CLOUD SEEDING IN THE AMERICAN TROPICS 
in the projects included in this study, by spray- 
ing either plain water in droplets with a mean 
diameter about 70 microns or by spraying 
sodium chloride solutions with a mean droplet 
diameter about 40 microns. In either case the 
spray was introduced at or near the bases of the 
clouds in the updrafts. In the discussion the 
process has been referred to simply as water 
seeding. 
In Table 1, which shows the results of cloud- 
seeding trials in the American tropics, distinc- 
tion has been made among three classes of proj- 
ect: (1) commercial, those conducted by a cloud 
seeder for a client where the cloud seeder pre- 
sumably is motivated to show a positive result; 
(2) practical, those conducted by an agency 
which is motivated by pecuniary interest to ob- 
tain an accurate evaluation of the result, either 
positive or negative; and (3) uninvolved, those 
conducted by an agency having no involvement 
other than the search for knowledge. The table 
is not exhaustive, and I know that there are 
other trials about which my present information 
is too meager to make inclusion of them in this 
examination useful. 
The individual percentages of increase cited 
in this table are extremely varied in reliability, 
some of them seem well founded, others are 
quite dubious. Hence it is unlikely that great 
confidence will be placed in the mean increases 
shown, but it is nevertheless interesting that the 
combined water and AgI seeding seems to show 
a clear advantage over AglI seeding alone. 
Certain among these data are worthy of closer 
attention, however. They represent the cases 
where a target-control relationship has been es- 
tablished, usually subsequent to a first season of 
seeding; and then the same relationship, un- 
415 
altered in any significant degree, has been used 
to evaluate subsequent seedings. There are six 
such cases, and they are tabulated separately in 
Table 2. These represent seeding trials where 
the game has been played according to pre- 
established rules. It will be noted that the mean 
increase of 19% among them compares closely 
with the 22% among all cases. 
These values, especially the ones representing 
probability of chance occurrence, should still 
probably be regarded with reservations. It is 
conceivable that there are secular changes in the 
climates of target and control zones that would 
alter the target-control regression sufficiently 
between the historical period and the seeding 
period to affect the value of the test, though such 
changes remain to be demonstrated, and it is 
doubtless true that other possible selections of 
control region or historical period or both would 
have produced different results. However, it 
seems that it would take an extraordinary de- 
gree of prescience on the part of the evaluator 
to select a control that would at some future 
time be biased in favor of a false indication of 
seeding influence. 
Leaving to the statisticians the question 
whether the figures tabulated afford reliable 
proof that the seeding increased the rainfall, one 
thing nevertheless is clear: a hypothesis that 
silver iodide seeding is incapable of increasing 
the rainfall in tropical clouds over land is open 
to serious doubt. The rest of this paper will be 
devoted to examining the implications of this 
statement. 
Should AgI seeding affect tropical Cumulus ?— 
The development of precipitation in shower 
clouds, both in the tropics and in middle lati- 
tudes, has been the subject of extensive study. 
TasLe 2—Evaluations of operations conducted after establishment of control area and 
historical period and evaluated according to established rules of the game 
>% 
Sais 
Location Year Seeding agent a $ Bs Remarks 
2g 3> 
Cie 24 
4 a 
pet 
Mexico, Necaxa 1950-58 | Agl +9 | 0.0001 | Orographic, maritime air 
Peru, Rio Chicama 1952-58 | AglI +25 | 0.0001 | Orographic, continental air 
Cuba, Baltony 1955 Agl +12 | 0.03 Valley location 
Puerto Rico, Fajardo 1955 AglI, water +27 | 0.07 Windward shore, mostly flat 
Cuba, Francisco 1956 AglI +25 | 0.005 | Flat, lee shore 
Cuba, Manati 1957 Agl +15 | 0.21 Flat, windward shore 
Average +19 
