NSF PR 96-37 - July 10, 1996
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RAINFALL ENHANCEMENT TECHNIQUE
TESTED IN DROUGHT-RIDDEN MEXICO
Scientists from the National Science Foundation-funded
National Center for Atmospheric Research in Boulder,
Colorado, are engaged in new research that will bring
more credibility to the science of weather modification
-- and possibly some rain to the drought-ridden Mexican
state of Coahuila.
Working with scientists from several Mexican universities,
the researchers are beginning the first field trials
in North America of a new technique for seeding clouds
to enhance rainfall. Field trials are planned for
June to October over the next four years in Coahuila.
The researchers will also transfer cloud-seeding technology
to Mexico and train Mexican scientists in its use
and evaluation.
"The technique tested in Mexico this summer uses pyrotechnic
flares mounted on aircraft to seed clouds," explains
Jewel Prendeville, coordinator of NSF's lower atmospheric
facilities section. "While the aircraft flies at the
base of the cloud, moisture-retaining particles produced
by the burning flares rise into the cloud. As the
cloud's water vapor is attracted to the particles,
droplets are formed, which then fall out as rain.
The wide range of droplet sizes produced by the particles
encourages and accelerates the precipitation process."
This method of cloud seeding was first tried in 1990
in South Africa. There it appeared to increase rainfall
by 30 to 60 percent over what would have occurred
without the seeding.
Cloud seeding has been practiced at various places
in the world since 1946, but most experiments have
produced inconclusive results. The initial optimism
that soared in the 1950s and 1960s has given way to
a much more cautious approach over the last 20 years.
Although there have been many rainfall-enhancement
programs around the world during the last two decades,
most have lacked a solid scientific basis and have
had no means to validate the results of the seeding.
Because of the natural day-to-day variability of clouds
across geographical regions, it has been difficult
for scientists to isolate the effects of seeding.
Brant Foote, director of the center's research applications
program, is coordinating the overall research program
that will test and validate the new seeding method.
He cautions that it is too early to tell whether the
seeding technique will be broadly applicable to Mexico
or any other rain-deficient area. "The results in
South Africa are striking," says Foote, "but there
is no guarantee of success elsewhere. These efforts
are clearly still in an early phase. We anticipate
that this research in Mexico will help us better understand
the physical processes that lead to increased rainfall."
The research program includes the following components:
- a scientific evaluation of seeded and non-seeded
clouds in the area;
- studies of cloud responses using high-resolution
numerical models;
- analysis of data collected by research aircraft
and ground-based radar;
- a training and technology-transfer program with
scientific collaborators in Mexico;
- strong collaboration with scientists in Mexico
in the areas of cloud physics, data analysis,
and numerical modeling.
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