218 
years a gradual increase of condensation nuclei 
oceurred which may have had some effect on 
cloud and precipitation physies along the lines 
Dr. Squires has indicated. The importance of 
sulfate for the composition of condensation nu- 
clei in remote places is for instance well demon- 
strated by the recent finding that in Greenland 
ice the concentration of sulfate is higher by one 
order of magnitude than that of all the other 
constituents, including sea salt. 
If it is true that the northern hemisphere is, as 
a whole, polluted to such an extent as indicated, 
it may be that the difference between ocean and 
continent with respect to nuclei concentration 
is more pronounced in the southern hemisphere 
where this pollution must be negligible, and the 
original natural conditions still prevail. This 
should result in a corresponding difference with 
respect to cloud behavior. I wonder if anybody 
has thought about this interesting problem? 
Dr. P. Squires—I am very happy that there 
are some observations in the northern hemi- 
sphere which confirm this. 
Dr. L. J. Battan—lIt is true that the concen- 
trations measured by the University of Chicago 
were similar, but there is one thing which I 
wanted to ask about. As you know, in the mari- 
time distribution there was a bimodality which 
was evident after averaging as well as in vir- 
tually every individual observation. There was 
one maximum in frequency at a radius of about 
15 microns and another at about 5 microns. I 
wonder if there is anything in your discussion 
which would suggest that there should be a dif- 
ference in the shape of the curves as well as in 
the concentration. 
Dr. Squires—The methods we have used are 
not good below five microns. The crux of the 
matter, however, is how many drops share the 
liquid water effectively, and the ones down 
around four or five microns are perhaps not im- 
portant. 
Mr. Jerome Namias—The problem of drought, 
on which I have been working for some 25 years, 
is very fascinating. Its incipience over the United 
States in summer is associated with the large- 
scale features such as I described earlier; namely, 
the presence of the upper-level contmental anti- 
cyclone with its neighboring anticyclones in the 
Atlantic and Pacific. If the continental cell van- 
ishes, it does so only temporarily so that it re- 
curs persistently. This happens so frequently 
that a forecast rule is: ‘All signs fail in times of 
drought.’ Some recent studies of mine dealing 
DISCUSSION 
with the problem of its maintenance suggest 
some influence of the condition of the soil below. 
(Discussed in the paper Persistence of Mid- 
Tropospheric Circulations between Adjacent 
Months and Seasons, to be published in the 
forthcoming Rossby Memorial Volume, Stock- 
holm, 1959.) But the only way I guessed this 
influence could operate was through a thermo- 
dynamic mechanism involving the availability 
or non-availability of the latent heat of vaporiza- 
tion, depending on the dryness or wetness of the 
underlying soil. Dryness would perpetuate the 
upper anticyelone through thermal heating. 
However, I am intrigued by the possibility im- 
plied by Dr. Squires that under very dry con- 
ditions rain might be inhibited because the 
clouds, once formed, contain too many nuclei 
and perhaps of an unfavorable kind. It could be 
that this is one of the feedbacks of nature to 
provide a sort of memory for long-term situa- 
tions. Certainly if we isolate the other factors 
such as I mentioned, and are able to do this 
quantitatively, the nuclei factor should receive 
consideration as a feedback. 
Dr. R. Wexler—Admitting the importance of 
the nuclei count, other factors may also cause 
considerable differences between land and sea 
clouds. The greater moisture content of the air 
over the sea should cause a greater liquid water 
content per unit depth of cloud than over the 
land. Entrainment of drier air into the cloud 
over the land may inhibit ram development. 
Measurements in Cumulus over both land and 
sea, reported by Battan and Reitan in the last 
Woods Hole conference, show only slight differ- 
ences in cloud drop size between those which de- 
veloped echoes and those which did not. Hence, 
the failure of ram to develop from these clouds 
cannot be ascribed to the lack of large drops. 
Dr. Squires—In this series of measurements, 
the liquid content was perhaps 20 to 30% higher 
in the maritime clouds. This shght difference 
would not go far towards explaining the radically 
different ability of maritime and continental 
clouds to produce rain by the coalescence proc- 
ess. Some much more drastic effect, such as 
Hocking’s calculations suggest, seems to be called 
for. 
The differences which cause some clouds to 
rain while others in the same air mass do not 
are probably too minute to be observed. At the 
present stage of knowledge, we merely wish to 
contrast continental clouds, which are so re- 
luctant to form rain by the coalescence process, 
