Dry ice was the seed of a wet discovery

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 Twitchell Reservoir and Lake Cachuma. Not only is cloud seeding widely used to increase rainfall, but also to suppress damaging hail from Alberta, Canada, as it moves southward through the Great Plains to Oklahoma. It’s also used to augment the snowpack in the many mountain ranges across the western United States.

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 (1967-1972). 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 cloudseeding 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” (ENMOD) 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. 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 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 acre- foot. By comparison, the cost of desalinated seawater can reach nearly $2,000 per acre-foot.

Today’s weather report

Friday’s thunderstorms produced periods of heavy rain, hail, lightning and gusty winds.

A 1,028-millibar eastern Pacific high will park itself about 500 miles to the west of San Luis Obispo later today. This condition will produce a classic type of spring weather pattern with strong to gale-force (25- to 38-mph) afternoon northwesterly winds and dry and mostly clear conditions through next weekend. There should be just enough north to northeasterly (offshore) winds during the late night and early morning to keep the marine layer out at sea.

Today’s maximum temperatures will reach the high 60s throughout San Luis Obispo County.

By Monday, maximum temperatures will reach the high 70s in the North County and the low 70s in the coastal valleys. Overnight lows will remain in the 40s throughout San Luis Obispo County. Little change is expected through next Sunday.

Even though the long-range models have struggled to reach a consensus on the details, rain is expected for the entire Central Coast on April 23.

Surf report

Today’s 6- to 8-foot northwesterly (300-degree deep-water) sea and swell (with a 7- to 13-second period) will decrease to 4 to 6 feet Monday.

A 6- to 8-foot northwesterly (300-degree deep-water) sea and swell (with a 5- to 17-second period) will arrive along the Pecho Coast on Tuesday and will remain at this height but with a gradually shorter period through Wednesday.

An extended period of northwesterly winds along the Northern and Central California coastline will produce a 4- to 6-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 12-second period) Thursday through next Sunday.

Preliminary extended surf analysis: At this time, no medium- or high-energy swell events are expected over the next few weeks.

Seawater temperatures will range from 49 to 51 degrees through next weekend.

Earth Day thanks

John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 25 years. If you would like to subscribe to his daily weather forecast or ask him a question, send him an email at pgeweather@pge.com.

This story was originally published April 14, 2012 at 8:33 PM with the headline "Dry ice was the seed of a wet discovery."