Abstract
It is already known that the observed terminal velocity of falling raindrops with a diameter larger than 1mm is considerably smaller than that which may be expected from the drag coefficient of a sphere. This fact is generally attributed to its deformation due to the non-uniform distribution of pressure upon the surface of the drop.
In this paper, the author shows theoretically that, if the flow around the drop is a potential one and the deformation is small, the shape of a falling raindrop is an oblate spheroid whose axis is vertical. He then computes the fall velocity, on the assumption that the increase in cross area due to the deformation only causes the decrease in fall velocity, and that the drag coefficient of an oblate spheroid with small eccentricity is approximately equal to that of a sphere whose radius is equal to the major radius of the former. The computed values of fall velocity agree very well with the recent measurements of R. Gunn and J. D. Kinzer in the range of diameters less than 2mm.
A more complete one of this paper in English is published in the Geophysical Magazine, Vol. 21, No.3, Tokyo.
