Laboratory Studies -28- 



Spontaneous Formation. Work done by Schaefer and others as early 

 as 1946 indicated that ice crystals formed spontaneously in water -saturated 

 air when the temperature reached the neighborhood of -35 or -40°C„ Schaefer 

 conducted quite a bit of research into this subject of spontaneous formation 

 and determined that the critical temperature was -38.9 ±0.1 degrees.^ 4 ' 



This phenomenon is probably of considerable significance in relation 

 to the formation of cirrus clouds and ice crystal fogs in the free atmosphere. 



Structure . Schaefer 5 s study of the various types of snow crystals, which 

 started before the establishment of Project Cirrus, continued throughout the 

 project. In 1948 he published a simple yet inclusive list of ten types of solid 

 precipitation for classification purposes. (39) In slightly modified form this 

 classification is now in use throughout the world. 



Crystal Growth and Multiplication. Experiments made by Schaefer in 

 1949 indicated that snow particles tend to shed minute fragments of ice when 

 they are placed in air slightly warmer than their own temperature. An ice- 

 forming nucleus appearing in a supercooled cloud grows rapidly, especially 

 in the temperature range of -12 to -16°C, where the difference between the 

 partial vapor pressure of ice and of water passes through a maximum. When 

 the crystal becomes large enough, it sheds a considerable number of ice parti- 

 cles as it falls through the cloud. These particles then serve as new nuclei 

 and repeat the cycle. In this manner, a few ice-forming nuclei in a cubic 

 meter of cloud may start a chain reaction which, within a few minutes, could 

 shift a supercooled cloud to a mass of snow crystals.^) 



A laboratory study was made to determine the factors of importance 

 for obtaining the maximum rate of snow crystal growth. 



SILVER IODIDE 



After the discovery that silver -iodide smokes serve as an excellent 

 nucleus for the formation of ice crystals, the project was faced With the prob- 

 lem of finding some way of generating the smoke efficiently and in quantity. 

 It was found that smokes consisting of exceedingly fine particles could be 

 easily produced by vaporizing silver iodide at a high temperature and then 

 rapidly quenching the vapor. This was readily accomplished by burning 

 silver-iodide -impregnated charcoal or injecting a spray of silver-iodide 

 solution into a hot flame. Simple generators based on this principle were 

 made which could produce 10 14 nuclei per second—enough to seed from 

 1000 to 10,000 cubic miles of air per hour. (65) 



