GROWTH OF CLOUD DROPS BY CONDENSATION 
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Drop Radius 
Fie. 5—Cumulative drop-size distributions after 
various elapsed times, Stratus Case A 
main smaller than 0.2 micron. Thus the separa- 
tion between the cloud droplets and the non-grow- 
ing nuclei is established. 
To see what this droplet growth means in terms 
of numbers of drops, the cumulative distribution 
curves are shown for various times in Figure 5. 
At 2400 sec there is only about one droplet per 
cm} larger than one micron; by 2700 sec this num- 
ber has increased to about 250, and there is al- 
ready almost one per em* larger than four 
microns; and at 3000 sec there are around 300 per 
cm’ larger than four microns, and about ten per 
liter greater than 10 microns. Thus the develop- 
ment from no cloud to a dense cloud occurs in a 
very few minutes. However, even after an hour, 
only one per liter has grown to 20 microns, where 
it might be expected, by the Langmiur theory, to 
start colliding with other drops. 
The development of the cloud, as distinct from 
the increasingly dense haze, is shown by the differ- 
ential frequency curves in Figure 6. While the 
computations for the relatively small number of 
groups do not establish these curves uniquely, the 
interpolation of the cumulative frequency curves 
199 
is sufficiently definite to indicate their general 
character. The initial nucleus distribution has a 
mode at about 0.03 micron. At 2700 sec there is 
just a slight suggestion that a second mode is 
developing at about two microns. By 3000 sec 
the mode is well established at 5.4 microns, and 
the gap between the cloud drops and the inacti- 
vated nuclei is conspicuous about three microns. 
The development of the second mode and the 
gap between the two modes is due to the fact 
that all the drops forming on nuclei above a criti- 
cal size (between 0.032 and 0.1 micron) grow, 
while those forming on smaller nuclei do not grow 
or actually shrink. This results in a decrease in 
number, and eventually development of a fre- 
quency minimum in the vicinity of the critical 
size. As the cooling continues the mode moves to 
larger values, reaching 12 microns at 6000 sec, 
and the gap appears to have moved upward to 
include seven microns. The nature of the gap is 
made clear if it is noted that between one and 
four microns at 3000 sec there are 90 drops, while 
between four and seven microns there are 310. 
At 6000 seconds there are 50 drops between two 
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Orop Radius 
Fie. 6—Differential drop-size distributions after 
various elapsed times, Stratus Case A 
