ELECTRON MICROSCOPY 



of water vupor in tlic undcrcooled cloud. The 

 nucleus is an aerosol in the atmosphere. 

 Aerosols consist of many kinds of materials 

 from various sources: from the ground, from 

 the ocean, from air pollution as a result of 

 human activities and from outer space (mi- 

 cro-meteorites) (1, 19). 



The nuclei of snow crystals can be classi- 

 fied as soil particles, hygroscopic particles, 

 combustion products, micro-organisms and 

 unkno\m materials. The last classification 

 shows that some nuclei were not identifiable 

 in this research. Table 1 shows the compari- 

 son of snow crystal nuclei observed in Hok- 

 kaido, Honshu and Upper Michigan. Some 

 differences are found, depending on the me- 

 teorological conditions or the locations where 

 the specimens have been collected, in the 

 percentage of nuclei substance among Hok- 

 kaido, Honshu and Upper Michigan. The 

 data concerning snow-crystal nuclei in Hon- 

 shu agree well with that of Upper Michigan. 

 Soil particle nuclei occupied 57 per cent in 

 Hokkaido, 87 per cent in Upper Michigan. 

 There is a smaller percentage of hygroscopic 

 nuclei in Upper Michigan than in Hokkaido. 

 All hygroscopic nuclei in Upper Michigan 



Table 1. The Comparison of Snow Crystal 



Nuclei were Observed at Hokkaido and 



Honshu, Japan and Upper 



Michigan, U. S. A. 



were potassium chloride, but in Hokkaido 

 most were sea salt particles; the remainder 

 were potassium chloride. This is because 

 Upper Mi(;higan is located far from the 

 ocean, Honshu is a large island, but Hok- 

 kaido is a relatively small island. The per- 

 centage of combustion products or carbon 

 particles is smaller for Upper Michigan than 

 for Hokkaido. Micro-organisms, that is bac- 

 teria, are not found in Upper Michigan. In 

 Hokkaido five per cent of the center nucleus 

 was unobservable; in Upper Michigan one 

 per cent was unobservable. These facts show 

 that the nuclei in the form of snow crys- 

 tals are mainly soil particles. 



Acknowledgments. The investigation in Upper 

 Michigan was conducted as a part of Ice Nuclea- 

 tion Research under a grant from the National 

 Science Foundation at the University of Chicago. 

 The writer is much indebted to Drs. H. R. B3'ers, 

 R. R. Braham, Jr. and U. Nakaya, to Miss B. J. 

 Tufts and to The Snow Ice and Permafrost Re- 

 search Establishment, U. S. Army Corps of En- 

 gineers. 



REFERENCES 



"The influence of meteoritic 

 Austral. J. Phys., 6, 490-497 



1. Bowen, E. G. 



dust on rain 

 (1953). 



2. Byers, H. R., Sievers, J. R., and Tufts, B. 



J., "Distribution in the atmosphere of cer- 

 tain particles capable of serving as con- 

 densation. Artificial stimulation of rain." 

 H. Weickman (ed.), Pergamon Press, New 

 York, pp. 47-72, 1955. 



3. Burton, E. F., Sennett, R. S., and Ellis, 



S. G., "Specimen changes due to electron 

 bombardment in the electron microscope," 

 Nature, 160, 565 (1947). 



4. IsoNO, K., "Microphysical processes in pre- 



cipitation mechanism," Japanese Journal of 

 Geophysics, 1-57 (1959). 



5. JuNGE, C, "Die Rolle der aerosole und der 



gasformigen beimengungen der luft in 

 spurenstoffhanshalt der troposhare." Tellus, 

 5, 1-26 (1953). 



6. KoENiG, L. R., "The chemical identification 



of silver iodide ice nuclei." Technical Note, 

 No. 19. Dept. of Meteor., Univ. of Chicago, 

 1959. 



7. KuMAi, M., "Electron-microscope study of 



snow-crystal nuclei." /. Meteor., 8, 151-156 

 (1951). 



258 



