212 
to be consistent with the observed predominantly 
maritime oceurrence of warm rain. The most 
significant conclusion was that observations of 
the spectra of both cloud droplets and cloud nu- 
clei would be required in order to resolve these 
basic questions in the physics of warm rain. 
Measurements of droplet  spectra—Cloud 
droplet spectra have been measured by several 
observers, but generally without any specific 
reference to the problems mentioned above. Ob- 
servations were begun near Sydney in 1950 and 
evidence quickly appeared which seemed to be 
consistent with the suggestions drawn from the 
distribution of the occurrence of warm rain and 
the characteristic difference in Aitken count 
between oceanic and continental air; there ap- 
peared to be a significant difference in micro- 
structure between Cumuli in maritime and conti- 
nental air masses. Maritime Cumuli contained 
both fewer and larger droplets. The picture was, 
however, confused by other effects: as Zaitsev 
[1950] found, there is a tendency for the drop- 
let concentration to decrease upwards in clouds. 
Furthermore, large Cumuli tend to have smaller 
average droplet concentrations than small ones. 
It so happened that among the earliest observa- 
tions, the maritime Cumuli sampled were signifi- 
cantly larger than the continental Cumuli, and 
furthermore, the sampling runs in the former, 
for operational reasons, had not been evenly dis- 
tributed throughout the vertical extent of the 
3250 ft 
50 
4250 ft 
Nn 
o 
PERCENTAGE OF DROPLETS 
10 
DROPLET 
DIAMETER 
P. SQUIRES AND 8S. TWOMEY 
clouds, but were mostly taken in their upper 
regions. 
It seemed therefore that some or all of the 
apparent difference between maritime and conti- 
nental Cumuli could have been due to these two 
factors: (1) cloud size, and (2) the height of 
the sampling run above cloud base. In 1953 it 
was found that there was a serious deficiency in 
the method then in use for measuring droplet 
diameter, although the droplet concentrations 
were not affected. This difficulty was not finally 
solved until 1954. Further observations in con- 
tinental clouds in southeastern Australia and of 
maritime clouds off the Australian coast, as far 
south as latitude 42°S, and off Hawaii in lati- 
tude 19°N left no doubt that the suspected con- 
trast between maritime and continental Cumuli 
was real. The median droplet concentrations 
found were 45 and 228 droplets em™ respec- 
tively. These results are in very good agreement 
with those of Battan and Reitan [1957] who 
found median droplet concentrations over the 
central United States and over the Carribbean 
which are very close to the medians quoted 
above for continental and maritime clouds re- 
spectively. The observations taken in 1954 and 
later yielded drop sizes as well as concentrations, 
and confirmed the expected result that a strong 
negative association exists between droplet size 
and concentration. Figure 1 shows typical drop- 
let spectra found in a maritime Cumulus, and 
20 40 
(») 
Fie. 1—The mean droplet spectrum at four levels in a trade-wind Cumulus off the east coast of Ha- 
wali on October 23, 1954; cloud base was at 2200 ft, cloud top at 9500 ft; there was some light rain in 
parts of the cloud, and turbulence was moderate; droplet concentrations (n) per em# and liquid water 
content (w) g per m3: 3250 ft, n = 74, w = 0.50; 4250 ft, n = 51, w = 0.53; 5250 ft, nm = 27, w = 0.50; 
and 6250 ft, n = 30, w = 0.42 
