Weather Watch

Cloud seeding drawing more attention after dry winter

Special to The TribuneMarch 16, 2013 

It’s been dry in 2013. So far, Cal Poly, home of climatology for San Luis Obispo, recorded a meager 2.6 inches of rain since Jan. 1. From the first day of the year through mid-March, Cal Poly historically averages about 11.3 inches of rain. In fact, it’s been very dry this year throughout California.

Fortunately, we entered into this year in respectable shape; in fact, we had above-normal rainfall in December. However, the chance of getting back to normal gets increasingly smaller every day as we move closer to the dry season.

With the threat of continued dry years ahead with the cool phase of the Pacific Decadal Oscillation firmly entrenched in the Pacific, people are expressing an interest in cloud seeding or weather modification to increase rainfall.

Mark Twain once said, “Everyone talks about the weather, but no one does anything about it.” If he was writing today, his famous quote would probably have to be altered. You see, for more than half a century, the Santa Barbara County Water Agency has been seeding clouds to coax more rainfall from the sky and increase runoff to Lake Cachuma. Numerous studies have shown that cloud seeding can increase rainfall between 5 to 20 percent.

Clouds form from the condensation of invisible water vapor on nuclei from dust, pollen, salt from ocean spray, even 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.

Cloud seeding was 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 (1967-1972). 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 relatively 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 Wood stock Festival in upstate New York. More than half a million concert attendees joined in chants of “No more rain!” during a performance.

Today, most cloud-seeding operations inject silver iodide into clouds either from aircraft or land-based stations on mountaintops where updrafts carry the silver iodide into passing clouds. Effective cloud seeding is only possible in the presence of clouds that are capable of producing rain. The silver iodide is very 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.

Environmental studies have shown that accumulated silver iodide concentrations are less than 100 times below natural background concentrations in cloud-seeding areas. The average cost of water produced by cloud seeding is less than $100 per acrefoot. By comparison, the cost of desalinated seawa ter can reach nearly $2,000 per acre-foot.

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Did you know? PG&E delivers some of the nation’s cleanest power. Nearly 60 percent of the electricity the company provides to customers comes from sources that are renewable and/or emit no greenhouse gases.

John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. If you have a question, send him an email at pgeweather@pge.com .

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