WMOD00388

• Title: Feasibility of Snowpack Enhancement from Colorado Winter Mountain Clouds: Emphasis on Supercooled Liquid Water and Seeding with Silver Iodide and Propane
• Date: 9/30/2005
• State: CO
• Weather Modification - Doc Type: Report

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of crystals fomled too close to a barrier crest, especially if a broad valley exists upwind of the
next barrier (Reynolds 1996).

3) Several investigations have demonstrated the best sites for ground-based release of either
AgI or liquid propane are at high altitudes, at least midway up the windward slopes from the
valley floor to crestline elevations. Propane dispensers must be located high enough to be within
or just below liquid cloud base, and at temperatures at least slightly below ooe. While AgI
generators are usually run below cloud, it is best to operate them within liquid cloud at _60C or
colder so the forced condensation freezing mechanism can function. This mechanism results
from high water vapor concentrations immediately downwind of generators, a byproduct of the
combustion of propane and acetone (Finnegan and Pitter 1988, Chai et aI. 1993). The mechanism
provides large concentrations of seeded crystals just downwind of the generators, maximizing
both output and crystal growth times. Operating AgI generators below cloud base can also be
effective but only if the seeding agent is vertically transported to altitudes where temperatures are
colder than about -80e.

4. Seeding by Expansion of Liquid Propane

1) It has long been recognized that AgI seeding is ineffective for mildly supercooled cloud,
that is, warmer than about -6 to -80e. The "threshold temperature" at which a tiny fraction of a
large population of AgI particles can produce ice crystals has long been stated as near -60C, well
below the OOC freezing point of bulk water. Some recent papers suggest that newer formulations
of AgI have threshold temperatures of -50C or even -4OC. Unfortunately, the Colorado State
University (CSU) cloud chamber facility, used for many years as the standard for testing AgI
effectiveness as functions of SL W cloud temperature and other variables, is no longer
operational. Consequently, such claims are difficult to verify. The authors are unaware of any
published observations which show significant AgI nucleation at such warm temperatures in
natural (not laboratory) cloud.

2) A far more important point is that some authors have presented threshold temperatures in a
manner which erroneously suggests that AgI seeding can be effective at temperatures 2: _60e.
That is not the case except for the important exception of forced condensation freezing discussed
above. But that mechanism requires that AgI be released directly into SL W cloud, requiring use
of high altitude remote-controlled generators. Few operational seeding projects operate AgI
generators high enough to be in cloud at temperatures of _60C or colder. Therefore, the following
discussion is generally applicable.

3) It is well known that AgI effectiveness is highly temperature dependent, with the percent of
the total AgI particle population able to nucleate ice crystals increasing by orders of magnitude
from about -6 to -l2OC. For example, DeMott et al. (1995) used the CSU cloud chamber facility
to show that a particular commercial AgI generator with unusually good warmer temperature
yield (effectiveness) had an increase in ice crystal production over its -60C output which was 33
times higher at -80C and 314 times higher at -120C. The same comparison for the type of AgI
e-enerator used in the Bridger Range Experiment (Super and Heimbach 1983) showed the increase
,m the _60C effectiveness was 770 times higher at -80C and about 7000 times higher at -l2OC.
o tie Bridger Range Experiment strongly suggested significant snowfall increases for ridgeline
I ~mperatures of -90C and colder. But the half of all experimental periods with ridge temperatures
warmer than -9.50C showed no significant seeding effect. And it is likely that much of the
,uccess of the Bridger Range Experiment was due to frequent AgI release directly into cloud,