To measure the falling velocities, the diameters and the aspect ratios of raindrops, consecutive images of raindrops in the open air outside of our laboratory were captured by an ultra-high-speed video camera. For automatic image capturing and avoiding the damage to the image sensor of the video camera due to the strong continuous illumination, a new trigger device was newly introduced to the capturing setup.
As a result of the image analysis of the falling raindrops images, the following findings were obtained, when the equivalent raindrop diameter is smaller than 3mm. (1)The relationship between the falling velocity and the raindrop diameter determined by our outdoor measurement is similar to that between the terminal velocity and the water drop diameter determined by Gunn and Kinzer's indoor experiment. (2)The aspect ratios of raindrops determined by our measurement is similar to that calculated by Pruppacherr's model of raindrop.