Weather Watch

Cloud seeding can only go so far in fighting California drought

Visitors to California’s Yosemite Valley look toward clouds forming around El Capitan after a rainstorm following the celebration for the 125th Anniversary of Yosemite National Park in October.
Visitors to California’s Yosemite Valley look toward clouds forming around El Capitan after a rainstorm following the celebration for the 125th Anniversary of Yosemite National Park in October. Fresno Bee/TNS

Despite the strongest El Niño event on record and the well above normal amounts of rainfall it brought to Northern California, the worst category of drought (D4 Exceptional Drought) continues to persist throughout most of San Luis Obispo and Kern counties and all of Santa Barbara and Ventura counties.

Since the start of 2016, nearly all the long-range models have consistently advertised that a La Niña condition — which historically produces below average rainfall for the Central Coast — will develop in the eastern equatorial Pacific this upcoming winter.

This dire prediction has left many Central Coast residents to ask about cloud seeding. Unfortunately, you just can’t go out and seed clear skies; you have to have the right atmospheric conditions with plenty of clouds that are capable of rain.

The concept of cloud seeding was discovered by Vincent Schaefer in 1946. At the time, he was working at the General Electric laboratory to create artificial clouds in a large, cold chamber kept below freezing. He thought that the chamber was too warm and placed dry ice inside to further cool it. Suddenly a cloud formed around the dry ice. It turns out, the microscopic ice crystals in the dry ice had provided a near-perfect seed on which the water vapor could condense.

Working with Schaefer, physicist Bernard Vonnegut discovered that silver iodide got even better results in nucleating clouds than did dry ice. Over the decades, no other molecule has been found that rivals it for creating rain.

Clouds form from the condensation of invisible water vapor on very small nuclei from dust, volcanoes, pollen, forest fires, pollution from cars and factories, salt from ocean spray or sulfite particles from phytoplankton in the oceans. Cloud seeding increases the number of these available nuclei. Raindrops or ice crystals might not form without these added nuclei.

After these discoveries, Project Cirrus, a combined effort involving the government and private industry, was organized in 1947 to explore the possibilities and limitations of cloud seeding.

Twenty years later, cloud seeding would be used in the Vietnam War. The Department of Defense seeded clouds to extend the monsoon season along the Ho Chi Minh Trail during Operation Popeye (1967-72). The goal of the program was to deny the use of this major supply route by flooding it. The continuous rainfall slowed the truck traffic and was considered successful. The U.S. Air Force 54th Weather Reconnaissance Squadron carried out most of the cloud-seeding operation to “make mud, not war.”

About this time, it was rumored that cloud-seeded activities contributed to the heavy rains over the Woodstock Festival in upstate New York. More than half a million concert attendees joined in chants of “No more rain!” during Neil Young’s performance.

After the war, the “Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques” treaty was adopted by the United Nations General Assembly in 1976. This treaty sought to ensure that weather modification would be used only for peaceful purposes.

Today, most cloud-seeding operations inject silver iodide into clouds either from aircraft, which can be very expensive, or land-based stations on mountaintops where updrafts carry the silver iodide into passing clouds. Land-based stations are less expensive and are remotely activated.

Effective cloud seeding is only possible in the presence of clouds that are capable of producing rain. Meteorologists look for the top of the cloud formations where “supercooled” water vapor may exist. This means that the water vapor is suspended in the cloud at temperatures that are below freezing.

At these low temperatures, the silver iodide is exceptionally hydroscopic, and water vapor immediately condenses on these microscopic seeds as ice crystals. The crystals grow larger and larger until they become large enough to overcome the forces of “uplift” in the cloud and fall as rain, snow or hail.

For about half a century, the Santa Barbara County Water Agency has been seeding clouds to coax more rainfall from the sky and increase runoff to Twitchell Reservoir and Lake Cachuma. Not only is cloud seeding widely used to increase rainfall, but it is also employed to suppress damaging hail from Alberta, Canada, as it moves southward through the Great Plains to Oklahoma and to augment the snowpack in the many mountain ranges across the western United States.

Environmental studies have shown that accumulated silver iodide concentrations are less than 100 times below natural background concentrations in cloud-seeding areas. Overall, numerous studies claim that cloud seeding can increase rainfall between 5 and 20 percent.

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John Lindsey is the PG&E Diablo Canyon meteorologist and media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.

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