WMOD00473

• Title: Evaluation of Bridger Range Winter Cloud Seeding Experiment Using Control Gages
• Date: 12/12/1983
• Weather Modification - Doc Type: Report

DECEMBER 1983

ARLlN B. SUPER AND JAMES A. HEIMBACH, JR.

1993

experimental day. the first author then subjectively
estimated the missing data, insuring that the sum of
the 6 h estimates agreed with any precipitation total
of unknown time distribution for the period in ques-
tion. It is believed that this procedure did not introduce
human bias into the experiment because of the esti-
mator's ignorance of the status of each period. It is
again emphasized that 96% of all possible data was
measured. All precipitation data were reported in Part
II in 1974 and the same data are used in this analysis.

b. Rawinsonde system

A Weather Measure 1680 MHz RD-65 rawinsonde
system was operated about 10 km west of the Main
Ridge crest during the 1970-71 and 1971-72 winters.
While data were occasionally lost due to system prob-
lems, reliability was generally high. All launch prep-
arations were made and data recorded in accordance
with procedures contained in Federal Meteorological
Handbook No. 3(1969).

c. Main Ridge thermographs

Two recording thermographs were maintained in
the same standard weather shelter atop the crest of the
Main Ridge at 2595 m (see Fig. 1). A precision mer-
cury-in-glass thermometer was also in the shelter with
its bulb thermally lagged to closely correspond to that
of the thermographs. The thermometer was read im-
mediately upon opening of the shelter door for each
weekly servicing. The mean difference in readings be-
tween the thermometer and each thermograph for the
winter season was used to establish the final calibration
for the chart readings. Chart temperatures were ex-
tracted at 1 h intervals and then averaged for the 6 h
means reported in Part II. With the use of two ther-
mographs, a complete data set was obtained without
any missing temperature measurements.
While shelter temperatures are known to be influ-
enced by solar radiation, this effect should be limited
by the cloud cover during storms and by the well ven-
tilated exposure of the Main Ridge site. Moreover, the
snow-covered terrain should reduce the effects of the
underlying surface.

d. Seeding generators

The first attempts to seed clouds over the Bridger
Range took place during the winter of 1968-69 with
AgI generators at two foothill locations at 1550 m,
well below the 2600 m crestline. However, measure-
ments obtained that winter revealed the presence of a
persistent stable layer during snowfall that often ex-
tended from the upwind valley floor to about midway
up the west or windward slope of the Bridger Range
(Super et a/., 1970). Consequently, there were no ap-
parent means for the AgI to be transported into the
inter. 1 vrographic cloud region during a large portion

of the seeded periods. Because of this, generator sites
were later established at two remote sites located about
two-thirds of the way up the slope of the. Bridger Range,
near the 2150 m level (see Fig. 1). These sites were
difficult to maintain, with winter access limited to foot
travel and helicopter. However, they were judged nec-
essary for achieving reliable transport and dispersion
of the AgI into the orographic clouds.
The Montana State University AgI generators used
during the 1970-71 and 1971-72 winters were among
the highest output ground-based units tested at the
CSU Simulation Laboratory as reported by Garvey
(1975). They were particularly effective at warmer
temperatures. Fig. 2 shows their effectiveness values,
as reported by the CSU Simulation Laboratory, for
two wind speeds past the burner: natural draft (- 1-
2 m S-I) and maximum fan (-10 m S-I).
Wind speeds were measured at generator stack height
during all seeded periods of the 1971-72 winter. A v-
erage speeds at the northern and southern sites were
only 1.3 and 1.8 m S-1 respectively, as both generators
w~re in relatively sheltered locations to minimize
flameouts. Since average wind speeds seldom exceeded
3 or 4 m s-J it is probably appropriate to use the
natural draft curve in Fig. 2 to approximate field con-
ditions. It can be seen that generator output increases
very' rapidly as temperature decreases until about
-120C. At lower temperatures the output reaches only
about twice the -120C value. However, the values at
-80C and -lOoC are factors of approximately 150
and 5 less than the -120C output respectively. This

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FIG. 2. Calibration of Montana State University
Silver Iodide Generator.