176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



brevity and continuity no specific references will be made to papers 

 and reports describing the results of seeding experiments. The sum- 

 mary of the results as required for the present discussion is well known 

 to most readers. Those who wish to read the original accounts are 

 referred to the excellent bibliography recently published (1).^ It is 

 not the intent of the writer to attempt to refute or support specific 

 claims of individuals; rather it is proposed to survey the subject as 

 impersonally as possible. Lack of complete information makes the 

 inclusion of certain personal opinions of the writer inevitable. 



METHODS AND RESULTS 



Modern rain making owes its origin to the discovery by Schaef er (2) 

 that the insertion of dry ice or any object colder than about —40° C. 

 into a supercooled water-drop cloud converts the cloud to ice crystals. 

 Later Vonnegut (3) showed that the same result could be obtained by 

 introducing tiny crystals of silver iodide, which are presumed to act 

 as sublimation nuclei because of the dimensional similarity of the 

 crystal lattices of silver iodide and ice. The mechanism by which dry 

 ice forms ice crystals is still in debate, as is the whole problem of 

 sublimation nuclei, but it is a demonstrable fact that either agent will 

 convert a supercooled cloud to an ice cloud. Knowledge of the mech- 

 anism of ice-crystal formation is desirable, however, for an estimate 

 of the number of ice crystals created by a known amount of the 

 nucleating agent. 



The application of the nucleating agents to rain making is based on 

 the Bergeron-Findeisen ice-crystal theory of precipitation. Accord- 

 ing to this theory, all moderate to heavy precipitation is initiated by 

 the appearance of a few ice crystals in a supercooled cloud. By virtue 

 of the fact that the vapor pressure over ice is less than that over water 

 at temperatures below freezing, the ice crystals grow by sublimation 

 at the expense of the supercooled water drops. If the number of ice 

 crystals is very small compared to the number of water drops, the ice 

 crystals will become large enough to fall. After further growth by 

 sublimation and collision they leave the cloud as precipitation ele- 

 ments. It was suggested that many natural supercooled clouds do 

 not release precipitation because of a failure by nature to provide ice 

 crystals. The use of the newly discovered methods for producing 

 ice crystals in a supercooled cloud to initiate precipitation in such 

 clouds was a logical step. 



The reported results of cloud-seeding experiments have been quite 

 diverse. In a vast majority of the tests there has been visible evidence 

 of the transformation of supercooled clouds to ice ciystals. It seems 

 safe to conclude that the occasional absence of such effects has been 



* Numbers In parentheses refer to bibliography at end of this article. 



