agreement was found between the measured and the theoretically determined raindrop shape calculated by a force balance model. A new method was developed to determine the equivalent drop diameter with the help of the oscillation frequency. The drop size determination by means of the frequency method was found to be three times more precise than by volumetric methods. The time-averaged axis ratio was found to be equal to the equilibrium axis ratio in the investigated raindrop size range. The analysis of the oscillation frequency of the raindrops revealed that the drops undergo multimode oscillations and are oscillating in a transverse mode in addition to ah axisymmetric oblate-prolate mode. Experiments are included in which the infernal circulation associated with drop oscillation was investigated and compared to theory.

An expression is derived for the total volume of rain absorbed in terms of the speed of the traveler. It will be helpful for a calculus class to show that maximums and minimums are not always at the points where the slope of the function is zero.