DISCUSSION 
brought about by chance. This brief note has 
been written as a progress report for others 
working on similar problems. After more data 
are collected, it is hoped that some of the press- 
ing questions in the important area of cloud 
seeding can be given definite and unequivocal 
answers. 
Acknowledgments—The authors gratefully ac- 
411 
knowledge the contributions to this program 
made by personnel of the central and the Tuc- 
son offices of the U. 8. Weather Bureau. 
The main financial support for this work has 
come from the National Science Foundation and 
the State of Arizona. In 1957, the research pro- 
gram was conducted as a cooperative effort with 
the University of Chicago. 
Discussion 
Mr. J. Namias—The evidence looks very in- 
teresting here, and very convincing at first 
glance. I would just like to ask about the method 
of selection. You pointed out, if I understood 
correctly, that the criterion of whether you 
would have clouds was determined from the 
amount of precipitable water? 
Dr. L. J. Battan—That is correct. 
Mr. Namias—And then you chose a pair of 
days with the seeded day determined by the 
process of randomization. The question is 
whether you had a bias when you seeded on 
either the first or the second day. It is therefore 
very important that the data on which the evi- 
dence is based is evenly distributed between the 
first and the second day. 
Dr. Battan—The data were very well dis- 
tributed. This was one of the things the statis- 
ticians looked up right away. 
Mr. Namias—That would be a rather impor- 
tant consideration, for if there were any bias, 
your criterion would imply a certain synoptic 
situation, the second day of which is not at all 
independent of the first, so you could have a 
certain situation that would either augment or 
inhibit. But if the evidence is evenly distributed 
between the two days, my point does not hold. 
I was thinking also of the possible factor in- 
volving the preceding day’s precipitation and its 
effect on the surface. As I pointed out earlier, a 
relationship between thunderstorms occurring in 
southern Texas on one day and their lack in the 
same area on the subsequent day has been as- 
cribed to wetting of the ground the first day. 
Dr. Battan—This is exactly why we used ran- 
domization by pairs of days, because if you ran- 
domized by day, one might be led to seed or not, 
seed on three or four consecutive days. With the 
present scheme the most you can get is two con- 
secutive seeded or not-seeded days. 
Dr. R. D. Elliott—I might add that the State 
of California Forestry Service has conducted 
cloud seeding experiments in the northern part 
of the State for a number of years to see if 
lightning could be reduced, and the answer so 
far seems to be that the hghtning is actually in- 
creased on the seeded days. The experiments are 
randomized, but the outstanding factors are that 
the precipitation is mereased, and this fits very 
well also with what Dr. Singer said about the 
Swiss Hail Suppression Experiment. 
Mr. C. EB. Anderson—I want to raise a point 
about whether or not these experiments are truly 
randomized from the seeding-agent standpoint. 
This would apply to both the Swiss experiment 
and to the ones you conducted in New Mexico, 
where you released silver iodide on a particular 
day for seeding, and then did not seed the next 
day. I recall the seeding experiment that the 
Weather Bureau conducted on the West Coast as 
part of the Artificial Cloud Nucleation Project 
which the United States Government sponsored 
some five years ago. During this time we had 
made some trials in cooperation with Ferguson 
Hall who was in charge of that project in the 
State of Washington, with release of zinc sul- 
phide tracer materials from the aircraft at 15,000 
ft. We were quite surprised to find that the zine 
sulphide turned up (and this was during pre- 
cipitation at the release level) in the rain gages, 
not that day, but the next. Therefore I am just 
wondering whether or not, when you release 
this in the atmosphere, you can depend on its 
being removed overnight, so that the next day 
is truly a fresh sample. 
Dr. Battan—We do not have measurements, 
which can dispel this argument; but I find it hard 
to understand how it can happen. 
