Early History -7- 



after he took the dry ice out. 



Following this discovery, Schaefer conducted a number of experiments. 

 These showed that even a tiny grain of dry ice would transform the super- 

 cooled cloud in the cold box to ice crystals. -Quantitative experiments were 

 conducted which showed that many millions of crystals could be produced 

 in this manner. 



In order to find out if there was something peculiar to dry ice which 

 produced this effect, he worked with other cold materials. For example, 

 he showed that, by dipping a common sewing needle into liquid air and then 

 passing it momentarily through the supercooled cloud in the cold box, sim- 

 ilar spectacular effects occurred. This demonstrated that the presence of 

 a sufficiently cold substance was all that was required to produce the effect. 

 Schaefer devised methods and equipment for determining, with considerable 

 accuracy, the critical temperature at which the supercooled cloud changed 

 to ice crystals. ^6) This temperature was found to be -38.9C±0.1 degree. 



Schaefer 's discovery changed the whole situation. It meant, first, that 

 it was not the dry ice or the needle as such that was responsible for the ef- 

 fect, but the temperature. Anything could be used having a temperature of 

 -40 C or colder. 



VONNEGUT'S EARLY WORK 

 CLOUD STUDIES AT M.I.T. 



Meanwhile the stage had been set for another important contribution 

 to this pioneering work in meteorology. Before Dr. Bernard Vonnegut be- 

 came associated with the General Electric Research Laboratory, he was 

 employed at Massachusetts Institute of Technology, where he had been en- 

 gaged in various studies during the early years of World War II. In the 

 laboratory of the Chemical Engineering Department he worked on smokes 

 for the Government's Chemical Warfare Service. He measured smokes, 

 smoke penetration, and smoke filters. Then he became interested in the 

 problem of icing of airplanes and went to work on that in the Meteorology 

 Department, for the Air Force. 



SUPERCOOLING 



Meanwhile he had been doing some work on the side in supercooling. 

 He found that by making an emulsion of water drops suspended in oil, he 

 could cool water far below the normal freezing point, and it would not 

 freeze until a certain point was reached, when the whole mass froze very 

 rapidly. ^) 



