238 
of the picture with large natural openings in the fore- 
ground.) These two examples illustrate that even the 
most favorable seeding missions in the Ohio tests re- 
Fria. 3.—Radarscope photograph of seeding, October 5, 1948, 
showing L-shaped area of light rain. 
sulted in only slight precipitation amounts and in dis- 
sipation only when the clouds were breaking up due to 
natural conditions. A study of the data presented in 
the Ohio report indicates without exception that such 
RE Sadan 
Fia. 4.—First seeded area as seen from 10,000 ft above cloud 
deck, photographed thirty minutes after seeding (October 5, 
1948). Note natural breaks in clouds. 
large-scale seeding effects, as illustrated above, were 
possible only when the cloud deck treated was under- 
going natural dissipative action. On the “unfavorable” 
days, seeding resulted in the production of narrow lines 
of ice crystals which did not diffuse through the super- 
cooled cloud deck. 
The Sierra Nevada tests were conducted for the 
purpose of evaluating the importance of dry-ice seeding 
in orographic type stratus clouds. It was felt that the 
clouds formed along the Sierra Nevada Range by the 
mechanical lifting of moist Pacific air would offer the 
most favorable conditions for seeding. The report for 
CLOUD PHYSICS 
the Project’s operations near Sacramento [6] indicates 
that its seeding results were no more successful than 
those in Ohio [4]. First of all, it was found that non- 
frontal orographic clouds existed only a fraction of the 
time, and that the majority of cloud systems over the 
Sierra Nevada Range were of frontal origin and con- 
tamed natural ice-crystal clouds before lower super- 
cooled decks were formed. On only three days during a 
two-month operational period was it possible to find 
nonfrontal clouds and, on each of these days, the lack 
of large-scale horizontal convergence in the general 
area resulted in somewhat broken cloud decks. A dis- 
cussion of the seeding of February 15, 1949, is in- 
cluded here since on this particular day fairly large- 
scale dissipation results were observed. An altocumulus 
deck extending from 9800 ft to 10,650 ft where the 
temperature was —7C was seeded in the shape of an 
L along the base of the Sierras near Auburn, California. 
Dry ice was used as the seeding agent and was dropped 
at the rate of four pounds per mile. Immediately after 
seeding an ice-crystal pattern conforming to the dry- 
ice pattern was observed and within some 20 min the 
ice-crystal area had spread about 114 miles. Within one 
hour after the seeding, definite holes through to the 
ground, especially along the borders of the seeded pat- 
tern, were observed and at the apex of the L a large 
opening some three to four miles across was observed. 
However, the significance of this dissipation becomes 
questionable in view of the natural dissipation occur- 
ring simultaneously. The natural openings could easily 
be seen, and within a half-hour after the observation 
of the large seeded opening, the whole cloud deck be- 
gan to break up rapidly and disappeared shortly there- 
after. No precipitation of consequence could have fallen 
as a result of seeding, since its effects did not spread 
over a large area nor did it cause any large-scale con- 
vection. The only amount of moisture that could have 
fallen was that which could have been extracted from 
the 800-ft thick cloud. 
Other experimenters in the field have indicated that 
they believed that the Cloud Physics Project was not 
successful in causing large-scale dissipation because of 
its use of excessive amounts of dry ice, usually two to 
five pounds per mile, and they recommended that 
rates of one pound per mile or less be used. They felt 
that overseeding had occurred in most of the cases 
studied by the Cloud Physics Project and that, as a re- 
sult, the ice crystals which formed did not grow large 
enough to precipitate from the cloud deck in sufficient 
numbers actually to cause it to dissipate. As a matter 
of fact, they suggested the use of only one pellet of 
dry ice to open relatively large areas. Four such single 
pellet drops by the Cloud Physics Project during its 
California tests gave no results on a scale large enough 
to be recognized. There is no doubt that there exists 
some optimum ratio of the specific number of droplets 
to ice nuclei for most favorable seeding results. Ber- 
geron [3] has discussed this in some detail in his paper 
and gives examples of the effects of the various ratios 
from underseeding to overseeding. It is readily appar- 
ent there is no one seeding rate which will give the 
