As California’s unprecedented four-year drought rages on, I’ve been asked about cloud seeding at an ever greater rate.
This is understandable because, for many California communities, the situation has become dire.
Here at home, Cal Poly — home of climatology for San Luis Obispo — is 38.5 inches behind its normal four-year total. Consequently, to make up this deficit, the rainfall this coming season at Cal Poly would need to be 60.9 inches. The all-time record for Cal Poly is 54.5 inches, which occurred in the 1968-69 rain season.
Mark Twain once said, “Everyone talks about the weather, but no one does anything about it.” If he were 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.
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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. 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 (1967-72). 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 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 Woodstock Festival in upstate New York. More than half a million concert attendees joined in chants of “No more rain!” during a 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 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.
Despite the promise of a strong El Niño this winter and the anticipated heavy rains it may bring, many climatologists believe that most of the western United States is in a long-term dry spell that could last for decades.
The forecast is calling for a chance of thunderstorms Sunday and Monday. Lightning accompanying these events can spark wildfires. PG&E is serious about working with Cal Fire and other state and local agencies to do our part to reduce that risk. To learn more about safety, please visit www.pgecurrents.com/