New Developments in Hail Research

Men have long been curious about the natural phenomenon of hail, and it is therefore not surprising to find that hailstones have become the object of intensive study anlong scientists during the last century (1). What is astonishing is, rather, the variety of observational material collected and the conclusions derived fronl it. As a result of wars on the continent of Europe, however, a large part of this knowledge has been lost, and although leading centers of research have been built up in England and the United States, their traditions do not go back so far. A perusal of the scant statements about hail formation in the standard work The Physics of Clouds (2) makes this situation quite clear. Where observational material was lacking there was all the more imaginative speculation about the way in which hail is formed. The first major hail theory in mathematical form was given by Schumann in 1938 (3). Although it has not been borne out in every detail, it contains nlost of the assumptions necessary for a procedure of this kind. Subsequent work has pointed out very little that is new in this respect. On the contrary, further development has tended to be trivial, degenerating into mere exercises in calculation based on apparently reasonable assumptions. The effects and countereffects that have been calculated have led to a series of propositions and counterpropositions, all of them the work of the same researchers, who can continually invoke them to demonstrate the correctness of this or that point of view. It is consoling that nature has allowed unforeseen and surprising discoveries to be made, which result in the more or less complete collapse of these proud intellectual constructions.