216 
hundreds of miles inland. It has been observed 
that the most continental clouds, that is those 
with the highest droplet concentrations, occur 
during dry weather. In such weather also, the 
highest counts of cloud nuclei have been ob- 
served; unfortunately, simuitaneous observa- 
tions have not been obtained in these conditions. 
The salt content of an air mass moving inland 
is redistributed by vertical mixing during dry 
weather, so that the concentration in sub-cloud 
levels is somewhat reduced; but in wet weather 
the greater part of the salt is washed out. Thus 
when the air mass concerned has not rained since 
leaving the coast, there may be a tendency for 
both the salt and cloud droplet concentrations 
to be high. As regards the association between 
dry weather and high cloud nucleus and cloud 
droplet concentrations, it may be noted that dry 
weather would favor the formation of nuclei 
from fires or perhaps by the drying out of soluble 
salts [Twomey and McMaster, 1955]. 
It seems almost self evident that the release 
of particles into the atmosphere from the land 
surface would be most efficient in dry weather. 
For when the surface is dry, and evaporation 
slight, the surface temperature rises to very high 
values during the day, so that the surface of the 
soil and the air layer above it are very dry, 
while the lowermost layer of the atmosphere is 
strongly unstable. 
REFERENCES 
Barran, L. J., anp C. H. Rerran, Droplet size meas- 
urements in convective clouds, Artificial Stimu- 
lation of Rain, Pergamon Press, pp. 184-191, 1957. 
Hocxine, L. M., The collision efficiency of small 
drops. Q. J. R. Met. Soc., 85, 44-50, 1959. 
DISCUSSION 
Houcuton, H. G., A preliminary quantitative anal- 
ysis of precipitation mechanisms, J. Met., 7, 363- 
369, 1950. 
Howe tt, W. E., The growth of cloud drops in uni- 
formly cooled air, J. Met., 6, 184-139, 1949. 
Junce, C., Die Rolle der Aerosole und gasférmi- 
gen Beimengungen in der Luft im Spurenstoff- 
haushalt der Troposphire, Tellus, 5, 1-26, 1953. 
Lanomutr, I., The production of rain by a chain 
reaction in Cumulus clouds at temperatures above 
freezing, J. Met., 5, 175-192, 1948. 
Squires, P., The growth of cloud drops by conden- 
sation, pt I: General characteristics, Aust. J. Sci. 
Res., Ser. A, 5, 59-86; pt II: The formation of 
large cloud drops, /bid, 5, 473-499, 1952. 
Squires, P., The microstructure and colloidal sta- 
bility of warm clouds, Tellus, 10, 256-271, 1958. 
Squires, P. anv S. Twomey, Some observations re- 
lating to the stability of warm Cumuli, Tellus, 
10, 272-274, 1958. 
Squires, P., anp J. Warner, Some measurements 
in the orographic cloud of the island of Hawaii 
and in trade wind Cumuli, Tellus, 9, 475-494, 
1957. 
Tevrorp, J. W., A new aspect of coalescence the- 
ory, J. Met., 12, 436-444, 1955. 
Twomey, 8., Condensation nuclei at low super- 
saturations, pt I: The chemical diffusion method 
and its application to atmospheric nuclei, Geo- 
fisica pura e applicata, 1959a (in press). 
Twomey, S8., Condensation nuclei at low supersatu- 
rations, pt Il: The supersaturation in natural 
clouds and the variation of cloud droplet con- 
centration, Geofisica pura e applicata, 1959b (in 
press). 
Twomey, S., anp K. N. McMaster, The produc- 
tion of condensation nuclei by crystallizing salt 
particles, Tellus, 7, 458-461, 1955. 
Wretanp, W., Die Wasserdampfkondensation an 
natiirlichem Aerosol bei geringen Ubersittigun- 
gen, E.T.H., Ziirich, Promotionsarbeit 2577, 1956. 
Zaitsev, V. A., Water content and distribution of 
drops in Cumulus clouds, Glavaia Geofiziche- 
skaia Observatoria, Trudy, 19, 122-132, 1950. 
Discussion 
Dr. C. E. Junge—This paper seems to me a 
very important one and I would like to con- 
gratulate the authors for their work. I would like 
to comment on two items: 
(1) Concerning the supersaturation spectra, 
the reason for the difference in these spectra in 
continental and maritime air is the difference in 
the size distribution of the condensation nuclei. 
The relationship between supersaturation and 
size can be calculated on the basis of the well- 
known growth curves of salt droplets. This rela- 
tionship is somewhat influenced by the chemical 
composition of the nucleus but not too much as 
long as a good portion of the nucleus consists of 
soluble salts. This relationship was recently con- 
firmed indirectly by measurements of the growth 
curves of nuclei with relative humidity down to 
sizes of about 0.01, radius (Ord, Hurd, Hendrix, 
and Junge, The Behavior of Condensation Nuclei 
under Changing Humidities, J. Met., 15, 240- 
242, 1958). The relationship between supersatu- 
ration and size can therefore be regarded as re- 
liable and can be used to convert nuclei size 
spectra into supersaturation spectra. 
In our measurements of the size distribution 
of nuclei, which were confirmed by others. we 
