WMOD00393

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

614 WEATHER MODIFICATION

that none of them grow very large. Another, the
trajectory-lowering hypothesis, holds that the
seeding causes the growth of precipitation at
lower elevations in the cloud than would other-
wise be the case, thus, it rains rather than hails.
However, careful study of these hypotheses has
shown practical limitations in all of them.
Although hail suppression has been practiced
in 15 to 20 countries, the most extensive applica-
tions have taken place in the Soviet Union. So-
viet scientists have conducted hail suppression
projects to protect fruit crops in the Moldavian
S.S.R., vineyards in the Caucasus Mountains,
and cotton crops in the Soviet republics of Cen-
tral Asia. Soviet pioneers in hail suppression,
notably G. Sulakvelidze, identified radar echoes
aloft in newly developing convective cells and
decided that they consisted of accumulations of
supercooled raindrops that would support rapid
hailstone growth. A desire to seed such accumu-
lation zones rapidly led to the development of
the rockets and artillery shells used in the Soviet
hail suppression arsenal.
A typical Soviet hail suppression program is a
massive effort involving as many as 100 to 300
people. Radar crews scan threatening cloud for-
mations and apply objective criteria based on
radar echo heights and intensities to determine
when seeding should be initiated. The firing of
rockets or artillery shells follows quickly once
threatening conditions appear.
The Soviet hail suppression programs have
been evaluated principally by comparing hail
damage within protected areas with that occur-
ring in surrounding regions. Large amounts of
crop loss data have been compiled. On the basis
of these analyses, Soviet scientists have re-
ported hail suppression effects ranging from 50
to 90% of the damage that would have been ex-
pected in the absence of seeding.
The claims of Soviet scientists regarding the
effectiveness of their hail suppression tech-
niques led to the establishment of the National
Hail Research Experiment in the United States,
with seeding from 1972 to 1974 inclusive, and of
Grossversuch IV in Switzerland, with seeding
from 1977 to 1981. Neither of these randomized
experiments gave any evidence of a hail sup-
pression effect. In both cases, aircraft flights
through accumulation zones detected by radar
showed that they consisted mainly of hailstones
or graupel rather than supercooled raindrops.
The Russian technique of direct injection of
seeding agents into threatening hail clouds has

been adopted in some other countries of eastern
Europe and has been adapted to locally manu-
factured rockets in Argentina. Direct injection is
also used in projects in Alberta, Canada, and in
South Africa in which AgI pryotechnics are
dropped near cloudtop by high performance air-
craft. There are insufficient data available to de-
termine the effectiveness of these programs, al-
though the practitioners believe the seeding has
some effect.
An alternative approach to hail suppression is
to broadcast the seeding agent into the subcloud
layer from ground generators or from aircraft
flying below cloud base. Typical AgI consump-
tion rates are 20 g/hr for a ground-based genera-
tor and 300 g/hr from an aircraft. The seeding
agent has some time to disperse before it is in-
gested into the threatening cloud formations, but
the rapid reaction to hail threats is lost. This
approach has been followed in operational pro-
grams in Canada, France, the United States, and
elsewhere. The choice between locating genera-
tors on the ground or on aircraft is based often
on local conditions. For example, in southwest-
ern France, where cloud bases are close to the
mountain tops and visibility is often poor due to
haze in the maritime air masses, an extensive
ground generator seeding program has been un-
der way since the 1950s. About 500 AgI genera-
tors are deployed over an area of roughly
100,000 km2. Consumption of AgI has ap-
proached 4000 kg in a single summer season.
Several hail suppression programs of the type
just described have been evaluated statistically.
The large program in France has provided some
evidence of a hail suppression effect in recent
years. Analyses of commercial hail suppression
by aircraft in the south-central United States,
also using target control analyses, have indi-
cated a suppression effect amounting to about 25
to 50% of the hail damage that would have oc-
curred without seeding. Large-scale aircraft
seeding programs operated by the states of
South Dakota and North Dakota in the north-
central United States have also provided evi-
dence of reductions in total hail damage. How-
ever, the results of seeding cannot be evaluated
on a storm-by-storm basis. In any case, many
atmospheric scientists regard the data as merely
indicative rather than proof of a net hail suppres-
sion effect.
The large variability of natural hailfalls and
the complexities of the hailstone growth process
make it unlikely that definitive answers about