162 
kilometers of cloud depth is needed, and a little 
longer time, too. Even in this very simple case, 
the actual pattern of the motion is important. 
The motion is important, and the time span is 
important. 
Dr. H. G. Houghton—I am sure Dr. Wexler 
agrees with this. 
Dr. J. Smagorinsky—It would seem that the 
question of efficiency not only arises in the trans- 
formation from cloud particles to precipitation 
particles, but also enters into the formation of 
condensed particles from water vapor. I think 
one rarely finds that the entire mass is cooling 
at the moist adiabatic rate. If one is to consider 
the budget for the entire water content, that is 
water vapor, suspended liquid water, and pre- 
cipitating water, the question of the efficiency 
should be considered in each transformation. 
Dr. Horace R. Byers—I was very glad to see 
Dr. Wexler give a definition of efficiency. I think 
this is something that has been used in too loose 
a sense by many people. I hope that those of us 
who are called upon to review papers for jour- 
nals and other publications insist that the au- 
thors very carefully define what they mean by 
efficiency and use terminology that will not fur- 
ther confuse the issue. 
Dr. Helmut Weickmann—As I feel Dr. Wex- 
ler’s lecture has a very important bearing on 
the problem of rain making, I may be permitted 
to elaborate a little on my own thoughts on this 
particular subject. Figure 2 shows another ex- 
pression of the water-budget equation with the 
terms on the left side representing the water 
source, and those on the right side representing 
the water sink. Of course, the growth processes, 
condensation and coalescence of the precipitation 
particles constitute the water sink; the updraft 
and water storage in the cloud constitute the 
water sources. Note that the number of the pre- 
-———_ 
SOURCE 
WATER SUPPLIED BY 
SINK 
WATER STORED| WATER EXTRACTED FROM CLOUD 
WEATHER MECHANISM| IN CLOUD | BY PRECIPITATION MECHANISM 
| do i) KoWV 
dane w 27K 2 
anes + N400r (= Apr <P ) 
tL \ \ 
(SUFPLY)) (STORAGE) | (CONDENSATION) (ACCRETION ) 
da 
= "ABS. HUM. TEMP. 
dT BS. HUM, VS. TE 
yY = TEMP. LAPSE RATE 
us UPDRAFT 
W*CLOUD WATER CONTENT 
N= CONCENTRATION OF PRECIP. PART. 
r= RADIUS 
K,* VENTILATION FACTOR 
D = DIFF. CONSTANT 
Ap*VAPOR PRESS. DIFF. 
K2a= COLLECTION EFFICIENCY 
W*CLOUD WATER CONTENT 
V«FALLING VELOCITY OF PARTICLES 
P*PARTICLE DENSITY 
DISCUSSION 
cipitation particles is a factor on the right side, 
therefore, increasing their number through seed- 
ing increases only the efficiency of the sink, but 
unless there exists water to be depleted on the 
left side, no increase in the rate of precipitation 
will occur. This may be illustrated by a simple 
model for the precipitation process which Louis 
Battan has called the ‘Weickmann plumbing 
model.’ Figure 3 gives the conditions in a releaser 
cloud. Here the particles grow through sublima- 
tion; humidity is at or near ice saturation, and 
the liquid or solid water content is negligible. The 
water which is released through the updraft dif- 
fuses to the snowflakes and, if there are many, 
each stays small; if there are a few, each grows 
large. This process is illustrated by a container 
which has a perforated base. Each hole represents 
a snow crystal and the water which flows into the 
container supplied by the updraft mechanism 
runs out through the holes. If the number of the 
holes is increased the droplets become smaller, 
but the rate of rain does not increase. If a rain- 
drop recorder could be exposed to such a type of 
precipitation and we would plot the number NV 
of precipitation particles falling within a certain 
small period of time, their median volume diame- 
ter D, and the rate of precipitation R, then the 
curves for N and D should be inverse propor- 
tionally since R does not change. 
In the case of the spender cloud the conditions 
may, however, be different (Fig. 4). Here the in- 
n——_ 
FE ier epiee &s | ey Lene 
ap Ve VaN aor rma i 
6 
INFLUX = DRAINAGE ‘ —> time 
t) 
6 
6 NE=PARTICLE CONCENTRA- 
Se ee TION 
D=meoian volume 
DIAMETER 
RR=RATE OF PRECIPITATION 
Fia. 3—Releaser cloud, W = 0 
n—— 
D————— 
ida rapeihews 
at 7 VtWe New? —S— w 
a — 
INFLUX +STORAGE=DRAINAGE —time 
N=PaARTICLE CONCENTRA- 
TION 
Demeoian voLUME 
DIAMETER 
i i 
R= RATE OF PRECIPITATION 
Fia. 2—Water budget of precipitation 
Fig. 4—Spender cloud, W ¥ 0 
