214 
10,000 
1000 
N m-3 
160 
0 100 200 aS 300 400 
nocm 
Fic. 3—Seatter diagram showing a positive cor- 
relation between the concentration of cloud drop- 
lets in inland Cumuli (7) and that of giant sea salt 
nuclei (NV), near cloud base level; corresponding 
points for maritime Cumuli would fall in the 
hatched area 
dicated by the theoretical analysis quoted above, 
the effect of the giant nuclei is not great enough 
to achieve this, one would expect to find no cor- 
relation between the concentrations of sea-salt 
nuclei and cloud droplets. In fact, observations 
taken to test this hypothesis showed a strong 
positive correlation between sea salt and cloud 
droplet concentration as shown in Figure 3 
[Squires and Twomey, 1958]. While emphatically 
indicating the absence of the negative correla- 
tion, and so confirming the theoretical conclu- 
sion that the giant nuclei are not responsible 
for the maritime-continental contrast, these ob- 
servations indicated a positive association in- 
stead of a random scatter and so raised other 
questions which will be discussed later. 
These indirect considerations, although fairly 
conclusively disposing of alternative explana- 
tions, could never establish in a final manner 
whether variations in cloud nucleus spectra were 
really the major cause of variations in cloud 
microstructure. For this purpose, there is no sub- 
stitute for direct measurement of the spectrum 
of cloud nuclei. 
Measurements of cloud nuclei—The formidable 
P. SQUIRES AND 8S. TWOMEY 
difficulties in the way of this measurement need 
little emphasis. While it is a simple matter to 
produce a small expansion instead of a large 
one, it is not easy to be sure of the desired 
initial condition of saturation with the accuracy 
required, which was estimated from the cloud 
spectra data as being equivalent to a few thou- 
sandths of a degree C. Again, droplet growth at 
small supersaturations is naturally quite slow, so 
that a new problem appears: that of maintaming 
the supersaturation for periods of minutes. This 
problem is made the more difficult by the fact 
that even apart from the inevitable wall effects, 
the mere growth of the droplets tends to reduce 
the supersaturation. 
These problems were first overcome by Wie- 
land [1956] using a series of diffusion chambers, 
the top and bottom of which were held at slightly 
different temperatures, so that a region of small 
supersaturation was established, and maintained, 
in the centre of the chamber. Twomey [1959a] 
used diffusion between a water surface and a 
dilute aqueous solution of hydrochloric acid to 
establish and maintain a small supersaturation 
in a closed chamber. Using this method, Twomey 
[1959b] established that there are large and sys- 
tematic differences between the cloud nucleus 
populations of maritime and continental air 
masses, at ground level, and was able to show 
that a representative surface nucleus spectrum, 
assuming an updraft speed of 1 m see”, would 
produce a cloud with a droplet concentration of 
around 500 em™ in continental air and of about 
60 em“ in maritime air. (Observed median drop- 
let concentrations, 228 and 45 respectively.) 
This result seemed to leave little doubt that 
the difference in the cloud nucleus population 
was indeed the main cause of the difference in 
cloud microstructure. The fact that the com- 
puted values lay somewhat above the observed 
ones could be explained if, as would seem likely, 
the surface layers were richer in cloud nuclei 
than the air entering the bases of Cumuli. 
Simultaneous observations were needed to re- 
solve this question, and these were taken late in 
1958, mostly about 200 mi inland from Sydney. 
The result, comparing observed mean droplet 
concentrations found on one flight with those 
computed from the cloud nucleus spectrum ob- 
served in air sampled below cloud base, is shown 
in Figure 4 for an assumed updraft speed of 
1 m sec’, and in Figure 5 for a speed of 10 m 
see’. As will be seen, updraft has little effect; 
