Field observations of multimode raindrop oscillations by high-speed imaging

Periodic oscillations of raindrops falling at terminal velocity in natural rain are visualized for the first time by high-speed imaging. These images show the existence of an oscillation mode with the same frequency as the fundamental harmonic, but with shape different than that predicted by linear theory. These oscillations cause a lateral drift with a speed of approximately 20%-30% of the drop terminal velocity and without a preferred direction. These experimental observations serve as an insightful illustration of the potential benefit of applying high-speed imaging technology to investigate the dynamical microstructure of rainfall at the raindrop scale.