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10° 10° 10" 10° 10? lovcm 
0.01 (o 10 10 100 1000 microns 
Drop Radius 
Fie. 9—Cumulative drop-size distributions after 
various elapsed times, Stratus Case B 
the more numerous 0.056 micron group also con- 
tinues growing. 
The Cumulonimbus case—The more rapid cool- 
ing due to the vertical velocity distribution as- 
sumed in the Cumulonimbus model results in 
saturation being reached much more quickly than 
in Stratus Case A (1180 vs 2650 sec), and 
a slightly higher supersaturation being attained 
(0.16 vs 0.14 per cent, see Figure 12). Neverthe- 
less the separation between the activated and the 
non-activated groups of nuclei is the same; the 
0.1 micron group grows rapidly to 100 times its 
original radius and continues growing, while the 
0.032 micron group reaches less than eight times 
its original radius at the maximum supersatura- 
tion and then gradually shrinks as the degree of 
supersaturation goes down. 
By 1760 see (less than ten minutes after satura- 
tion) the average radius of all the growing groups 
exceeds ten microns, and by 3450 sec they all have 
grown to more than 20 microns. Thus, as shown 
in the cumulative distribution curves (Fig. 13), 
there are about 150 per em larger than ten mi- 
NEIBURGER AND CHIEN 
crons (although only 12 per 10° cm larger than 
20 microns) at 1800 sec; at 2400 sec the number 
larger than 20 microns has increased to about one 
per liter, and by 3600 seconds it exceeds 60 per 
em’. 
It should be noted that the numbers per unit 
volume are altered as the air parcel rises because 
of the expansion of the volume of the parcel with 
decreasing pressure as well as because of the drop 
growth. 
Assuming that the growth by accretion would 
become significant when the drop radii exceed 20 
microns, precipitation development by the warm- 
cloud process would begin at about 2400 sec (20 
min after saturation) when the parcel has reached 
2300 m (7500 ft), with a temperature of 12°C. 
The 0°C level (4750 m) is reached at 3180 sec, 
and the —10°C (6400 m) at 3400 sec. Thus there 
would be more than 15 min during which the 
warm process (collision and coalescence) would 
be active in producing raindrops before the ice- 
erystal process could begin, and radar echoes 
might be expected at about 2500 m (8000 ft). 
Drop Concentration (cm~>) per Centimeter Interval of Radius 
; \ 
10'cm 
1000 microns 
Drop Radius 
Fia. 10—Differential drop-size distributions after 
various elapsed times, Stratus Case B 
