Artificial Nucleation of Orographic Cumulus Clouds 
Louis J. Barran anp A. RicHarp KASSANDER, JR. 
Institute of Atmospheric Physics, University of Arizona, Tucson, Arizona 
Abstract—A brief summary is given of the results of randomized cloud-seeding ex- 
periments carried out durimg two summer seasons. Orographic Cumuli were seeded 
with an airborne AgI generator. The results to date have suggested that seeding 
causes important changes in the physical processes involved. It is planned to continue 
the experiments. 
Introduction—During the last two years, the 
University of Arizona has been carrying out a 
program of cloud-seeding research directed to- 
wards the study of the effects of silver-iodide nu- 
clei on supercooled orographic Cumulus clouds. 
Clouds over the Santa Catalina Mountains in 
southeastern Arizona were studied. 
Since the program is still in progress, this 
note is a brief summary of some aspects of the 
research. When the research is completed, the 
results will be published in the geophysical Jour- 
nals. 
Design of excperiment—The design of the seed- 
ing experiment was evolved with the assistance 
of K. A. Brownlee and W. Kruskal of the De- 
partment of Statistics of the University of Chi- 
cago. Briefly, the procedure involved an objec- 
tive prediction, made prior to 09h 00m MST of 
each day, as to whether or not Cumulus con- 
gestus or Cumulonimbus clouds would form over 
the Santa Catalina Mountains. The main cri- 
terion for the prediction was whether or not the 
precipitable water at Tucson, Arizona, equalled 
or exceeded 1.10 inches. When this occurred, the 
day was considered to be suitable for seeding, 
and an envelope was opened which specified 
which of two suitable days would be seeded. If 
more than one unsuitable day separated two 
suitable days, the first day of the pair was re- 
jected and a new pair was started. The scheme 
of randomized pairs was adopted in order to 
take into account day to day correlations and to 
assure that there would be an equal number of 
seeded and not-seeded days. 
The actual seeding was carried out with an 
Australian-type airborne silver iodide generator 
suspended under the wing of a Supercub air- 
plane. The generator has been made available 
through the cooperation of the University of 
Chicago. The flight plan involved repeated 
passes at about the —6°C-level along a track 
409 
upwind from the mountain range. The pilot nor- 
mally started the generator at about 12h 30m 
MST and continued his flight until all the seed- 
ing material was exhausted or the burner went 
out. Normally the seeding period was of the 
order of four hours. The generator consumed a 
20% solution of silver iodide in acetone at a rate 
of 2 to 24% gal/hr. 
Observations—In order to permit studies of 
cloud and precipitation processes the following 
observations were taken: (1) visual cloud prop- 
erties were recorded on a pair of carefully cali- 
brated ground-located K-17 aerial cameras from 
which accurate estimates of cloud locations and 
dimensions could be measured; (2) the location 
and spread of precipitation echoes were ob- 
served with a vertically scanning 3-cm radar 
set; (3) rainfall was noted with a network of 29 
recording rain gages; and (4) visual observations 
were made of the time and location of cloud-to- 
ground lightning strokes. 
Results—The experiments conducted during 
the first two years suggest that AgI seeding 
caused some important changes in the natural 
cloud processes. During each summer 16 pairs of 
days were studied. 
Rainfall—When data from both years were 
combined it was found that the mean rainfall 
per gage was 30% higher on the seeded days; 
however, the probability that the observed dif- 
ferences in the mean rainfall occurred by chance 
was quite high, about 0.14. This value was ob- 
tained from a sign-rank test which made use of 
a ranking of the differences of the mean rain- 
fall of pairs of days. A comparison of the ex- 
treme rainfalls on seeded and non-seeded days 
showed greater differences, but the statistical 
confidence of a real difference was still not suffi- 
ciently high to be acceptable. 
Heights of thunderstorms—An objective way 
to measure the relative frequencies of large 
