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 601

World War II was notable for work on the
creation of smoke screens, which is a form of
local weather modification, and for the FIDO
system, which evaporated fog by use of open
flames. The most interesting research into pre-
cipitation was carried out by Findeisen and his
colleagues at a German-controlled research in-
stitute in Prague, Czechoslovakia. They tested
various artificial ice nuclei in the laboratory and
on a number of cloud seeding flights. In one no-
table flight in September 1942, powdered diato-
maceous earth was dropped into a supercooled
altocumulus cloud deck. Virga ("fallstreifen")
were observed below the cloud after seeding,
followed by partial clearing along the seeded
tracks.
During World War II, a group was formed un-
der the leadership of Irving Langmuir at the
General Electric Research Laboratories in
Schenectady, New York, to conduct research
on such topics as aircraft icing and the creation
of smoke screens. Following the war, this group
explored the possibility of modifying super-
cooled clouds by adding artificial ice nuclei.
While cooling a cloud formed in a freezer, Vin-
cent Schaefer, a member of the group, discov-
ered in July 1946 that a dry ice (solid CO2) pellet
introduced into a supercooled cloud produces a
great number of ice crystals. His discovery was
followed on November 13, 1946, by the first suc-
cessful cloud seeding flight in the United States.
Dry ice seeding from an aircraft converted su-
percooled cloud near the Berkshire Mountains
in western Massachusetts into a shower of
snowflakes, which fell 600-1000 m below cloud
base before subliming away.
Additional seeding trials soon followed in sev-
eral countries. A particularly noteworthy exper-
iment took place in Australia on February 5,
1947. A cumulus cloud seeded with dry ice grew
spectacularly into a cumulonimbus cloud, which
produced peak rainfall amounts at the ground of
10 to 12 mm, while no other cloud in the vicinity
gave any rain. This may have been the first time
that a deliberate attempt to modify a cloud
yielded significant rainfall at the ground.
Meanwhile, in the continuing search at the
General Electric Research Laboratories for an
artificial ice nucleant, Bernard V onnegut found
in 1946 that silver iodide (Agl) crystals can nu-
cleate ice at temperatures as high as -40C. Von-
negut developed generators to produce large
numbers of AgI crystals. (Different types of AgI
generators and their products are discussed in
Section II.) It is sufficient here to note that AgI

generators are very efficient and can be used to
seed clouds from the ground at much less cost
than se,eding from aircraft.
The late 1940s saw great weather modification
research activity in the United States. Under the
umbrella of Project Cirrus, the U.S. Department
of Defense and other agencies sponsored a large
number of experiments. Experiments in Project
Cirrus and elsewhere included attempts to stim-
ulate rainfall and to modify fog, hail, and light-
ning. Numerous physical concepts were ad-
vanced to justify the various experiments. It was
recognized that, in principle, one could convert
a cloud entirely to small ice crystals, thereby
reducing the amount of precipitation falling from
it. Thus, the concept of overseeding was devel-
oped and was actually applied in a few in-
stances. However, the available technology for
observing atmospheric processes did not make it
possible to test the various concepts rigorously.
Although the final report on Project Cirrus was
optimistic in tone, it did not show conclusively
that cloud seeding could increase rainfall on a
predictable basis.
Meanwhile, cloud seeding had been adopted
for operational purposes. Farmers, power com-
panies, and others hurried to make use of the
new technology, principally to increase rainfall.
New York City, in 1950, was one of the first
political entities to enter into a contract for
cloud seeding services to increase water sup-
plies. The 1950s also saw operational programs
to suppress hail and to reduce wind damage to
banana plantations. Projects sprang up in Aus-
tralia, Mexico, and many other countries.

II. Clc)ud Seeding Agents and
Delivery Systems

A. ApPI.ICATION OF WATER SPRAY,
HYGROSCOPIC SOLUTIONS, AND POWDERS

Although cloud droplet coalescence into rain-
drops as a result of gravitational capture pro-
ceeds slowly at first, the process goes quickly
once some collector drops have exceeded 150-
200 p,m in diameter. Such drops are called rain-
drop embryos. In principle, one can hasten rain
formation in a cloud by 10 to 15 min by spraying
the cloud with water drops large enough to func-
tion as artificial raindrop embryos. Experimen-
tal verification has been obtained in Australia
and ov,:r the Caribbean. In both cases aircraft
were used to deliver the spray into or very close