WALLACE E. HOWELL 
Generator 
Scale of kilometers 
fe) 10 20 
Sl —— 
HODOGRAPH OF WINDS ALOFT 
Generator 0 10 20 30 
a 
Scale of knots 
Arrows represent 
one hour of mean wind travel 
_30 CARIBBEAN SEA 
Fra. 4—Isohyetal map of rainfall and hodograph of winds aloft for cloud 
seeding trial, October 18, 1951 
between the areas of rainfall and the positions 
of the smoke plumes at from one to two hours of 
wind travel from the generators. Another third 
of the maps showed some weak connection, and 
in the remaining third no connection was found. 
Figure 4 illustrates an exceptionally good con- 
nection. Furthermore, it was noticed that most 
of the days when there were isolated heavy 
showers showed a good connection, while most 
of the days when rain was more or less general 
showed weak connection or none at all. The 
analyses suggested that the seeding was most 
effective under Sequence II conditions, and it is 
these conditions that form the basis for a new 
model of shower development. Although it was 
not found possible to treat these results rigor- 
ously, they together with the pilot balloon runs 
and local cloud observations gave the field me- 
teorologist a feeling of considerable confidence in 
directing the seeding effects onto the target. 
The _ field-of-competition model—We have 
taken for our model not a single cloud but a 
portion of the atmosphere, overlying a uniform 
ground surface, extensive enough to contain sev- 
eral convective cells. We suppose that heat and 
moisture are added slowly at the bottom of this 
atmosphere, creating a ‘moist layer’ of condi- 
tional instability which gradually deepens 
through the upward transport of heat and mois- 
ture by convective clouds. Horizontal transport 
is presumed sufficient to maintain more or less 
horizontal uniformity throughout the region. 
Above the moist layer the air is assumed to be 
relatively dry and with slight positive stability. 
This part of the atmosphere constitutes a 
field within which a number of convective cells 
compete for the potential energy that they can 
convert to air motions. At each stage in the heat- 
ing and deepening of the surface layer there is 
an optimum size of convection cell that repre- 
sents a balance between the advantages of larger 
cell size for drawing energy from more air and 
the disadvantages of lengthening horizontal 
transports, the optimum size becoming larger as 
the surface layer deepens; and all the competing 
cells will tend to approach this optimum size. 
Within the field of competition there will there- 
fore be a number of clouds that in their mature 
stage are of nearly equal size and in which the 
probability of precipitation is nearly equal. 
Following the model developed by Ludlam 
[1956] and others, we consider the clouds as 
formed by a series of bubbles or ring vortices 
that entrain air from their environment, pro- 
ducing energy in the lower part of the cloud, 
while at the top and sides of the cloud exchange 
of air with the environment causes evaporation, 
cooling, and subsiding motion. The bubbles, while 
within the cloud, lose water by mass exchange 
but not by evaporation, but once they reach the 
cloud top they begin evaporating rapidly, cool- 
ing and falling back as they dissipate but leaving 
a moister environment for the next bubble. Cloud 
droplets form in a variety of sizes depending on 
