An Experimental Research on the Form of Falling Rain Drops

Abstract

T. Matukawa reported his observation on the form of frozen rain drops⁽¹⁾ which had a shape of a cup, sometimes filled with water and other times not. Later on N. Mori ascertained the fact by his own observation.⁽²⁾
The author carried out an experiment to make clear of the mechanism of this curious formation of frozen rain drops. From the top of the wind tower of the Central Meteorological Observatory, having the height 72 ft., above the ground he dropped drops of melted paraffin, naphthaline and sealing wax. The drops of the former two substances solidified on the way of falling and at the ground the author could collect grains of various forms as pictured in fig. 1 in plate 14. Besides, he observed paper-like thin films and also very thin threads which were formed and fell rather slowly. The author insists that such forms are produced mainly by the resistance of air on the falling drops when they were still in the liquid state, then according to cooling they solidified themselves retaining these forms. As illustrated in fig. 3, vortex motion of air surrounding the drops and also that in the liquid on the side of the drops may be the most powerful factors to form semispherical shells and other forms as shown by b, c, d, e, etc. He thought that “Australite” reported by Frof. Dunn⁽³⁾ having a form in fig. 2 Pl. 14 may be formed by some similar process. Vortex motion as shown in that picture supports the view above describ: d. The author's opinion on the formation of hollow shells as f, g and h in fig. 3 is that the liquid still occupying the interior part of the shells must have splash out at the moment when they stroke the ground. This opinion is, however, left to further criticism.
Discussion by S. Fujiwhara.-In Japan it was reported two or three times that very large hail stone fell, sometimes with diameter of about one foot, and rather small thickness, of the shape of a basin or of a flower of peony, with curved up brim sometimes with zigzag edges. Such a formation of hail stones may also due to the similar process. That vortex motion plays most important part in the above peculiar phenomena is also seen clearly in the picture of the section of an Australite shown by Prof. Dunn. (Pl. 14 fig. 2) Spiral figures as seen on both the sides of the stone are marked indications of the vortex motion existing before solidification.