The science of weather modification drew mixed reviews from practitioners here today just hours after cloud seeding of the Colorado mountains resulted in the first snow in more than two weeks in this drought-ridden state.
Up to eight inches of snow fell today in Aspen and up to four inches in Steamboat Springs, two of three locations seeded for the first time Monday night. The seedings were done with silver iodide blown up into the clouds from generators on the grounds and down into the clouds by twin-engined airplanes flying a racetrack course over the mountain passes.
Clouds above the San Juan Mountains in the southwest part of the state were also seeded Monday night but no snow fell, to the surprise of none of the weather modification experts attending the annual meeting of the American Association for the Advancement of Science.
"We didn't know how raindrops were formed until 30 years ago," said Dr. William A. Thomas of Chicagos American Bar Foundation. "Our knowledge of how to change the weather is still greatly limited to folklore and wizardry."
Modification takes in six forms of weather, Dr. G Brant Foote of the National Center for Atmospheric Research in Boulder said here today. Foote identified the six - all of them controversial - as cold fog dispersal, hurricane modification, rain enhancement, lightning suppression, hail suppression and snowpack buildup.
The main controversy about all six is whether they can be done, given the knowledge science has today about how weather happens and how it can be changed after it happens. For example, Foote said we still don't understand the mechanism by which silver iodide crystals form ice droplets in clouds, even though it was first demonstrated as possible 31 years ago.
"About all we can say with certainty is that silver iodide does make ice crystals," Foote said. "The important question of how it operates stands quite firmly in the way of a convincing framework for a weather modification technology."
Foote told the meeting a story of the hail suppression experiment in the Soviet Union five years ago. Central to the Soviets' claim that they had successfully and repeatedly suppressed hail was the Russian finding that hail grew rapidly in dense zones of liquid water consisting of oversized and supercooled drops.
Scientists at the South Dakota School of Mines, tried duplicating the Soviet'sachievement by seeding zones of large, cold water drops in three years of experiments in northeast Colorado. They failed to suppress hail by this type of seeding in Colorado, and in fact increased the size of hailstones in some types of storms by seeding the way the Soviets had. The five-year U.S. program to match the Soviet success was dropped at the end of three years.
The seeding that began in the Colorado Rockies Monday night to force snow to fall drew cautious praise from Foote, who said that in his experience seeding either increased snowfall 10 to 20 per cent or decreased it by the same amount.
"Why?" Foote asked. "The technology is decades away from being understood."
The history of weather modification is rife with apparent success, followed just as often by dismal failure, said Dr. Stanley A. Changnon Jr., of the Illonios State Water Survey.
Five-year experiments to increase rainfall in Missouri and Arizona almost 20 years ago produced just the opposite results, Changnon said. Rainfall in both states decreased after the clouds were seeded.
"All I can say is that in the beginning there were meteorologists, physicists and charlatans attempting to modify the weather," Changnon said. "They did it with chicanery and with amazingly little knowledge of the atmosphere."
For some reason still only dimly understood, Changnon said, hail suppression seems to work only in the Great Plains. The best way to approach hail anywhere else in the country, he said, is to buy more hail insurance.