ARTIFICIAL CLOUD-MODIFICATION AND PRECIPITATION 
be at 24,000 ft where the temperature was —25C was 
seeded with dry ice at the rate of five pounds per mile. 
This cloud contained only supercooled water at the 
20,000-ft seeding altitude and no radar echo was ob- 
served from it prior to the seeding. Within 10 min after 
the seeding, a small rain echo was observed from the 
cloud and light virga was observed by the low obser- 
vation aircraft. It was estimated that within about 
one-half hour after the seeding this cloud had dissi- 
pated approximately 75 per cent. Figures 7 and 8 are 
Fie. 7.—Third seeding, June 7, 1949 (before seeding). Altitude, 
20,000 ft; time, 1430; azimuth, 360°. 
pictures of the cloud immediately before and 14 min 
after seeding. They show its rapid conversion into ice 
crystals. 
“ as = 
Fic. 8.—Third seeding, June 7, 1949 (after seeding). Altitude, 
21,000 ft; time, 1444; azimuth, 360°. 
Considering the result obtained in the Ohio tests, 
and especially in the Gulf tests, the project report [7] 
concludes in part that 
...it seems a logical conclusion that seeding, by usually 
stopping the development of cumulus soon after they reach 
the freezing level, possibly interferes with the growth of some 
few of them to full-scale thunderstorms. All of our observa- 
tions seem consistent with this conclusion. Seeding then 
might actually be a deterrent to the production of precipita- 
tion over any particular area. It apparently inhibits the 
growth of cumulus clouds by initiating premature downdrafts 
241 
of ice crystals which subsequently choke off the necessary 
moisture inflow at lower altitudes and at lesser stages of 
development than those decreed by nature. f 
The large number of independent experiments con- 
ducted by the Cloud Physics Project have been gen- 
erally disappointing in terms of the practical and eco- 
nomic results hoped for. However, the treatment of 
natural clouds by large numbers of artificially dispersed 
sublimation nuclei has frequently induced weather per- 
turbations whose study has added notably to our sci- 
entific understanding of precipitation processes. The 
objective analysis of detailed changes occurring in the 
tremendous spaces of the atmosphere is extremely diffi- 
cult even with all the instruments and facilities avail- 
able. Future progress will depend, in no small measure, 
upon the invention and development of better air- 
borne instruments suitable for making rapid determi- 
nations of the detailed characteristics of each cloud to 
be explored. 
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