WMOD00274

• Title: Western Kansas Weather Modification Program 2006 - Final Report
• Prepared By: Walter E. Geiger III
• Date: 10/30/2006
• State: KS
• Weather Modification - Doc Type: Report

with the parent storm providing it both moisture and potential natural hail embryos. However,
the basic effect of seeding is the same in both cases---to promote the formation of abundant
numbers of ice crystals within a supercooled cloud volume before large hail can form in that
same cloud volume.

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AgI-based seeding agents promote ice crystal formation by heterogeneous nucleation,
when ice crystal numbers increase as temperatures decrease in growing clouds. Homogeneous
nucleation, however, is the instantaneous change of water droplets into ice crystals at all sub-
freezing temperatures such as what happens when dry ice pellets contact water droplets.

Although AgI produces greater numbers of ice nuclei than does dry ice, gram for gram,
large numbers of ice nuclei can be produced more quickly by dropping comparatively larger
amounts of dry ice directly into the moisture-laden cloud updrafts found in the new-growth
feeder cloud towers. Palletized dry ice is dispensed from a container which is augured into a
opening in the aircraft floor, subsequently falling directly into the clouds. The container carries
about 200 lbs. of dry ice pellets and is released at a rate of 5 lbs. per minute. Relatively large
amounts of dry ice are needed to produce the same number of ice crystals from a given mass of
AgI, roughly 1,000 to 2,000 grams dry ice is equivalent to a gram of AgI. Dry ice pellets impact
clouds water droplets as they fall through the cloud creating ice crystals by homogeneous
nucleation. Droplets also may be brought into the wake of the falling dry ice and freeze into ice
crystals as well. The main difference in their use is this: AgI based seeding agents, while rising
in the cloud, can only begin to create ice crystals at temperatures near -4C to -5C, or roughly
2,000 to 2,500 feet above the freezing level, whereas, dry ice immediately freezes all
supercooled cloud water it contacts even down to the freezing level as dry ice has a temperature
of -70 Celsius.

High numbers of ice nuclei can be produced by the liquid seeding agent being vaporized
in wing-tip generators and exhausted out its tail cone. Generators, one mounted to each wing tip
of our cloud base seeding planes, employs a combustion process in which a 3% silver iodide
(AgI) liquid seeding solution, produces at its peak, trillions of ice nuclei per gram of AgI
consumed. Each generator contains a built-in air pre$sure tank that forces the liquid seeding
solution through an aperture becoming a fine spray. The spray then flows into a combustion
chamber where it is vaporized by burning, producing very pure IN particles. These particles
exhaust out the tail cone of the generator into updrafts at cloud base where they can be carried
aloft by natural action into the cloud's supercooled region.

Prom 1987 through 1995 the liquid seeding agent was the same: The oxidizers, sodium
percWorate and ammonium percWorate, were added to a silver iodide-ammonium iodide- water-
acetone solution resulting in a solution containing 2% AgI by weight. However, in 1996 the
solution was changed to contain AgI, sodium iodide (NaCl), paradichlorobenzene (C6H4CI2)
and acetone. Cloud chamber test results indicated the number of ice crystals produced by the new
solution at -I OC were nearly equivalent to the old solution containing the percWorates. Also,
these new particles initially acted as hygroscopic CCN which continued to ensure that the vast
numbers of water droplets would have a strong tendency to contain IN particles. Another
seeding agent composition change came in 2002 as a new formulation came along promoting
even more hygroscopic ability as well as activating at a lower temperature. The new seeding
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