CLOUD SEEDING IN THE AMERICAN TROPICS 
creased. In exactly the same way as with the 
water-seeded warm cloud, then, the seeded cloud 
is given an advantage that expresses itself in the 
form of a much improved chance that the seeded 
cloud will be the one in the group that will first 
develop precipitation. It will be noted that this 
effect of AgI seeding operates through the coa- 
lescence mechanism and that it is unnecessary 
to postulate the independent growth of new 
rudimentary precipitation particles in the form 
of ice crystals entirely by sublimation in order to 
account for an effect of the AgI seeding. 
When the seeding releases precipitation within 
the field of competition, the ensuing chain of 
events draws to the seeded cloud the energy 
from a much larger share of the atmosphere 
than it could otherwise have reached. In effect, 
the ‘signal’ energy released directly by the seed- 
ing is amplified many times over, and the mag- 
nitude of the output is determined not so much 
by the strength of the signal as by the energy 
resources of the system. The outcome will be 
sometimes the initiation of rain on a day when 
rain would not otherwise have fallen, in which 
event the precipitation efficiency of the model as 
a whole is increased; and sometimes the outcome 
will be to direct onto the target area the center 
of shower development that might otherwise 
have been elsewhere, and to a certain extent to 
increase the precipitation efficiency of the shower 
mechanism. 
Discussion—Of course, natural occurrences 
present all gradations between the Sequence I 
and Sequence II types of development described 
above. On the other hand, certain locations are 
more suitable for the occurrence of one sequence 
or the other. For example, we may contrast the 
situation of the Francisco target, already dis- 
cussed, where the usual trajectory brings air for 
some considerable distance over a rather uniform 
level terrain, with the situation of the Necaxa 
Watershed in Mexico and with the Boca Raton, 
Florida, site of the Project Seabreeze studies. 
In the latter two, strong localized impulses are 
available to initiate convection, the orography in 
the Necaxa location and the sea breeze at Boca 
Raton, so that one might expect more frequently 
to find Sequence I followed at these sites. In- 
deed, examination of the plots of cloud top 
height against time for the Project Seabreeze 
observations [MacCready and others, 1957] 
shows that in most cases the clouds more than 
doubled in height and reached altitudes greater 
421 
than 25,000 ft within less than an hour. The ef- 
fect of AgI seeding might therefore be expected 
to be less at these locations than on the Francisco 
target. Referring back to Table 2 we note that 
the rainfall stimulation at Necaxa is indicated 
to be less than that at Francisco, in accordance 
with this expectation. 
It has often been noted that horizontal shear 
in the wind sometimes decapitates growing Cu- 
mulus clouds, carrying away the portion of the 
cloud containing the best-developed rudimentary 
precipitation particles and thereby preventing or 
delaying the onset of precipitation. However, 
shear also contributes to the energy available 
for convection when the cells reach an appropri- 
ate size; and if the field of competition is char- 
acterized by vertical shear, a sudden jump in 
the growth rate of the clouds will appear when 
they approach the size at which they can utilize 
the energy of the shear for their growth. How- 
ever, this jump will probably appear throughout 
the field at about the same time, and indeed it 
may have much to do with the occasional de- 
velopment of Sequence I clouds causing more or 
less general rainfall. However, our experience in 
the tropics suggests that the jump usually does 
not occur until a cloud has begun precipitating, 
and that the shear energy then contributes to 
the selection of the favored cloud for still more 
vigorous development. 
The operation of the  field-of-competition 
model suggests that the effects of cloud seeding 
will be extremely variable, often ineffective but 
sometimes extremely effective, with an average 
effectiveness that depends to some extent upon 
the habitual sequence of cloud developments over 
the seeded region. These characteristics are in- 
deed suggested by the data on actual seeding so 
far accumulated. The model further suggests 
that, if it is practical to isolate for experimenta- 
tion those situations characterized by Sequence 
IL developments, effects of seeding will occur 
that may be large enough to make their reality 
immediately apparent. 
Another implication of the model is that ex- 
periments performed in clouds situated over lo- 
calized sources of convective impulses, such as 
isolated mountain peaks or small tropical islands, 
while capable of making trial of the physical 
changes that may be produced within a single 
cloud by seeding, will tend not to demonstrate 
to full advantage the significance of the com- 
