CLOUD DROPLET SPECTRA AND THE SPECTRUM OF NUCLEI 
Figure 2 shows those found in a continental 
Cumulus of similar size. 
Interpretation of measurements of droplet 
spectra—This result was of course quite consist- 
ent with the observed distribution of warm rain. 
Indeed, when account was taken of other ob- 
servations such as those taken on the orographie 
cloud of Hawaii [Squires and Warner, 1957], 
there were five groups of clouds, namely ‘dark 
Stratus,’ Hawaiian orographic cloud, maritime 
Cumuli, Cumuli in transitional air masses, and 
continental Cumuli, arranged here in order of in- 
creasing median droplet concentration: this 
proved also to be the order of increasing col- 
loidal stability, as measured by the depth of each 
cloud type which normally yields rain [Squires, 
1958]. The recent calculations of Hocking [1959] 
on collision efficiencies indicate that the presence 
of a few relatively large collector drops is not 
sufficient to permit the coalescence process to 
proceed efficiently; it is essential that a signifi- 
cant amount of liquid water should be present 
in the form of relatively large drops, for only 
large drops can be efficiently collected. The ex- 
amples given in Figures 1 and 2 show that in 
the continental case there is practically no cloud 
water in the form of drops larger than 20 mi- 
crons diameter, while in the maritime case, the 
greater part of the liquid water is in the form of 
drops larger than 20 or even 25 microns diam- 
eter. 
The relationship between the colloidal stabil- 
ity of clouds and their microstructure seemed 
therefore fairly clearly established; it remained 
to be seen whether the difference in microstruc- 
ture between maritime and continental Cumuli 
could be explained in terms of the factors con- 
trolling the initial formation of the clouds. The 
suspicion mentioned in the introduction, that 
such contrasts in microstructure could only be 
explained in terms of differences in the popula- 
tion of cloud nuclei could not be directly con- 
firmed, since no method was available for meas- 
uring this small but important fraction of the 
Aitken nuclei, which becomes activated at very 
small supersaturations (rather less than one per 
cent), and consequently is effective in cloud for- 
mation. The only alternative was to examine the 
other possible factors which could influence the 
microstructure of clouds to see whether any of 
them seemed adequate to cause the observed dif- 
ference between maritime and continental Cu- 
muli [Squires, 1958]. Thus a comparison of flight 
notes of turbulence experienced in the two kinds 
213 
9800 ft 
605 
8200 ft 
11200 ft 
9100 ft 
PERCENTAGE OF DROPLETS 
8 
Ss 
Oo 10 20 0 10 20 0 10 20 0 10 20 
DROPLET DIAMETER (W)) 
Fic. 2—The mean droplet spectrum at four ley- 
els in a continental Cumulus over the Blue Moun- 
tains northwest of Sydney on November 2, 1956; 
cloud base was 7200 ft, cloud top at 12,000 ft; drop- 
let concentrations n per em* and liquid water con- 
ent wg per m*: 8200 ft, n = 490, w = 0.35; 9100 
fits 7 — 270s — 0.29; 9800itts 7, — 250, w' =. 0:31; 
11,200 ft, n = 350, w = 0.48 
of clouds indicated that over the clouds which 
had been sampled, the updraft velocities were 
on the average much the same, so that this fac- 
tor could not be responsible. Further, assuming 
equal updrafts, it could be deduced from ob- 
served droplet spectra that the supersaturation 
in the body of the cloud was higher in maritime 
than in continental Cumulh, despite the effect of 
giant sea-salt nuclei in reducing the supersatu- 
ration in the former. Thus there seemed little 
possibility that these giant nuclei caused the dif- 
ference in microstructure by depressing the max- 
imum value of the supersaturation and reducing 
the number of nuclei activated. A rather sur- 
prising confirmation of this latter point was 
given by simultaneous measurements of spectra 
of droplets and of giant sea-salt nuclei, made in 
southeastern Australia, 200 to 600 mi inland. 
Suppose for the moment that the cloud nu- 
cleus spectrum, that is, the largest few hundred 
nuclei, per em’, is invariant, apart from fluctua- 
tions in the giant sea-salt nucleus content (usu- 
ally less than one nucleus per em*). Then it 
seems possible that in air masses which are rich 
in giant nuclei the supersaturation may be de- 
pressed by their great absorptive power for wa- 
ter. This effect, if large enough, could lead to a 
negative correlation between sea-salt nuclei and 
cloud droplet concentration, such as is suggested 
by the continental-maritime contrast. If, as in- 
