CLOUD SEEDING IN THE AMERICAN TROPICS 
the updraft at the condensation level and on 
the size on the condensation nuclei, and we may 
regard some fraction of the largest cloud drop- 
lets as rudimentary precipitation particles that, 
if they survive long enough within the cloud, 
will become raindrops. 
The rate of formation of rudimentary precipi- 
tation particles in a given cloud depends on the 
concentration of giant hygroscopic nuclei, oc- 
currence of collisions, ete. Survival of them in 
the evaporating part of the cloud, or if they are 
thrown out of the cloud, depends upon their 
being large enough to maintain their existence 
until they are re-entrained or fall back into the 
cloud. Ludlam [1956] has estimated that a di- 
ameter of about 150 microns is critical for drop- 
lets carried by a bubble out the top of a cloud; 
those smaller will be lost; those larger will fall 
back into the cloud and continue growing. The 
establishment of precipitation, we assume, re- 
quires the survival of some critical number of 
rudimentary precipitation particles in the region 
of the cloud where the liquid-water content is 
high. Or, to express it another way, the cloud 
must accumulate a certain quantity of particle 
seniority in its rain factory. If the operating 
force is insufficient or the turnover is too high, 
this necessary quantity of seniority will not be 
accumulated, and although some few raindrops 
may fall, precipitation will not be established. 
But when this critical seniority is reached and 
precipitation is established, a mass of water that 
approximates, according to Braham [1952], 20% 
of the water entering the convective circulation 
(and probably with respect to the water in 
the active portion of the cloud a somewhat 
higher percentage) is removed by the rain and 
falls from the cloud. This water represents not 
only a loss of mass from the cloud but also 
a gain of the heat that would otherwise have 
gone to re-evaporate this water in the upper 
branch of the circulation. Both these effects 
operate in the direction of increasing the convec- 
tion in the cloud in which rain forms as com- 
pared with similar clouds nearby. 
Now, during the time that there are a num- 
ber of clouds having approximately the same 
size, the ascending air currents will tend to oc- 
cupy the maximum area consistent with unsta- 
ble motion according to the slice method of 
computing instability. But if one single cloud 
outstrips its neighbors and grows significantly 
higher, it will enter a slice of the atmosphere 
where it is the only rising current present, and 
419 
hence the instability it experiences will be much 
greater; its convective circulation will increase 
rapidly in magnitude and depth. The energy- 
producing part of the circulation is then able to 
draw on the whole depth of the moist layer as 
a source of moisture and energy, whereas for- 
merly only the lower portion of it was available 
so. These several effects working in concert cause 
the precipitating cloud to grow rapidly in size 
and in the intensity of its circulation, suppressing 
the unsuccessful competitors. Figure 5, which is 
a photograph of a Cumulus cloud 13 min after 
being seeded by dry ice published by Kraus and 
Squires [1947] and reproduced here through the 
courtesy of P. Squires, appears to show this ef- 
feet occurring under conditions similar to those 
of our model. 
At any time during the gradual growth of the 
convective clouds, we may describe their collec- 
tive approach to the rain stage as a frequency 
distribution of the property that we have called 
quantity of seniority in each cloud’s rain factory, 
which can be represented in the manner of Fig- 
ure 6 as an ogive of cumulative probability of 
the seniority having surpassed the value critical 
for production of rain in some one of the clouds 
within the model. If there are many clouds in 
the group, by the time the percentage of clouds 
surpassing the critical limit rises to a few per- 
cent (at a ‘seniority’ of about 12 on Figure 6), it 
becomes very likely that rain will have begun 
somewhere, in one of the clouds within the field 
of competition. We then presume that this oc- 
currence will be followed by rapid growth of the 
successful cloud and depress the level of conveec- 
tive activity elsewhere, causing the average sen- 
lority of the remaining clouds to diminish again. 
Let us consider the effect on this model of seed- 
ing one of the clouds with water droplets or 
hygroscopic particles during the time that the 
group of clouds is approaching the stage critical 
for the formation of rain somewhere in the group. 
The seeding in effect employs in the seeded cloud 
a large number of pre-aged rudimentary pre- 
cipitation particles and thereby increases the 
quantity of seniority in the operating force, and 
places the seeded cloud somewhat higher on the 
seniority scale than its neighbors, perhaps only 
by a few percent, or one unit higher on the arbi- 
trary scale of Figure 6. But it will be noted that 
before the group as a whole reaches the point 
where the formation of rain in it becomes likely, 
the seeded cloud will have a very good chance 
of becoming a rain-producer. 
