In Search of a Drought-Proof Source - The Ripple Effect: The Fate of Freshwater in the Twenty-First Century - page 284

Congress to pay him to start forest fires so that he could demonstrate how his on-de-
mand precipitation would snuff them out. Congress declined to fund Espy's project.
In the late nineteenth and early twentieth centuries, “pluviculturists” promised they
could draw rain from clouds by “puncturing” them with pointed balloons, blasting
them with percussive artillery barrages, releasing steam from chimneys, or sprinkling
clouds with “electrified sand” from a plane.
In 1946, two General Electric chemists in Schenectady, New York— Vincent Schaefer
and Irving Langmuir —discovered that when dry ice was dropped into a cold cloud, it
produced crystallized water vapor, better known as snow. Bernard Vonnegut , another
GE chemist (and the older brother of novelist Kurt Vonnegut), found that silver iodide
created snow in clouds. Their insight was that they didn't have to create new clouds;
rather, they could induce precipitation by adding crystallization to existing rain clouds.
They termed their methods cloud seeding. Langmuir, who won the 1932 Nobel Prize in
Chemistry, believed that this was his most important discovery and spoke of the poten-
tial to end drought and hail, control snowfalls, and turn the Southwest into a verdant
garden.
The trick is to fly above already moisture-laden clouds in an airplane and “seed”
them with a trail of silver iodide or dry ice (frozen CO 2 ) particles, which causes the pre-
cipitation to fall. Some scientists estimate that under perfect conditions spreading silver
iodide into a cloud can increase precipitation by 10 to 15 percent.
Cloud seeding has been used to produce rain around the world for over sixty
years, and today numerous dry nations—Morocco, Saudi Arabia, countries of West
Africa—are researching the technology as an answer to their chronic lack of water.
he Xinjiang region of China is home to the world's largest cloud-seeding initiative.
The China Meteorological Administration fires tens of thousands of rockets and cannon
rounds loaded with silver iodide into the skies to promote rain. From 1999 through
2006, China claims to have produced 36 billion metric tons of artificial rain per year,
reduced the size of hailstones (which destroy crops and houses), and suppressed forest
fires caused by lightning strikes. China's latest five-year plan calls for increasing artificial
rainfall to 50 billion metric tons a year.
But tinkering with the weather can go badly wrong. In November 2009, clouds were
seeded over China to alleviate a drought, but the temperature suddenly dropped, and
the resulting blizzard closed highways and the Beijing airport, while heavy snows in
other cities collapsed roofs, injured scores, and killed at least eight people.
Does weather modification really work? Proponents such as Herb Guenther believe
it does. But critics say it is impossible to distinguish cloud-seeded rain from natural
rain, that the amount of precipitation created by silver iodide cannot be measured, and
that controlled experiments to answer these questions are impossible to construct.