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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1. BACKGROUND

The purpose of winter cloud seeding over mountains is to increase the seasonal snowfall, thereby
augmenting water resources. The large majority of winter cloud seeding has used silver iodide (AgI) as
the seeding agent. But typical below-cloud AgI releases have not been demonstrated to be effective in
field projects unless the AgI particles are vertically transported to supercooled liquid water (SL W) cloud
colder than about _80C. A considerable set of observations has clearly demonstrated that SL W cloud near
mountain barriers is frequently too warm for effective AgI seeding in the western states. Because of
AgI's strong temperature dependence as an effective ice nuclei, many potentially seedable opportunities
are left unseeded when AgI is used as the only seeding agent. Development of a technology to seed such
mildly supercooled clouds would significantly increase the frequency of successful winter orographic
cloud seeding, not only in Utah, but throughout the western U.S. and elsewhere.

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The Utah Division of Water Resources, with primary support from the Bureau of Reclamation
(hereafter Reclamation), U. S. Department of the Interior, conducted an experimental cloud seeding
program using propane (C3Hg) on the Wasatch Plateau of central Utah. The goal was to demonstrate that
seeding winter mountain-induced (orographic) clouds by expansion ofliquid propane could result in
additional snowfall on the mountainous terrain, enhancing the seasonal snowpack. Spring and early
summer melting of mountain snowpacks is the major source for runoff and stream flow in most areas of
the western U. S., utilized by agriculture, municipalities, industry, and aquatic environments.

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This final report provides considerable detail about the 2003/04 winter field program conducted on the
Wasatch Plateau, and concerning the analyses of observations obtained, as well as analyses of
measurements from earlier Wasatch Plateau experimentation not previously accomplished. A publication
in a scientific journal (Super and Heimbach 2005a) provides a shorter but still technical version of the
infonnation within this report. A brief and less technical version can be found in Super and Heimbach
(2005b ).

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2. REASONS FOR TESTING AND IMPROVING PROPANE SEEDING TECHNOLOGY

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The earliest modem weather modification experiments occurred during 1946. They showed that dry ice
particles, much colder than the -40oC temperature threshold needed for homogeneous nucleation of ice
crystals, could seed SL W cloud. The tiny droplets which make up such clouds are colder than OOC, the
freezing point of bulk water, but are still in liquid rather than ice fonn. Such SL W clouds are common
over western U.S. mountain barriers during winter. Localized chilling of the air below -40oC can be
achieved by the rapid expansion of propane, a convenient compressed gas. The U.S. Air Force developed
the technology to create ice crystals in SL W fog by expanding propane (Vardiman et al. 1971). This
approach resulted in successful cold fog suppression at airports.

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During the past several years, propane dispensers have been used for winter orographic cloud seeding
on a limited basis, both experimentally and operationally. The main reason for this development is that
abundant observations have shown the frequent existence of SL W cloud at from OOC to about _80C, over
mountain barriers of the western United States. Conventional AgI seeding is usually not practical within
this temperature range because of the strong temperature dependence of AgI as effective ice nuclei. The
threshold temperature at which a tiny fraction of a large population of AgI particles produces ice crystals
is generally considered to be _60C. It may be slightly wanner for some recent exotic solutions of AgI and
other chemicals as tested in laboratories. The concentration of effective AgI ice nuclei increases by orders
of magnitude when exposed to SL W cloud only a few degrees colder than _60C.

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Commonly used AgI particles must be vertically transported to SL W cloud at temperatures of _80C or
colder in order to provide ice crystal concentrations sufficient to result in meaningful snowfall rates some
kilometers downwind of AgI generators. Silver iodide plumes undergo considerable horizontal and

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