WMOD00393

• Title: Weather Modification Glossary
• Prepared By: A. S. Dennis
• Date: 1/1/1987
• Weather Modification - Doc Type: Report

604 WEATHER MODIFICATION

tassium iodide (KI), and ammonium iodide
(NH4I). The AgI solution is sprayed into the
flame and burned either alone or with propane or
other fuel to vaporize the AgI and the carrier.
Silver iodide consumption rates in acetone gen-
erators have varied from as little as 5 g/hr to
almost] kg/hr. Some of the earliest acetone gen-
erators yielded 1016 particles per gram of AgI.
Some persons have used liquid fuels in which
AgI dissolves without carriers, but such fuels
tend to be hard to handle or toxic (e.g., isopro-
pylamine).
Liquid-fueled generators are used extensively
for seeding from the ground. Normally the tank
of AgI solution is pressurized (e.g., from a com-
pressed air bottle) to drive the solution into the
flame (Fig. I). Liquid-fueled generators on air-
craft can work with ram-air pressure to force the
fuel into the flame, but this ties AgI consumption
to aircraft speed and to ambient air density.
Therefore, many generator installations on air-
craft provide for an independent pressure source
and control system to provide for positive con-
trol. The AgI consumption rate for a typical air-
craft generator is 300 g/hr. Generators have also

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FIG. 1. A silver iodide generator in operation near the
Sierra Nevada in California. A solution of silver iodide
in acetone is contained in the tank (right). The propane
tank (left) provides fuel to support combustion and a
source of pressure to drive a jet of silver iodide solu-
tion into the flame. (Courtesy of Atmospherics, Inc.)

been constructed that burn solid fuels such as
coke pellets or string previously soaked in an
AgI solution. However, these have not been
used as widely as the acetone generators or the
pyrotechnic generators, which constitute the
second major type of AgI generator.
Pyrotechnics have been used extensively on
airplanes, where they eliminate the need for car-
rying flammable solutions. Extensive work was
carried out at the U.S. Naval Weapons Center at
China Lake, California, in the ]960s to improve
pyrotechnic generators. Some were developed
to be dropped from aircraft and others to be
burned in place on racks (Fig. 2). The droppa-
bles can be divided further into those with a con-
tinuous burn, typically over 500 to 1000 m of
fall, and those designed to explode after a given
time interval. Good pyrotechnics should burn
smoothly and not explode unexpectedly. Drop-
pable pyrotechnics preferably burn up com-
pletely, leaving no spent shell to fall to the
ground.
In pyrotechnic generators AgI-bearing com-
pounds are packed with others in an organic
binder, often a nitrocellulouse fuel with a plasti-
cizer. One commonly used compound is silver
iodate (AgIO}). The release of oxygen as the
AgIO} breaks down to AgI renders the oxidation
of the fuel relatively independent of air density

FIG. 2. On a hail suppression project in Kenya, a
technician installs silver iodide flares in a rack on the
trailing edge of the wing of a cloud seeding aircraft.
These flares are ignited electrically from a control box
in the aircraft cabin and are designed to burn in place.
(Courtesy of Atmospherics, Inc.)