Cloud Seeding in the American Tropics 
Wauuace E. HowEiu 
W. E. Howell Associates, Inc., Lexington, Massachusetts 
Abstract—Cloud-seeding projects that have been carried out in the American trop- 
ies are described, and evaluation of the results are summarized and discussed in the 
light of existing models of precipitation formation. A new model based on a field of 
competition in which convective clouds develop is proposed and discussed to explain 
the effectiveness of seeding under weather conditions where clouds develop relatively 
slowly toward the precipitating stage. 
Introduction—We should hope that an ex- 
amination of the results of cloud-seeding trials 
in the tropics will reveal essential agreement be- 
tween the results observed and the expectations 
based on observations of the natural mechanisms 
of rain formation in the tropics and in labora- 
tory and theoretical studies of natural and arti- 
ficial mechanisms for the release of precipitation. 
Most of the trials of cloud seeding in the 
American tropics have been of a practical na- 
ture, intended to increase the rainfall for a spe- 
cific purpose or need, or, in a few instances, to 
diminish the force of locally destructive winds. 
The operations have been for the most part 
conducted by commercial firms and in a few 
cases by the agencies directly interested. The in- 
stances of cloud seeding performed by persons 
entirely uninvolved with the outcome have been 
with few exceptions incidental to other aspects 
of the investigation of cloud physics. In no in- 
stance known to the author have such seedings 
resulted in data suitable for evaluation to de- 
termine whether seeding applied for practical 
purposes could be effective. 
Up to the present time, no evaluations have 
been performed by any competent independent 
agencies of the practical seedings carried on in 
the American tropics, nor has any specific re- 
buttal been made to the reports by the cloud 
seeders of practically successfully results. It 
would be desirable, perhaps, to limit examina- 
tion of seeding trials to those carried out under 
impartial auspices in conformity with certain 
standards as to seeding agent used, the control 
of its dispersal into the air or cloud, the ade- 
quacy of rainfall data, and the objectivity of 
the procedures of analysis. For the present, how- 
ever, such a limitation would end our examina- 
tion at once, and so we must examine what is 
at hand, and substitute suspicion for the luxury 
of impartiality. 
Seeding experiences—A series of commercial 
projects conducted for the Francisco Sugar Co. 
is described next because it will serve to illus- 
trate the conditions under which a majority of 
the projects were conducted and the manner of 
their operation. The target covers some 3800 sq 
mi of nearly flat land for about 20 mi along the 
south coast of Camagiiey Province in east cen- 
tral Cuba, and extending 15 to 20 mi inland. It 
is shown as the two right-hand areas in the map, 
Figure 1. The climate has been described in de- 
tail elsewhere [Howell, 1953]. Operations were 
begun in the summer and fall of 1951, using six 
ground-based silver iodide smoke generators of 
the acetone-propane type which dispersed about 
50 g/hr of silver iodide. These were located at 
various points in and immediately upwind from 
the target at points where instructions for their 
operation could be given by telephone, and they 
were operated in groups of two to four at a time 
on 33 days during a period of three months at 
the times and places judged appropriate by the 
field meteorologist on the basis of forecasts and 
local pilot-balloon observations. Subsequently, in 
1952, similar operations were conducted over 
the same target for 11 months, encompassing 
most of the dry season and all of the showery 
season, and during several of these months op- 
erations were conducted also for the adjoining 
properties of several neighbors. 
In subsequent operations a different type of 
smoke generator was used, which burned string, 
impregnated with silver iodide, in a propane 
flame and dispersed silver iodide at a rate of 
about 10 g/hr, with about 15 generators situated 
in and upwind of the target. Operations were 
conducted in 1953 for five months which showed 
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