WMOD00412

• Title: Final Report on Utah Cloud Seeding Experimentation Using Propane During the 2003/04 Winter
• Date: 3/1/2005
• State: UT
• Weather Modification - Doc Type: Report

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The remaining 35 plume passes were used to document the south and north plume edges relative to the
TAR. The upwind highway tracks nearly due north of the TAR for about 3.5 km and no SF6 was ever
detected further north. South of the TAR the highway tracks closer to southeast-ward than south. For
simplicity, plume edges were calculated to the nearest 0.1 km due north or south of the TAR, based on
GPS observations and plots of each pass.

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Figure 3 is an example ofa van passage through a SF6 plume. The tracer gas concentrations twice
dropped to zero within the plume. These "gaps" were caused by brief injections ofSF6-free gas at one
minute (hereafter min) intervals to document the detector baseline. In this case the plume extended from
0.4 km south of the TAR to 1.9 km north of it, having a width of2.3 km. Mean and median plume widths
for all 35 passes were 2.1 and 1.9 km, respectively.

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The typical 17 min plume transit time from the HAS to the TAR, documented by Holroyd and Super
(1998), was used to lag the time of matching HAS wind direction for each van passage by the TAR.
Early 1994 HAS data were recorded as 10 min averages. The 10 min block which included the time 17
min before van passage by the TAR was used to estimate mean wind directions. In a few cases that time
was the same as the sampling block boundary and the two adjoining sampling blocks were averaged.

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Figure 4 plots the southern edge found on each plume passage against the HAS average wind direction.
In two cases van sampling could not reach the southern edge as the upwind highway turns northeastward
2.5 km south of the TAR. These edges were assigned a value of2.5 km. A least squares linear regression
line was fit to the 35 data points as shown on the plot. While a fair amount of scatter exists, 55 percent of
the southern edge position variance (R-squared where R is the correlation coefficient associated with the
regression line) was explained by the HAS wind direction.

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Instrumented TAR = 0,0 km
Gauge GSO = -1.07 km
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200

220 240 260
HAS Mean Wind Direction (deg true)

280

300

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Fig. 4. Plot of south em SF6 plume edges south of the TAR versus HAS wind direction.

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