DISCUSSION 
427 
Discussion 
Dr. W. E. Howell—May I speak to this ques- 
tion from the point of view of rain-making ex- 
perience. Of course anyone in my position makes 
90% of his new contacts during the 10% of the 
dryest weather. One is always asked the question, 
“Can you make it rain in the middle of a 
drought?” I can honestly say that our reply is 
“No.” However, that is not the end of the mat- 
ter. These people do not come to us in the first 
five dry days; they are not particularly ap- 
palled by any five-day period without rain. It 
is when they have gone 30 or 60 days with only 
a fraction of the normal precipitation that they 
begin to worry about drought. By then, the 
question is not, “How suitable is a typical 
drought day for cloud seeding?” but “How fre- 
quently during an extended drought do seeding 
opportunities present themselves?” We have 
made, therefore, a rather detailed study for 
Louisiana defining a drought situation as a pe- 
riod beginning with the sixth day after a gen- 
eral rain, and continuing as long as the cumula- 
tive amount of precipitation was less than a 
tenth of an inch a day. In the 39-month period, 
January 1949-March 1952, we found 400 drought 
days, covering a trifle over a third of the time, 
in 37 individual drought periods, mostly of a 
few days duration; but there were several more 
extended periods of drought: for instance, Au- 
gust 26-December 20, 1950; April 28 to June 
10, 1951; and November 8 to December 17, 1951. 
Now, in the longest of these, August 26 through 
December 20, 1950, there were 31 out of 116 
days, about “4, during which small amounts of 
rain fell somewhere on or near the target. It was 
similar for the other drought periods. 
It is our feeling that these occasions that punc- 
tuate the typical drought are quite good for 
cloud seeding when the precipitation mechanism 
almost reaches the stage of rain but does not 
quite get there; our experience working in this 
kind of condition is on the whole quite favorable. 
We feel that we can not actually break a drought, 
but we also feel that we can considerably di- 
minish its intensity during certain critical parts 
of the drought. 
Mr. Jerome Namias—I would like to say a 
little about this problem of drought, and also 
mention a few things about criteria for the for- 
mation of rain. The criteria Mr. Semonin used 
were the Showalter stability index and the total 
precipitable water. Those two indices are cer- 
tainly not the best criteria for the occurrence of 
rain. It is well known that the vertical compo- 
nent of air motion is the central factor in the 
formation of rain, and Mr. Semonin’s criteria 
would often not be very wise choices. Also, varia- 
tions in precipitable water of five per cent have 
to be considered against the knowledge of the 
normal variability; and not considered small. We 
have to know the frequency distribution of it, 
particularly in the Midwest, where this distribu- 
tion is extremely important. Now, with regard 
to the situation Mr. Semonin described; I have 
tried to look it up, although the precise dates 
were not mentioned in his abstract. I could get 
just a broad seale picture, but I have studied 
that drought period as well as many others. 
Figure 1 depicts the 700-mb conditions at- 
tending the great Dust Bow] drought of 1936. It 
is somewhat similar in nature to the period that 
arose during the seven-year drought from 1951 
to 1956 over Texas and adjacent areas of the 
southern Plains. The main thing to note in this 
figure is the great anti-cyclone in the mid-tropo- 
sphere into which dry air from the westerlies is 
recurrently injected. A good deal of subsidence 
takes place, resulting in lack of clouds, and the 
excess insolation during the daytime raises tem- 
peratures as much as 10° above normal; this 
means that temperatures between 100 and 110°F 
are frequent over the drought area. The two 
conditions, very hot and very dry, go hand in 
hand. 
An important point is that this great drought- 
producing cell depends in large part on the exist- 
ence of two neighboring cells, one in the Atlantic 
and one in the Pacific; both have to be anoma- 
lously strong, so that they are, so to speak, in 
resonance. If one or both of the oceanic cells 
should vanish, the U.S. cell would die. But once 
this mechanism is set up, there are also some 
life-sustaining properties for the continental cell 
which remain to be explained. It might be, for 
example, that Squires’ suggestion of the differ- 
ent nuclei counts and kinds in continental and 
maritime regions might be important here. There 
are also some other factors which might be 
raised, such as changes in the characteristics of 
the underlying surface itself, and presence or 
lack of water on this surface. In other words, 
solar heat may be used for evaporation or for 
