Radius (cm) 
° 300 600 300 
1200 1500 
Time (seconds) 
Fig. 16—Variation of relative humidity and 
growth curves for various drop-size groups, Trade- 
Wind Cumulus Case 
this case the very large number of nuclei in this 
group makes it the largest contributor to the 
liquid content. 
Conclusions—All four cases for which computa- 
tions were carried out, representing three widely 
differing rates of decrease of temperature and two 
different nucleus distributions, showed the tend- 
ency for the separation of cloud drop sizes from 
the inactivated nuclei. While the number of 
groups used in the computation is not sufficient 
to define it uniquely, it appears definite that a 
mode in the cloud drop sizes is established shortly 
after saturation occurs, and that this modal size 
increases as cooling continues. The mode is in the 
same general size range as in observed cloud drop 
distributions, but tends to move with time to 
larger sizes than in the observed distributions. On 
the other hand, there are fewer very large drops 
in the computed distributions than in size distri- 
NEIBURGER AND CHIEN 
butions observed in Cumulus congestus and Cu- 
mulonimbus [Diem, 1948, Weickmann and aufm 
Kampe, 1953). 
Since the initial conditions and rates of cooling 
for the observed cases were doubtless not the same 
as those assumed for the computations, the com- 
puted distributions cannot be expected to corre- 
spond closely to the observed. Nevertheless it is 
interesting to note that the processes neglected, 
namely coalescence and in the case of the Cumu- 
lonimbus the ice-crystal (three phase) process, 
would tend to keep the modal size small and in- 
crease the number of very large drops. 
As an example of the comparison of the com- 
puted distributions with observed values, Table 
3 gives the drop-size distributions observed for 
three cases of Stratus off the coast of California, 
taken by blimp (lighter-than-air ship) in the sum- 
mer of 1945, and the data for the Stratus Case B 
computation for 3000 and 6000 sec, as taken from 
Figure 9. The 3000-sec data correspond in a gen- 
eral way to the observed cases, but even this dis- 
10 Sel | 
Tee ata 
NN I 
(Ne i ast 
19 E- 7 ‘ - 11200 sec dat 
C -0 \6o oo} 12900 sacl |} 171800 sec 
F ce al 
o 
Si 
S =| 1 
ao 
3 fe 
R 10" ae = 
= |e 
is] 
& E 
BSE 
6 = 
2 \ 
$ 10° —— \- ———) 
HE 
$ WW 
ae ven bee 
E 10 E wil it 
Fi \ \\\ 
\\\th 
10° =! ee 
i Vi] 
\ \\ 
\ Mi 
We 
108 1 u eli 11 
1o® 10° 10* 10° 10? 10' cm 
oo! ol 10 10 100 1000 microns 
Drop Radius 
Fic. 17—Cumulative drop-size distributions af- 
ter various elapsed times, Trade-Wind Cumulus 
Case 
