236 
ground include (4) the height of the cloud base above 
the terrain, and (5) the humidity of the air below the 
cloud. In view of all the specialized conditions and fac- 
tors involved, one of the objectives of conclusive tests 
of cloud-seeding potentialities must certainly be the de- 
termination of the frequency of such conditions and the 
importance of such factors. The question and impor- 
tance of particular seeding rates, the one variable under 
control of the experimenter, are somewhat nebulous 
and will be discussed later in the light of some experi- 
mental results. 
The Cloud Physics Project 
In view of the divergent claims made by both quali- 
fied and unqualified experimenters in the rain-making 
field, the requirement for a long series of well-controlled 
objective tests became obvious. Such a program, the 
Cloud Physics Project, was initiated by the U. S. 
Weather Bureau, the U. 8. Air Force, and the National 
Advisory Committee for Aeronautics in 1947. Since 
the 170 tests of that project are the only well-docu- 
mented series yet reported [4, 5, 6, 7], the following dis- 
cussion of seeding results will be based upon them. 
The first requirement for an evaluation program is 
that it be able to separate results of seeding from natu- 
ral effects. This is especially important in view of the 
fact that the necessary conditions for successful seeding 
operations approach those required for natural pre- 
cipitation processes. The Cloud Physics Project experi- 
ments were designed to use the maximum number of 
controls and observational tools available for such an 
atmospheric vestigation. Tests were first conducted 
for a period of about one year in the vicinity of Wil- 
mington, Ohio, where ideal facilities were available [4, 
5]. Further tests were then conducted from Sacramento, 
California [6] m the Sierra Nevada Range, and from 
Mobile, Alabama [7] along the Gulf Coast. At Wilming- 
ton, all of the project seeding operations were controlled 
from a central radar site. The high-powered 10-em V- 
beam radar was of utmost importance im these tests. 
Not only was it capable of indicatimg and marking per- 
manently on film the paths of each of the aircraft m- 
volved in the investigation, but also it mdicated the 
presence of precipitation areas, whether they were 
formed naturally or as a result of seeding. This radar 
then was an invaluable tool which indicated if the cloud 
chosen visually for seedmg, or for that matter other 
clouds in the vicinity, had precipitation echo sources 
before seeding, while, on the other hand, it mdicated 
if echoes resulted exclusively from seeding. This radar, 
of course, was also of great help in keeping the obser- 
vational aircraft properly positioned with respect to 
the cloud area being investigated. 
Whenever possible, the project’s operations were car- 
ried out over a 55-station surface network south of 
Wilmington. The rain gages at these stations, along 
with recording gages of the hydrometeorological sery- 
ice, were used to measure precipitation amounts. Also 
available in the Wilmington area were two rawinsonde 
stations which provided, as required, information re- 
CLOUD PHYSICS 
garding the temperature, pressure, moisture, and winds 
of the upper atmosphere. 
A number of aircraft were used in most of the seeding 
flights. A B-17 usually performed the seeding operation 
in an area such that its effects would be noted and meas- 
ured over the surface rain-gage network. Its seeding 
conveyor was carefully calibrated and indicators of the 
rates and periods of seeding were photographed con- 
tinuously from remote-reading instruments installed in 
a photo panel along with mdicators of other important 
parameters, such as temperature, pressure, altitude, 
and electric field. Also installed in the seeding aireraft 
was a low-powered X-band radar which would indicate 
the presence of light-precipitation areas. Photographs 
of the seeded cloud were taken from this aircraft both 
before and after seeding. Other aircraft which partici- 
pated in the flights were high-altitude photo planes 
which took vertical and oblique photos of the seeded 
area, and observation planes which flew above, within, 
and beneath both the seeded cloud and neighboring 
clouds to determine their characteristics. Paths of all of 
the aircraft participating in the missions were recorded 
on the photographs of the ground radarscopes. From 
these photos, it was possible to determine the exact 
location of any of the planes at any instant, since each 
of them was identified by its own radar-triggered beacon 
code. All oral descriptions of seeding operations and 
results were recorded on continuous records, making it 
possible to determine easily the exact time of any ob- 
servation. Since the position of the particular aircraft 
at the time of the observation was known from the 
radar data, the location of the observation was also 
known. 
In summary, observations of the results of the seeding 
operations were made from several points of vantage: 
the radar, the aircraft flying above and below the seeded 
cloud, and on the ground (by visual observers). The 
quantities measured were the height of the base and 
top of the seeded cloud, the relative humidity above and 
below, the temperatures inside and outside the cloud, 
lapse rates, optical characteristics, and the extent and 
character of resulting precipitation. In the Sacramento 
and Mobile operations, not all of these quantities were 
available. However, the basic observational plan was 
followed with a high-powered 10-cm radar installed in 
a B-17 used to obtain the important radar information. 
Experiments with Stratus Clouds 
The first results to be discussed will be those obtained 
by the Cloud Physics Project in stratus clouds. Alto- 
gether 37 seedings in such clouds were conducted in the 
Wilmington area in 1948 and 15 seedings into orographic 
clouds were made in the Sacramento area in 1949. The 
objectives of these tests, as set forth in 1948, were to 
determine “the practical limits and general utility of 
cloud modification processes in producing or suppress- 
ing precipitation and increasing the visibility for flying 
aircraft.” 
On January 21, 1948, over one inch of snow fell in 
the vicinity of Wilmington, Ohio, within eighteen hours 
