WMOD00393

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

606 WEATHER MODIFICATION

1015

geometric standard deviation being in the range
of I to 2.
We have already referred to the use of NaI
and KI to dissolve AgI in acetone. Depending on
flame temperatures, degree of dilution, etc., the
particles produced by burning such solutions are
not pure AgI but are instead aggregates with
some NaI or KI crystallites mingled with the
AgI crystallites in each particle. In the presence
of moisture weak chemical bonds develop to
form numerous compounds such as 2 AgI . NaI,
all of which may take on water to form hydrates.
Ammonium iodide (NH4I), however, tends to
break down to ammonia (NH3) and hydrogen
iodide (HI), so generators charged with AgI .
NH4I solution yield relatively pure AgI.

E. ICE NUCLEATION BY AGI PARTICLES

AgI particles can nucleate ice in anyone of
four ways: (1) contact freezing, (2) condensation
freezing, (3) bulk freezing, and (4) deposition
from the vapor. A given particle can act in any
one of these four ways depending on ambient
conditions, although different modes are fa-
vored for different types of particles. Therefore,
it is extremely difficult to determine in advance
how given AgI smokes will react with a cloud.
Cloud chamber tests are used to measure the
effectiveness of AgI generators, but do not ex-
actly simulate the conditions in natural clouds.
The ice crystal yield from nuclei produced by
vaporizing I g of AgI in a typical generator, as
estimated from cloud chamber measurements, is
shown as a function of temperature in Fig. 4.
The activity of the nuclei typically begins at
about - 50C and increases markedly as the tem-
perature decreases to about -200C. In general,
acetone generators yield more ice nuclei active
at a given temperature per gram of AgI con-
sumed than do the pyrotechnics.
Recent work has emphasized that the nuclea-
tion of the ice phase by AgI crystals is a time-
dependent process. Obviously, collision freez-
ing depends on Brownian motion and phoretic
forces; as a result, the time constant for collision
freezing may be on the order of tens of minutes.
AgI particles formed with NaI or KI tend to act
through condensation freezing, and the time
constant for this process is typically shorter,
usually 5-10 min. There are also indications that
the condensation-freezing process becomes ex-
tremely rapid in regions oflocal supersaturation.
Complete simulations of the activity of AgI
smokes must take account of these complexities

10'4

'"
""

~ 10"
0:
'"
0:
W
a.
U) 1012
-'
""
l-
(/)
>-
0:
U
W lO"
U

1010

o

-10

-15

-20

-25

-5

TEMPERATURE (.C)

FIG. 4. Ice nucleation activity curve for the aerosol
from a typical silver iodide generator.

as well as others, such as partial dissolution of
AgI crystals while rising through warm clouds to
supercooled regions and changes in the chemical
structure of the AgI complexes with time.

F. OTHER ICE NUCLEANTS

Many inorganic and organic compounds act as
ice nucleants. In addition to those already men-
tioned, there are cupric sulfide, lead oxide, and
many organic compounds, including metalde-
hyde. Cloud seeding generators for delivering
organic compounds have also been developed.
Nevertheless, these seeding agents have not
been widely adopted.

G. DELIVERY AND DISPERSION
OF SEEDING AGENTS

Regardless of the type of seeding agent used,
its transport and distribution are controlled by
the atmosphere. The dispersion of seeding
agents in the atmosphere can be studied using
models designed to simulate the diffusion of air
pollutants. However, because cloud seeding is
normally done under stormy conditions, care
must be used to select the appropriate versions
of the dispersion models. The bursting ofa pyro-
technic or artillery shell containing seeding
agent is modeled as an instantaneous point
source, while a ground-based generator is mod-
eled as a continuous point source. An aircraft
carrying a seeding generator is a moving point