WMOD00393

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

II. CLOUD SEEDING AGENTS AND DELIVERY SYSTEMS 603

substances have been used to produce the same
effect, including liquid propane, liquid air, and
solid objects previously chilled by immersion in
some very cold substance. The effect can also
be produced by decompression, as in an explo-
sion or at certain positions near some types of
aircraft in flight.
Although chilling agents must freeze preexist-
ing cloud droplets and activate additional nuclei,
these effects are insufficient to account for the
myriads of ice crystals produced. The vast ma-
jority of the ice crystals are produced by homo-
geneous nucleation from the vapor phase. It is
not completely settled whether they form by ho-
mogeneous deposition from vapor to ice or
whether liquid droplets condense by homoge-
neous nucleation and immediately freeze, also
by homogeneous nucleation. In any case, the
final product is a cloud of very small ice crys-
tals, each much less than I ILm in maximum di-
mension.
Although homogeneous nucleation of the ice
phase can be produced by a large number of
chilling agents, dry ice remains the most popu-
lar. Apart from a few cases in which dry ice has
been elevated from the ground by either teth-
ered or free-floating balloons, most applications
of dry ice have involved dropping pellets from
airplanes. In some projects the dry ice has been
crushed before being loaded into the aircraft.
Other projects have utilized on-board dry ice
crushers. The disadvantage of the former
method is loss of dry ice by sublimation. During
the crushing, sieving, and subsequent storage,
both prior to flight and in hoppers on the air-
craft, more than half of the dry ice originally
provided may sublime away before any opera-
tions are conducted. The latter method poses
the requirement of carrying additional equip-
ment on board the aircraft, namely, the dry ice
crusher. Another method, which has been used
in the Soviet Union, is to transform liquid CO2
into dry ice on board the aircraft. This method
produces a very fine "snow" or "dust" of dry
ice.
The effectiveness of dry ice is usually ex-
pressed as the number of ice crystals produced
per gram of dry ice sublimed. It appears that the
effectiveness of small pellets with low fall
speeds is somewhat greater than that of large
pellets. It appears too that the number of crys-
tals produced is reduced at ambient tempera-
turesjust below ooc. With these caveats, experi-
mental data indicate the effectiveness of dry ice
to be in the range of 1012 to 1013 ice crystals per

gram. A common technique in dry ice seeding is
to drop the dry ice from, for example, the
-120C level, using pellets sized so that they will
compl<~tely sublime away near the - 50C level.
This procedure requires the dropping of pellets
1.5 to 2 cm in diameter. In view of the very large
numbers of ice crystals produced per gram of
dry ic<~ used, it is possible to affect large vol-
umes of cloud by dropping modest quantities of
dry ic<~, for example, 100 kg per flight. There-
fore, dry ice seeding missions can be carried out
by small single-engined aircraft.

C. TYPES OF SILVER IODIDE GENERATORS

We have already referred to the ice nucleating
properties of silver iodide (AgI) crystals. Unlike
natural ice nuclei, most of which are operative
only at temperatures below -15 to - 20oC, AgI
particles can nucleate ice at temperatures as
high as -4 to -50C. The most common form of
AgI is a yellow hexagonal crystal with a density
of 5.68 g/cm3 . In the hexagonal crystal the silver
and iodide ions assume positions analogous to
those of the oxygen atoms in an ice lattice, and
the spacings are very similar. The separation be-
tween the oxygen atoms in the c-plane of an ice
lattice is 0.452 nm, and the corresponding spac-
ing of silver and iodide ions in the AgI crystal is
0.459 nm. There exists a cubic form of AgI,
which is also an effective ice nucleant.
Any device designed to produce small AgI
particles is called a silver iodide generator. Most
generators work on the principle of vaporizing
AgI and allowing it to solidify upon cooling into
small particles. The vaporization of the AgI is
sometimes accomplished with electric arcs, but
usually with a flame that may be either continu-
ous or associated with an explosion. The melting
point of AgI is 5520C and its boiling point at a
pressure of I atm is 15060C. Typical generators
work with flame temperatures around 10000C so
that the AgI sublimes or evaporates without ac-
tually boiling. It is important that the AgI not be
exposed to a reducing atmosphere in the flame
for any length of time, lest it decompose into
silver and iodine. The tendency to dissociate can
be suppressed by use of moderate flame temper-
atures, by adding iodine vapor to the flame, and
by minimizing the period during which the AgI is
exposed to the flame temperature.
A very common type of generator uses AgI
solutions in a flammable solvent, usually ace-
tone. Because AgI is not soluble in acetone, a
solubilizing agent or carrier must be used. The
most common ones are sodium iodide (NaI), po-