WMOD00097

• Applicant: Western Kansas Groundwater Management
• Project Name: Cloud Seeding
• Date: 5/1/1989
• Weather Modification - Doc Type: Report

~

J

To produce high numbers of ice nuclei from silver iodide, it must
first be vaporized. On theWKWM Program we employ a combustion process
which produces trillions of ice nuclei per gram of silver iodide
consumed. Carley-type acetone-silver iodide generators are mounted in
the position of wing-tip tanks on each cloud base seeding aircraft.
The generators use pressurized air contained in an air tank built into
the generators. The pressurized air forces the liquid seeding
solution through an aperture and produces a fine spray that flows into
a combustion chamber where it is vaporized by burning. Upon burning,
very pure silver iodide particles are formed and exhaust freely from
the combustion chamber into the cloud base updrafts which carry them
aloft into the cloud by its own natural action,

In 1989 we used the same seeding agent chemical formulation in
use since 1987. The formulation contains quantities of the oxidizers
sodium perchlorate and ammonium perchlorate added t'o a standard 2%
silver iodide-ammonium iodide-acetone-water solution. A chemical ratio
of 4 moles sodium to 1 mole silver results in particles acting
initially as strongly hygroscopic condensation nulei that help insure
the formation of vast numbers of water droplets containing silver
iodide ice nuclei in the lower parts of 610uds. Silver iodide ice
nuclei not trapped in water droplets hygroscopically, near cloud base,
will be collected in other droplets by "contact nucleation". Contact
nucleation occurs when ice nuclei rise inside moist cloud updrafts,
make random collisions with water droplets and are captured in those
droplets. The combined hygroscopic condensation-contact nucleation
process produces greater numbers of ice crystals formed at relatively
warmer temperatures within a cloud than when simply using the older
formulation depending solely upon contact nucleation. Contact
nucleation allows silver iodide activity to .linger within a treated
cloud after seeding ends and helps clouds precipitate more and for a
longer time; usually, silver iodide is the preferred seeding agent for'
rainfall stimulation.

Finally, in order to obtain the desired result from cloud seeding
with silver iodide, each cloud must be treated within a proper time
interval---a "window of opportunity"---not only to produce the optimum
ice crystal concentrations in clouds naturally deficient in ice nuclei
but also to be able to get the. proper concentrations into that volume
of the cloud where they can become effective. In the case of using
silver iodide, a cloud which grows to maturity and collapses must be
treated in time for the ice nuclei to be able to be lifted by natural
cloud action into the appropriate temperature and moisture regime and
kept there for a sufficient time to react with the super-cooled water
in that volume or the cloud may collapse prematurely and the effort
wasted. This "residence" time of silver iodide in the supercooled
cloud volume is critical to the success of both rain stimulation and
hail reduction.

In both weakly and moderately growing cumuliform clouds their
behavior can be altered through what is called the "dynamic effect".
When atmospheric conditions are right, cumulus clouds may be
stimulated to grow larger and precipitation persist longer than
normally would be the case if left unseeded. This is done by placing
the seeding agent (dry ice or silver iodide) into the supercooled part
of the cloud to promote the rapid change of water droplets into ice
crystals. When this water-to-ice conversion process occurs rapidly
within a cloud, latent heat of fusion is released on such a scale that
the cloud becomes slightly warmer and more buoyant causing the cloud
to grow. As the cloud grows, it draws in greater amounts of air, the