Proceedings PaperOptical Discontinuities Produced By Raindrops Falling In The Ocean
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In 1875 Reynolds published a paper "On the action of the raindrops to calm the sea" where he showed that raindrops entering a water surface turned into vortex rings which propagated some 10 cm below the surface. In so doing, surface horizontal momentum was transported to lower levels thus reducing the velocity gradient and the tendency for short wave formation. The occurrence of such vortex rings is readily demonstrated as dyed drops ' mm diameter enter a water surface from a height of a few cm. Two aspects of this phenomenon are of importance in carrying over this simple demonstration to real raindrops falling in the ocean. First, raindrops are falling at terminal velocity as they reach the surface of the ocean, which increases with diameter up to a maximum of 9.1 m 5-1 for 6 mm drops; larger drops than this break up (Figure 1, Terminal Velocity). Second, the density of sea water relative to rain is 1.025 so that raindrops may experience upward buoyant force on entry. The purpose of the study reported here was to investigate the mechanism of the splash in relation to propagation of the raindrop as a vortex ring, and to assess the nature and persistence of the resulting optical discontinuity.