230 
large cumulus formations an increase in temperature of 
several degrees within the cloud is quite possible. 
The Use of Silver Iodide as a Seeding Agent. An 
entirely different process for seeding the atmosphere 
with ice nuclei involves the use of submicroscopic silver 
iodide particles produced with a smoke generator. Since 
this substance is most effective at temperatures below 
—10C, techniques which are somewhat different from 
those used with dry ice are employed. Silver iodide 
lends itself particularly well to the use of ground genera- 
tors and the modification of large cloud systems. By 
the use of the generators described by Vonnegut [39] it 
is possible to produce invisible smokes of silver iodide 
particles in such high concentrations as to infect many 
thousands of cubic miles of the atmosphere. Not only 
is it possible to exceed the highest concentrations found 
in the free atmosphere, but the silver iodide particle is 
a more effective ice-crystal nucleus than any which has 
thus far been found in nature. 
The Seeding of Clouds with Water. Under certain con- 
ditions the precipitation cycle may be initiated in large 
cumulus clouds by introducing large water drops into 
actively convective portions of the cloud. This seeding 
initiates a chain reaction [13] involving the growth and 
breakup of water drops. Since this seeding mechanism 
is not related to snow, it will not be discussed here. 
Cloud Seeding 
Effects of Seeding Operations. Laboratory experiments 
show that a concentration of about 50 ice nuclei per 
cubic centimeter will exhaust all the moisture in a 
supercooled cloud in about 10 sec. Since supercooled 
clouds are sometimes invaded by ice crystals falling 
from cirrus clouds, it is interesting to determine their 
effect on a supercooled cloud. Such crystals have a 
falling velocity of about 1 m sec“! and a concentration 
in the air of about 1 X 104 m~’. Assuming typical 
atmospheric conditions with the invading crystals mov- 
ing through the supercooled cloud at a rate of 1 m 
sec!, not more than 20 min would be required to use 
up the supercooled cloud droplets in a specific region of 
a cloud. This process may often be observed in the 
free atmosphere. 
The Overseeding of Clouds. In most natural clouds the 
number of cloud droplets ranges from 100 to 500 em-. 
This is about one hundred times more than the number 
of potential ice nuclei thus far observed under natural 
conditions. Supercooled droplets can be transformed to 
ice crystals without a decrease in the number of the 
individual particles when they reach the spontaneous 
nucleation temperature of —39C or when they are 
artificially seeded with dry ice or silver iodide. Concen- 
trations of ice nuclei exceeding 10,000 cm? may be 
induced in natural supercooled clouds by proper seeding 
methods. This fact is most easily demonstrated in a 
supercooled ground fog [23]. Since one gram of dry ice 
may generate 1 X 10!* ice crystals, this minute quantity 
effectively distributed could fill 1 & 107 m3 of air (a 
volume of fog 100 m thick, 100 m wide, and 1000 m 
long) with a concentration of 1000 cm. 
The optical effects produced by overseeding of clouds 
CLOUD PHYSICS 
are very striking. Very beautiful halos, sun pillars, sun 
dogs, ete., of brilliant color and high intensity occur in 
overseeded clouds. Sometimes a peculiar bluish hue 
develops in an overseeded cloud because the particles 
are small with respect to the wave length of visible 
light. 
As suggested by Bergeron [3] and Schaefer [31], over- 
seeding may eventually be used to cause the transport 
of moisture from one region to another or may be 
utilized to form large reservoirs of ice nuclei. Such 
formations could affect clouds some distance from the 
seeding point or any clouds in the infected region which 
became large enough to form precipitation. It follows 
therefore that overseeding of supercooled clouds may 
become an effective method for preventing the forma- 
tion of rain or snowstorms in specific areas. 
Figure 13 illustrates the active formation of an ice~ 
crystal cloud by dry-ice seeding in clear air. The early 
etapa cenmenrmrerms res 
Fie. 13.—The formation of an ice-crystal cloud by dry-ice 
seeding of air supersaturated with respect to ice at a tempera- 
ture of —18.5C. 
stage of a stratus-cloud seeding, producing removal of 
cloud cover over a local region, is illustrated in Fig. 
14. Not enough moisture was contained in these clouds 
to produce substantial amounts of precipitation. 
The Effective Seeding of Clouds. To achieve maximum 
effectiveness in the form of energy release and precipi- 
tation, it is necessary to seed young, actively growing 
cumulus clouds in a region of the atmosphere which is 
potentially unstable. The clouds should be seeded when 
they have a vertical thickness above the freezing level 
of 3000-5000 ft. Care must be exercised that the dry-ice 
particles penetrate the entire supercooled region. 
As mentioned earlier, the seeding of cumulus clouds 
with dry ice to effect a sudden release of the heat of 
sublimation should produce unusually intense convec- 
tion in growing cumulus clouds. Such effects have been 
observed in experiments conducted in Australia [10], 
South Africa [1], Canada [17], and New Mexico [32]. 
Under optimum conditions, judicious seeding to produce 
this effect could lead to a local convergence by breaking 
through a limiting inversion layer or other restrictive 
atmospheric condition. At times it is conceivable that 
