240, 
tained within the cloud itself. The methods are certainly not 
promising for the relief of drought. 
In the Weather Bureau report are included detailed 
examples of three different cases in which the radar, 
in particular, and other operational tools available to 
the Cloud Physics Project proved invaluable in assess- 
ing the results of seeding. One of the examples illus- 
trates a case in which a small shower which resulted 
from the seeding was differentiated from other natural 
showers occurring almost simultaneously within five 
miles. In the second example, there is discussed a case 
in which a small radar echo and accompanying shower 
resulted from seeding when there were no other show- 
ers indicated by the radar within fifty miles. In the last 
example covered in the report, an untreated tower 
which was part of the seeded cloud showed an echo 
previous to the dry-ice drop, indicating that precipita- 
tion had already started in that portion. The tower 
that was seeded did not give any precipitation. How- 
ever, observers underneath the cloud reported rain 
which, as a result of the detailed analysis procedure, 
was definitely attributed to the tower that showed an 
echo previous to the seeding. These cases show that 
the use of all available controls, such as radar, is most 
important in properly evaluating the results. Without 
a radar in the first example, the adjacent natural show- 
ers might either have been overlooked or, on the other 
hand, attributed to the seeding operation. In the last 
example, the rain which was already falling from the 
seeded cloud might easily have been attributed to the 
effect of the dry ice. This third example illustrates a 
real difficulty in the seeding of cumuliform clouds. In 
the Ohio area many of these clouds reached the spon- 
taneous freezing level soon after surpassing the OC iso- 
therm, thus occasionally making it impossible to find 
clouds extending above the freezing level which did not 
have ice crystals in them before seeding. The following 
results of the tests conducted at Mobile, Alabama, 
indicate this to be a particularly important difficulty 
in the seedings conducted in that area. 
In clouds over the Gulf States [7], ice crystals would 
usually form naturally just a few thousand feet above 
the freezing level, often when the temperature at the 
tops of the clouds was not below —6C. Such clouds 
would appear to be composed of only water droplets 
when observed visually from outside, but the occur- 
rence of radar echoes and subsequent visual observa- 
tion of ice crystals within these clouds proved external 
visual observation to be unsatisfactory and somewhat 
misleading. Thus, there was only a short period of time 
during cloud development in which seeding was likely 
to be effective; that is, between the time the cloud 
reached the freezing level and the time it reached the 
natural crystallization level, from 3000 to 6000 ft 
higher. As in the Ohio tests, there was no appreciable 
building as a result of seeding, but rather dissipation 
of the cloud top was the rule. Occasionally, clouds con- 
taining only ice crystals were seeded because of the 
unavailability of strictly supercooled ones. The report 
of the Cloud Physics Project indicates that even in 
these cases seeding hastened the process of dissipation, 
CLOUD PHYSICS 
possibly by increasing the number of nuclei available 
within the cloud. 
Three specific examples of the Mobile seeding opera- 
tion seem of interest. On June 5, 1949 the cloud under 
study had its base at around 5600 ft with the top at 
18,000 ft where the temperature was —6C. It was 
seeded twice with dry ice at the rate of five pounds per 
mile. On the first run, only supercooled water was ob- 
served to freeze on the aircraft. This seeding resulted 
in the dissipation of the pmnacle traversed during the 
run. On the second seeding run, 13 min later, both 
supercooled water and ice crystals were observed. 
Shortly after the dry-ice drop, a faint radar echo ap- 
peared in the seeded area, while at the same time an- 
other echo formed in a cloud four miles away. This 
natural echo grew to a much larger size than that m 
the seeded cloud. Figures 5 and 6 show the cloud before 
and after the second seeding and illustrate its rapid 
dissipation into a small stratified cloud. The second 
picture shows a rainbow from the light precipitation 
which fell from the cloud. It is interesting to note in 
Fig. 5 the concurrent thunderstorms in the near back- 
ground. 
Fig. 5.—Second seeding, June 5, 1949 (before seeding). 
Altitude, 18,000 ft; time, 1452; azimuth, 300°. 
A very favorable situation for seeding is illustrated 
in the third seeding of June 7, 1949. A cloud whose 
base was near 5000 ft and whose top was estimated to 
- os » 
Fic. 6.—Second seeding, June 5, 1949 (after seeding). Note 
rainbow under cloud. Altitude, 18,000 ft; time, 1503. 
