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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separation, aircraft instrumentation sometimes detected one or both plumes and sometimes overflew
them. In other words, aircraft sampling was typically near plume tops. Because no aircraft sampling was
practical during the 2003/04 experiments, and because of expected veering of the wind with altitude, the
aircraft observations have not been considered here. These are mentioned because they documented that
plumes released from the site 4 km south of the HAS were typically further south than could be sampled
by the instrumented van, that is, more than 2.5 km south of the TAR. An instrumented observatory was
maintained during early 1994 very near the southern end of the upwind highway. It provided continuous
surface observations of AgI while the van and aircraft were sampling. Holroyd et al. (1995) presented a
detailed case study of the southern plume which shows it meandered back and forth, sometimes detected
at the surface observatory, and sometimes south of it, depending on the wind direction.

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The five days with isolated HAS-released plumes ofSF6 crossing the upwind highway provided a total
of 42 van traverses (passes) through the SF6 gas plume. Seven of these were rejected because oflow peak
SF6 concentrations and associated narrow plume widths, likely related to limitations in instrument
sensitivity. There were problems with SF6 release rates during some of the rejected cases, or SF6 release
had terminated shortly before the passes. Typical SF6 plume peak concentrations exceeded 500 parts per
trillion (ppt) by volume, with several near or above 1000 ppt. But six of the rejected plumes wer,e below
100 ppt and the seventh reject was 190 ppt. The next lowest value was over 310 ppt, providing a natural
breakpoint for accepting or rejecting plumes for further analysis.

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Pass 21 of 27 Feb 94
600 Concentration of SF6
released from HAS V5.
N-S Distance from TAR
500
~
Q.
.eo
c
0
:p 400
g
c
B
c
0
<.> 300
t/)
~
If
U)
200

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-2.0

-1.0 0.0 1.0
Kilometers North or South from TAR

2.0

3.0

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Fig. 3. An example ofSF6 tracer gas plume detection by the passage of an instrumented van on 21 Febmary 1994.

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