Artificial Precipitation Potential during Dry Periods in Illinois 
RicHARD G. SEMONIN 
Illinois State Water Survey, Urbana, Illinois 
Abstract—The macroscale meteorological conditions of the atmosphere were studied 
during 31 dry periods which occurred in 1953-1955. A dry period is defined as at least 
five consecutive days with less than ten per cent of the normal precipitation over an 
area in east central Illinois. 
The parameters investigated, measured at Rantoul, Illinois, were: precipitable water, 
low cloudiness, and the Showalter stability index. The upper-air flow, surface tem- 
perature, and general synoptic conditions were considered in individual case studies. 
The results indicate that although there is near normal water vapor in the atmosphere 
during the majority of the dry periods, there was a deficit of low clouds. It is con- 
cluded from the study that in addition to present cloud-seeding techniques, much re- 
search is needed to determine means of initiating clouds, since, during dry periods in 
Illinois, large quantities of clouds desired for seeding are not available. 
Introduction—Bergeron [1935] postulated 
nearly 30 years ago that precipitation was the 
result of microphysical processes in clouds con- 
taining a mixture of ice particles and subcooled 
water. However, little progress was made to- 
ward a better understanding of the precipitation 
process during the ensuing 20 years. Langmuir 
[1948] and Schaefer [1948] began to make fur- 
ther progress in the study of precipitation phys- 
ics with their laboratory and field experiments 
in cloud seeding. These experiments definitely 
illustrated the possibility that man could arti- 
ficially affect the microphysical processes taking 
place within clouds. 
Following these poineer experiments, con- 
siderable research has been directed toward 
modifying clouds by attempting to increase the 
efficiency of the physical processes which initiate 
or enhance precipitation. While it is of the ut- 
most importance to determine the physical struc- 
ture of the clouds, it is equally important to ex- 
amine the macroscale conditions attending these 
cloud formations. 
The President’s Advisory Committee on 
Weather Control [Orville and others, 1957] re- 
cently disclosed that the most effective cloud- 
seeding experiments have been performed in 
areas where pronounced orographic effects are 
present. Little evidence has been found for es- 
tablishing the suecess of seeding clouds to in- 
crease rainfall in flat-land areas, such as the 
Midwest. 
This paper summarizes an attempt to ex- 
amine some of the more obvious macroscale 
parameters of the environmental atmosphere, 
such as precipitable water, low cloudiness, and 
stability durmg periods when artificial stimula- 
tion of precipitation is most needed; that is, 
during periods when the natural precipitation 
process is either not functioning or is very in- 
efficient. 
Analysis and results—The dry periods chosen 
for study in this investigation consisted of at 
least five consecutive days during 1953-1955 in 
which no measurable precipitation occurred at 
Rantoul, Illinois. To assure that no portion of 
overlapping wet periods was included in the 
dry-period analysis, the beginning and ending 
days of each dry period were deleted from the 
dry period. Thus, a defined dry period of five 
days was, in reality, a period of seven days with- 
out precipitation at Rantoul. 
Inasmuch as a point observation of rainfall 
sometimes can be unrepresentative, the dry 
periods, as determined from the rainfall records 
at Rantoul, were investigated further on an 
areal basis. An average areal value was calcu- 
lated from the precipitation values for seven 
stations within a 50-mi radius of Rantoul. The 
dryness of each period was then evaluated by 
comparing the daily average areal precipitation 
of the period to long-term daily averages. If less 
than ten per cent of the average amount was 
observed, the period was accepted as a dry 
period. This selection yielded 31 cases for in- 
vestigation as dry periods. 
To conduct this study, the Illinois State Water 
Survey obtained IBM punch cards from the 
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