426 
illustrate a comparison between the dry periods 
and the median values obtained from upper-air 
observations during 1953-1957. 
The spring months are clearly below normal 
in cloudiness while the precipitable water is still 
realtively high with 77% of the normal value. 
The remaining two seasons have over 90% of 
the normal precipitable water, but have less than 
50% of the normal cloudiness. Since cloud modi- 
fication practices to initiate or increase precipi- 
tation are dependent on the existence and 
amount of low cloudiness, the data presented 
indicate that investigations on methods of ini- 
tiating low clouds are equally as important as 
the seeding of existing clouds. van Straten [1958] 
recently attempted seeding cloudless air with 
carbon black to initiate clouds. The results of 
these experiments are encouraging, and certainly 
justify more research on the physics of cloud 
initiation. In all the dry periods investigated, 
there appears to have been an ample supply of 
moisture in the gaseous state, but for some rea- 
son the condensation process was not operating 
efficiently. It was suspected that the lack of 
cloudiness was due to the absence of vertical 
motions which are necesesary to the condensation 
process. However, an inspection of the individual 
dry periods does not indicate an absence of the 
conditions which would support convective ac- 
tivity m a number of cases. As an example, a 
ten-day dry period in July-August of 1953 was 
above normal in precipitable water content, and 
relatively unstable with a median stability index 
of +0.5. However, only 383% of the normal low 
cloudiness was observed. The surface maximum 
temperatures during the period were above 90°F, 
and a visual inspection of the thermodynamic 
structure of the atmosphere did not reveal any 
unusual, stable layers. Therefore, many of the 
conditions conducive to convective cloud forma- 
tion were observed, but the clouds did not form. 
An examination of the upper-air flow during 
this dry period revealed predominantly north- 
westerly flow over Illinois. In view of the fact 
that the macroscale conditions leading to cloud 
initiation and precipitation were present, it 
seems likely that the flow from the northwest 
was deficient in condensation nuclei. The Illinois 
State Water Survey, under a grant from the 
National Science Foundation, is now engaged in 
an airborne particulate sampling program to de- 
termine the distribution of nuclei during a wide 
variety of synoptic conditions. The results of 
RICHARD G. SEMONIN 
the particulate sampling research will provide 
data on the distribution of condensation nuclei 
during dry periods similar to that described. 
It is evident from the data presented that, in 
the majority of the dry periods investigated, the 
clouds and instability which are necessary for 
initiation of precipitation were not present and 
therefore, cloud-seeding practices would not 
have helped to alleviate the problem. 
Summary—This study has brought to light 
some of the interesting features of the conditions 
of the large-scale atmosphere during periods of 
little or no precipitation in the Midwest. The 
study is presently in its initial stages and will be 
developed more fully, but the preliminary re- 
sults present some interesting aspects of the 
problem of weather control. If we are to alleviate 
drought in the agricultural areas of the Mid- 
west, it appears that in addition to seeding 
existing clouds to increase precipitation, meth- 
ods for initiating clouds must be developed. 
If Ilhnois is considered representative of the 
Midwest, a large quantity of clouds desired for 
seeding are just not available during dry periods. 
Acknowledgments—The writer wishes to thank 
Wilham C. Ackermann, Chief, and Stanley A. 
Changnon, Climatologist, Illinois State Water 
Survey for their helpful comments, discussions, 
and careful review of the manuscript. This work 
was accomplished under the immediate super- 
vision of Glenn E. Stout, Head of the Meteorol- 
ogy Section of the Illinois State Water Survey. 
REFERENCES 
Berceron, T., On the physics of clouds and pre- 
cipitation, Proc. 5th Assembly, U.G.G_1., pp. 156- 
178, Lisbon, 1935. 
Cuaneonon, 8S. A., ano F. A. Hurr, Cloud distribu- 
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Rep. Invest. 33 Illinois State Water Survey, Ur- 
bana, Illinois, 83 pp., 1957. 
Lanemuir, I., Studies of the effects produced by 
dry ice seeding of Stratus clouds, G. E. Res. Lab., 
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York, pp. 121-135, 1948. 
OrvitLe, Howarp T., anv orHers, Final Report of 
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