CLOUD DROPLET SPECTRA AND THE SPECTRUM OF NUCLEI 
the agreement is slightly better for the rather 
more realistic assumption of 1 m see. 
The surprisingly small effect of updraft speed 
on the computed droplet concentration is ex- 
plained by the shape of the observed spectra of 
critical supersaturations of cloud nuclei. In most 
eases these spectra could be represented by an 
equation of the form 
Neus" 
where N is the number of nuclei, per unit of vol- 
ume, with critical supersaturations less than S, 
and C and k are constants. On the basis of this 
equation, it has been shown [T’womey, 1959b] 
that the computed droplet concentration just 
above the activation region near cloud base is 
proportional to V"’°"® approximately, where 
V is the updraft speed. The observed spectra 
were fitted by values of k ranging from 0.2 to 
0.4. Thus the computed cloud droplet concentra- 
tions were proportional to a power of V between 
0.14 and 0.25. The conclusion of some authors 
that updraft speed has a more considerable ef- 
fect on cloud microstructure than is indicated 
by Figures 4 and 5 is essentially due to assuming 
cloud nucleus spectra which correspond to values 
of k much greater than those which have been 
found to fit measured spectra. While observed 
cloud nucleus spectra are occasionally found 
which rise steeply over a portion of their range, 
1000 
800 
600 
400 
200 
No. cm? (observed) 
100 
60 
60 100 200 400 600 800 1000 
No. cm3 (computed, for V=1m /sec ) 
Fic. 4—Comparison of mean droplet concentra- 
tions observed in cumuli with those computed from 
observed spectra of cloud nuclei, with an assumed 
updraft speed of 1 m sec’; the dashed line repre- 
sents exact agreement between observed and com- 
puted values 
1000 
600 
600 
400 
200 
No. cm73( observed) 
60 100 200 400 600 800 1000 
No. em™3 (computed, for V=10m/sec ) 
Fic. 5—Comparison of mean droplet concentra- 
tions observed in Cumuli with these computed 
from observed spectra of cloud nuclei, with an as- 
sumed updraft speed of 10 m sec™!. The dashed line 
represents exact agreement between observed and 
computed values 
the use of values of & of the order of 1 or 2 is 
unrealistic. In some instances the empirical law 
of Junge [1953] for nucleus size distribution has 
been invoked to deduce a supersaturation spec- 
trum for cloud nuclei. This corresponds, for 
soluble particles, to k = 2. However, Junge’s 
law was not intended to represent the supersatu- 
ration spectrum, but the size distribution of the 
heterogeneous natural aerosol particles down to 
0... The critical supersaturation of a nucleus 
is influenced by its chemical composition and 
surface properties, as well as by its size. Even 
micron-sized particles will remain unactivated at 
the shght supersaturations occurring in clouds if 
they are at all hydrophobic. 
Conclusion—These latest observations seem to 
provide definite confirmation of the view that 
cloud microstructure is primarily determined by 
the spectrum of cloud nuclei. It appears that 
some continental surfaces at least are source 
regions of cloud nuclei, so that Cumuli forming 
in air masses which have passed over them have 
large droplet concentrations and consequently 
consist of small droplets. They are therefore 
relatively inefficient in releasing warm rain, 
which can form only by the coalescence process. 
In the light of this result, a tentative ex- 
planation may be offered for the positive corre- 
lation observed between cloud droplet concentra- 
tions and sea-salt content found at places some 
