Cloud Seeding

 Passage 1 is adapted from "'Cloud Seeding' Not Effective at 
Producing Rain as Once Thought, New Research Shows." 
©2010 by American Friends of Tel Aviv University. Passage 2 
is adapted from Janet Pelley, "Does Cloud Seeding Really 
Work?" ©2017 by American Chemical Society.
Passage 1
In many areas of the world, including California's 
Mojave Desert, rain is a precious and rare resource. 
To encourage rainfall, scientists use "cloud seeding," a 
weather modification process designed to increase 
precipitation amounts by dispersing chemicals into 
the clouds.
But research now reveals that the common 
practice of cloud seeding with materials such as silver 
oxide and frozen carbon dioxide may not be as 
effective as it had been hoped. In the most 
comprehensive reassessment of the effects of cloud 
seeding over the past fifty years, new findings from 
Prof. Pinhas Alpert, Prof. Zev Levin and Dr. Noam 
Halfon of Tel Aviv University's Department of 
Geophysics and Planetary Sciences have dispelled the 
notion that cloud seeding is an effective mechanism 
for precipitation enhancement.
During the course of his study, Prof. Alpert and his 
colleagues looked over fifty years' worth of data on 
cloud seeding, with an emphasis on rainfall amounts 
in a target area over the Sea of Galilee in the north of 
Israel. The research team used a comprehensive 
rainfall database and compared statistics from periods 
of seeding and non-seeding, as well as amounts of 
precipitation in the adjacent non-seeded areas.
"By comparing rainfall statistics with periods of 
seeding, we were able to show that increments of 
rainfall happened by chance," says Prof. Alpert. "For 
the first time, we were able to explain the increases in 
rainfall through changing weather patterns" instead 
of the use of cloud seeding.
Most notable was a six year period of increased 
rainfall, originally thought to be a product of 
successful cloud seeding. Prof. Alpert and his fellow 
researchers showed that this increase corresponded 
with a specific type of cyclone which is consistent 
with increased rainfall over the mountainous regions. 
They observed a similarly significant rain 
enhancement over the Judean Mountains, an area 
which was not the subject of seeding.
Last year marked the conclusion of a massive six-
year study that has been the most comprehensive and 
rigorous to date to investigate whether cloud seeding 
actually increases precipitation. Called the Wyoming 
Weather Modification Pilot Project (WWMPP), the 
study was run by a team of researchers from 
government, academia, and private industry. In the 
end, WWMPP wasn't able to provide a definitive 
answer. "But the results do provide a body of 
evidence that cloud seeding is working under certain 
conditions," says Roelof Bruintjes, an atmospheric 
scientist at the National Center for Atmospheric 
Research (NCAR), who was not part of the project 
although his colleagues at NCAR were deeply 
involved.
 Earlier studies would inject silver iodide into 
clouds, then compare precipitation gauges in areas 
inside and outside the seeding zone. But the studies 
weren't repeatable, and they didn't include enough 
trials to guarantee that observed increases in 
precipitation weren't sue to chance. The challenge 
with measuring the effect of weather modification is 
that natural rain and snowfall variability is 10 to 100 
times as large as the amount of precipitation 
augmented by seeding, Bruintjes says.
 Still, the WWMPP researchers thought they could 
address the drawbacks of past studies. The 
researchers designed their $14 million project to run 
for six winter seasons in the mountains of Wyoming. 
They conducted more than 150 tests, randomly 
selecting clouds to seed and clouds to be their 
unseeded controls.
 Measurements from the high-resolution snow 
gauges on the ground indicated that seeding elevated 
snowfall by 5-15%. But this result was achieved only 
after the researchers threw out some of the tests 
where silver iodide drifted into control clouds or 
where not enough seeding material was released, so 
the final results weren't statistically significant. 
"Nevertheless, all the results provided evidence for a 
positive trend," Bruintjes says.
 The scientists also took advantage of new 
developments in remote-sensing and atmospheric 
modeling to examine dynamics inside a small subset 
of seeded clouds.

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