WMOD00061

• Sponsor Name: Utah Department of Natural Resources
• Title: Propane Cloud Seeding Expiriment Overview: Wasatch Plateau, Utah, During Winter of 2003-2004
• Prepared For: Utah DNR
• Prepared By: Dr. Arlin Super and James A. Heimbach, Jr.
• Date: 3/1/2005
• State: UT
• Weather Modification - Doc Type: Scientific Study

Establishing whether propane seeding could
produce meaningful net snowfall increases from a
population of severa! storms was a significant
advance in the development of this potentially
important seeding technology. Propane seeding has a
very important advantage over AgI in being able to
create ice crystals within "mildly" SL W cloud, at
temperatures slightly below OOC. Unless SL W cloud
is colder than -80C, AgI seeding is usually
impractical, because its ability to nucleate sufficient
ice crystals is highly temperature dependent. Silver
iodide can be effective as warm as -60C if released
directly into SL W cloud, but this requires high
altitude generators not used by most operational
seeding projects. Because propane releases can
provide abundant seeded ice crystals in the "warm
temperature window" from _20C to -6 or -80C, its use
can provide an important adjunct to AgI seeding.
Winter observations from several ranges in the
Intermountain West and the Sierra Nevada of
California have documented that the SL W cloud zone
near the mountainous terrain is frequently at
relatively warm temperatures where AgI seeding is
not effective.

Silver iodide causes "hetrogeneous nucleation"
involving the bringing together of tiny AgI particles
with SL W cloud droplets resulting in formation of
seeded ice crystals. However, propane seeding
involves "homogeneous nucleation" as tiny SLW
cloud droplets cannot continue to exist in liquid form
at temperatures less than -40oC, but freeze into ice
crystals. Propane expansion is a convenient means of
locally chilling the air well below that temperature
and, thereby, producing vast numbers of tiny cloud
droplets which immediately freeze. As with AgI
seeding, the resulting seeded ice crystals will grow
within SL W cloud. When growth times are
sufficient, as when the seeded crystals are transported
by forced airflow up the windward slope of a
mountain barrier, snowfall can result. The same
forced airflow produces the orographic SL W cloud
which is the "raw material" needed for winter
mountain-induced cloud seeding to work.

As true for many gases, propane is technically a
"greenhouse gas" but its lifetime in the atmosphere is
under a month, too short to function in this manner.
In contrast, CFCs have atmospheric lifetimes in the
range of 60 - 500 yrs. For comparison, methane 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 ozone layer.

~

The 2003/04 randomized experiment was limited in
duration and target area size by available funding. It
was not designed to answer all remaining questions,
in particular the magnitude of seeding effects more
than about 6.5 kilometer (hereafter kIn) downwind of
the seeding site. (One kIn = 0.62 miles.) The gauge
furthest downwind was only 2 kIn east of the plateau
top's west (upwind) edge because of targeting
uncertainties when using a single seeding location
under varying wind directions. However, the
experiment was intended to test whether routine
seeding could produce meaningful additional
snowfall over the upwind slopes and western portion
of the plateau top. The experiment succeeded in
demonstrating such increases to a reasonable level of
statistical certainty, thereby providing justification
for further use and development of this important
emerging seeding technology.

Experimental Design and Conduct

Because the randomized experiment was
"exploratory" in nature, without firm analysis rules
stated beforehand, the convention will be to refer to
all findings as "suggestions" rather than "proof" in
the sense the latter word is used by scientists and
statisticians. Establishment of firm hypotheses, rules
and procedures usually requires knowledge not in
existence prior to analysis of an exploratory
experiment, and is used for follow-up "confirmatory"
statistical experiments. On the other hand, testing
methods for this experiment were specified in
advance as was the automated randomized treatment
scheme which was based on the glaciogenic
hypothesis and previous research on propane seeding.
When the entire experimental unit population is
examined without partitioning into subpopulations,
and when a partition is specified in advance as "the
wind direction partition appropriate for targeting,"
referred to in the experiment's proposal, it could be
argued that those portions of this experiment pass
somewhat beyond the usual exploratory experiment
limitations. That is, the statistical suggestions may
have somewhat more weight than the usual
exploratory experiment where many different tests
are made searching for a possible seeding signal.

Seeding was done by expanding liquid propane
through nozzles from a high elevation seeding
location known as HAS for High Altitude Seeding
location. Comprehensive plume tracking during
earlier winter projects demonstrated routine targeting
of the mountain top observatory, known as TAR for
target, and nearby precipitation gauge, GTR, when
HAS winds were from the southwest quadrant. All
equipment locations are shown on Fig. 1, a map of

2