Early History 



with silver iodide had been caused by the fact that the silver iodide he had 

 used was impure. Powdered silver iodide worked very well when it was 

 reasonably pure. He also found that the reason for the successful use of 

 iodine was again impurity --contamination with silver. 



The problem then became one of finding out something about how 

 silver iodide worked and of finding methods of generating silver-iodide 

 smoke of small particle size on a large scale. So many nuclei could be 

 produced with silver -iodide smoke that calculations indicated all the air 

 of the United States could be nucleated at one time with a few pounds of 

 silver iodide, so that the air would contain one particle of silver iodide 

 per cubic inch--far more than the number of ice nuclei occurring nor- 

 mally under natural conditions. (65) 



LANGMUIR 'S EARLY SEEDING CALCULATIONS 



Meanwhile Schaefer and Langmuir had continued their study of 

 the effects of dry ice. In August of 1946 Langmuir made a theoretical 

 study of the rate of growth of the nuclei produced by dropping pellets 

 of dry ice through clouds of supercooled water. ( 8 0) He calculated the 

 velocity of fall and time of dissipation of the dry ice, the amount of ice 

 particles that would be formed, their size, the amount of snow which 

 would result, etc. With a reasonable number of pellets dropped along 

 a flight path into the top of a cloud, the limiting factor would not be the 

 number of nuclei but the rate at which they could be distributed through- 

 out the cloud. 



He also showed that such a formation of ice and snow particles 

 would raise the temperature of the cloud, and he calculated the amount 

 of temperature change. Thus the air in the cloud would be caused to 

 rise, increasing its upward velocity because of the seeding. The result- 

 ing turbulence would spread the ice nuclei throughout the cloud. He 

 anticipated that it would only be necessary to seed a stratus cloud along 

 lines one or two miles apart in order to give complete nucleation of the 

 cloud within a period of 30 minutes or so. 



FIRST MAN-MADE SNOWSTORM 



Thus the stage was set for actual experiment with an airplane in 

 real clouds. On November 13, 1946, a Fairchild airplane was rented at 

 the Schenectady airport, piloted by Curtis Talbot, and Schaefer went 

 aloft in search of a suitable cloud. (38) xt was found over Pittsfield, 

 about 30 miles east of Schenectady, at an altitude of 14,000 feet and a 

 temperature of -20OC. What happened next is best described by the 

 following extract from Schaefer's laboratory notebook entry for that 

 day: 



