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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vertical dispersion as they are transported over rugged mountainous terrain, thereby greatly reducing
initial AgI concentrations. An important exception to the _80C limitation is "forced condensation
freezing" which can produce abundant ice crystal concentrations at temperatures as high as _60C
immediately downwind of AgI generators located within cloud, as demonstrated by Finnegan and Pitter
(1988) and Chai et al. (1993). But most operational seeding programs release AgI at relatively low
elevations, well below SL W cloud.

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There is no doubt that propane seeding under appropriate conditions can produce light snowfalI,
demonstrated by cloud chamber and field tests reported by Hicks and Vali (1973), and laboratory work by
Kumai (1982). Some short-term physical experiments over the Wasatch Plateau of central Utah during
the mid-1990s provided obvious and irrefutable evidence of propane seeding effects (Super and Holroyd
1997). Holroyd and Super (1998) showed evidence that about 20 ice crystals L-1 would be sufficient to
produce light snowfall, and such concentrations were routinely achieved with a two nozzle propane
dispenser. Reynolds (1989, 1991, 1994) developed a radio-controlled propane dispensing system for
California because of the high frequency of mildly SLW clouds over the Sierra Nevada. Super et al.
(1995) described Utah tests of the same propane dispenser system which showed that it was inexpensive
and very reliable, primarily because of its simplicity. A propane dispenser needs only a tank of propane,
readily available from commercial sources, an insulated exhaust tube to release liquid propane, a
particular type of nozzle at the end, and a remotely-controlled on/off value in the tube. Inclusion of a
temperature sensor immediately beyond the nozzle, and a flowmeter in the exhaust tube, provide
monitoring that propane is indeed being dispensed and at the desired rate. Other compressed gasl;:S, like
liquid carbon dioxide, can be used in a similar manner (Fukuta 1996). In contrast, radio-controll(~d AgI
generators are much more complex, susceptible to clogging, and seldom are equipped to monitor actual
Agl-in-solution flow but only flame temperature. Therefore, one cannot be certain from afar that AgI is
actually being released.

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There is a great deal of propane (C3Hg) and butane (C4HIO), another hydrocarbon, being released by
human activities at a scale far larger than for propane seeding. I Propane does not present an
environmental hazard because of its rapid oxidative degradation. Although it is technically a grel;:nhouse
gas, its lifetime in the atmosphere, about one month, is too short to function in this manner. In contrast,
CFCs have atmospheric lifetimes in the range of 60 - 500 yrs. For comparison, methane (CH4) has
stronger bonds which results in slower oxidizing allowing a buildup sufficient to contribute to the
greenhouse process. There is no known influence of propane on the ozonosphere (ozone layer). Ethyl
mercaptan, a sulfur compound added to propane in minute quantities to make it detectable by smell, poses
no hazard.

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Super et al. (1995) reported on the first totally automated propane seeding system for winter orographic
cloud in the early winter of 1994. Observations of SL W presence at the central propane dispenser were
used to automatically turn it, and two crosswind dispensers, on and off as SL W was available or not. An
icing sensor was used to detect SL W. Storm phases with available SL Ware usually short-lived, seldom
more than a few hours duration. This automated seeding system continues to be used operationally in
Utah by the Emery Water Conservancy District. A nearly identical system has been in use by th~: Eden
Valley Irrigation District in Wyoming since the winter of 2000/0 1. One propane dispenser was used for
the first two winters in Wyoming, and two thereafter. The propane generators were added to thre:e AgI
generators, which have been in use by that district since the mid-1970s.2

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Results of experimental propane seeding on the Wasatch Plateau during the 1994/95 and 1995/96
winters have been reported by Super and Holroyd (1997) and Holroyd and Super (1998). Most
experimentation during these winters used AgI seeding and propane seeding was attempted only when

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I Personal communication with Dr. Bert E. Holmes, Chair, Dept. of Chemistry, U. of North Carolina at Asheville,
20 Feb. 2005.
2 Personal communication with Bonnie Moody, Manager, Eden Valley Irrigation District, 22 Dec. 2004.

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