WMOD00058

• Contract/Permit #: #98-1
• Applicant: Western Kansas Groundwater Management
• Project Name: Western Kansas Weather Modification Program
• Date: 4/28/2001
• Weather Modification - Doc Type: Report

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The seeding agents used on the WKWM Program are delivered to clouds by aircraft Both
liquid and solid (flares) complexes of silver iodide (AgI) are used as our cloud base seeding
agents of choice. They are vaporized in updrafts at cloud base to produce ice nuclei rising into
clouds through the natural action of the cloud's own updraft. Another seeding agent, dry ice, is
dropped directly into growing clouds at altitudes above freezing up to -10C while flying through
the growing cloud towers feeding the main updraft. These "feeder" clouds quickly merge 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.

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 dry ice directly into the moisture-laden cloud updrafts found in the new-growth feeder
cloud towers. Pelletized dry ice is dispensed from a container which is augured into an 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 cloud 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.

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 2% 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 pressure tank: which forces the liquid seeding
solution through an aperture becoming a fine spray. That spray then flows into a combustion
chamber where it is vaporized by burning, producing very pure IN particles. These particles
exhaust out of the tail of the generator into updrafts at cloud base where they can be carried aloft
by natural action into the cloud's supercooled volume.

From 1987 through 1995 the liquid seeding agent was the same: The oxidizers, sodium
perchlorate and ammonium perchlorate, were added to a silver iodide-ammonium iodide-water-
acetone solution resulting in a seeding solution containing 2% AgI by weight. However, in 1996

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