ARTIFICIAL PRECIPITATION POTENTIAL DURNG DRY PERIODS 
U. 5. Weather Bureau containing upper-air 
soundings at 12-hr intervals during 1953-1957 
at Rantoul, Hlinois. The precipitable water con- 
tent for the layer from the surface to 400 mb 
was computed from the punch-card data by a 
digital computor at the University of Ilhnois. 
The Showalter stability index was computed 
in the standard manner for the twice daily ob- 
servations at Rantoul during the five-year pe- 
riod. A negative index value indicates unstable 
conditions and the numerical value is related to 
the degree of instability. Monthly median values 
were determined from these computations and 
are used as normals throughout the remainder 
of this paper. 
Cloud data for this study were obtained from 
a previous study of cloud distributions in II- 
linois made by Changnon and Huff [1957] of the 
State Water Survey. The hourly observations of 
clouds were summed to provide a total number 
of tenths per day of individual cloud types. In 
discussing cloudiness during dry periods, only 
low-cloud data were used since the observation 
of higher clouds is very dependent on the amount 
of low cloudiness. Furthermore, cloud seeding 
techniques to initiate or increase precipitation 
are more readily performed on the low clouds. 
The clouds used in this investigation are: Stratus, 
Stratocumulus, Cumulus, Cumulonimbus, and 
Nimbostratus. 
Daily normal values of precipitable water, 
stability index, and low clouds were calculated 
and used as a basis for determining the normal- 
ity of observations obtained during the dry pe- 
riods. The data obtained for this investigation 
were studied on an annual, seasonal, and indi- 
vidual case basis. Table 1 illustrates the median 
values of precipitable water, cloudiness, and sta- 
bility for the 31 dry periods during the years 
1953-1955. 
The Table indicates that, although the pre- 
cipitable water is within ten per cent of normal 
during dry periods, the cloudiness is well below 
the normal value. The absence of cloudiness is 
not a surprising feature during dry periods, but 
the near normal values of precipitable water in- 
dicate that a mechanism causing the initiation of 
clouds is absent. The investigation of the Sho- 
walter stability index was made in an effort to de- 
termine some information on the thermodynamic 
state of the atmosphere during these dry periods. 
A total of 610 calculations of the stability index 
were made for the dry periods, and only nine 
per cent of them were equal to or less than zero. 
425 
At the same time nearly 80% of the observations 
were more stable than the five-year median val- 
ues. Thus, very stable atmospheric conditions, 
which are unfavorable for the formation of pre- 
cipitating clouds, are predominant during dry 
periods. 
An examination of the 31 cases involved in 
this study disclosed that in 90% of the dry pe- 
riods the cloudiness experienced was less than 
the normal amount. However, in only 71% of 
the periods was the amount of atmospheric mois- 
ture below normal. Eighty per cent of the pe- 
riods investigated were more stable than the 
five-year seasonal median values of the stability 
index. 
In an attempt to determine facts more perti- 
nent to the problem of weather modification, 
these data were studied on a seasonal basis. Ta- 
ble 2 illustrates the results of the seasonal in- 
vestigation. 
It is noted in the Table that during dry pe- 
riods in winter there is considerable more cloud 
cover than in any of the other seasons, even 
though the winter dry periods are 31% below 
normal. The percentage of precipitable water 
in winter is relatively high, indicating a supply 
of moisture available for the precipitation proc- 
ess if it is initiated. However, the stability index 
is more stable than would be expected for the 
winter months which have a five-year median 
of +12.7. Admittedly the meaning of the Sho- 
walter stability index in the colder months of the 
year is somewhat dubious, but it does serve to 
TaBLE 1—Median values of environmental 
conditions during 1953-1955 Dry Periods 
Duration | Precip. water® Clouds® | Stability Index 
ho |- 
days 
8.0 | 90.8 SNe? | EEN 
* Per cent of normal determined from daily 
averages. 
TABLE 2—Seasonal median values of environmental 
conditions and duration of dry periods 
Item Winter Spring | Summer Fall 
Duration, days 8.0 8.0 8.5; 10.0 
Clouds, per cent 69.0 | 19.2 | 49.5] 49.3 
of normal | 
Precipitable 81.8 | 76.8] 95.4] 91.0 
water, per cent 
of normal 
Stability index +16.5 | +8.5 | +4.0 | +8.0 
