A Physical Explanation of the Hollow Structure of Waterspout Tubes

Observations of the distribution of minute foreign particles introduced into a small, mechanically produced air vortex are briefly described. Ideas that grew from these observations are developed quantitatively into a physical explanation of the hollow structure of waterspouts that depends on the balance of centrifugal and drag forces acting on condensed water particles. It is also suggested that the mechanism may operate in tornado vortices.