Western States Seed Clouds in Search of New Water

On a remote mountaintop in the Sierra Nevada, as thunderheads gather in a dark mass above the peaks, a thin propane flame burns against the pale backdrop of snow. The generator, perched on top of a spindly tower, vaporizes a solution of silver iodide, wafting invisible particles upward into the clouds.

From his office at the Desert Research Institute (DRI) in Reno, Nevada, Associate Research Scientist Arlen Huggins eyes the temperature and wind direction as the data stream across his computer screen. He’s not waiting for the weather to change. He’s changing it.

Huggins’ project, funded by Nevada water users, is one of dozens of operational programs that resumed cloud-seeding efforts in November 2011. Generators placed upwind of a target area cast silver iodide particles into winter clouds, where their crystalline structure encourages the formation of ice that otherwise might not occur.

For more than half a century, cloud-seeding has captured the imaginations of scientists and water managers alike. Its effects have proven elusive—difficult to measure and little understood despite decades of putting the process into practice. Yet cloud seeding continues to tantalize western states with the promise of more water for the tenuous supply in the Colorado River Basin.

Diminishing water
Six of the seven U.S. states that rely on water from the Colorado River—Nevada, New Mexico, Utah, Colorado, California, and Wyoming—practice cloud seeding. Arizona, the one remaining state, has no projects of its own, but agencies there fund cloud-seeding efforts in upstream states.

The urgency to supplement the natural water supply stems from recent climate change studies that predict higher temperatures will increase evaporation and decrease mountain snowpack in the Southwest. That is bad news for the Colorado River system, which already is over-allocated between the seven U.S. states and Mexico.

Scientific research on the effectiveness of weather modification remains scarce. Federal funding for scientific studies, largely driven by interest in military applications, declined sharply after the 1970s. In 2003, the National Academy of Sciences published a report stating “there still is no convincing scientific proof of the efficacy of intentional weather modification efforts,” even though 66 operational programs existed in 10 U.S. states.

The report prompted a backlash from researchers, who pointed out the authors demanded a level of proof rarely achieved in atmospheric research. Scientists reduce uncertainty by repeating and randomizing experiments, a difficult task when they have to wait for the right weather to arrive.

Dan Breed, project manager of weather modification projects for the National Center for Atmospheric Research (NCAR), labeled the 2003 report as “pessimistic.” The report focused primarily on seeding convective clouds, dramatic thunderheads common in spring and summer. Breed agrees that convective cloud seeding remains controversial, but points toward orographic clouds, which form in winter as air is pushing over mountainous terrain, as far more promising.

Wyoming experiment
As part of his research, Breed analyzes data from a Wyoming experiment designed to tease out the subtle differences in snowpack, if any exist, between a mountain range that receives silver iodide and a nearby range left alone. Researchers began collecting case studies in the winter of 2008 and hope to receive another year of funding to obtain five seasons of data. Scientists herald the project as one of the most rigorously designed experiments on cloud seeding ever undertaken in the U.S.
Breed points out that an observer on the ground can’t tell the difference between seeded and unseeded snowfall. “The randomization is needed because you’re looking for a small signal in a fairly large natural variability,” he said.

Wyoming’s geography allows for a close comparison between two mountain ranges that experience the same winter storms. When favorable conditions exist in both the Sierra Madre and Medicine Bow ranges, located in south-central Wyoming, generators randomly seed one or the other. The arrangement allows scientists—who are not told which range has received silver iodide particles—to complete a statistical analysis and quantify any differences they find. A third mountain range, Wind River, will provide corroborating case studies.

Breed’s team at NCAR will lead the statistical analysis, while Huggins and other scientists at DRI will look for elevated levels of silver in snowpack. That will help them understand where the seeding material falls so operational programs can improve the placement of their generators. Weather Modification, Inc., a private corporation based in Fargo, ND, controls the satellite-operated, solar-powered generators.  

Breed says the data so far shows “strong suggestions of positive seeding effects,” but it is too soon to know the magnitude. “What we want to do is provide a quantitative idea, or at least a range of effects, so a hydrologist could use that to figure out streamflow,” Breed said.

Challenges  
Cloud seeding doesn’t create rain. It merely enhances natural storms. That means it will not work effectively when there is not enough moisture in the air to form around the silver iodide particles. In addition, Breed cautions that not all winter storms have the correct conditions to make cloud seeding successful. In the past, operational programs have seeded any storm in the area with little regard to its suitability, because the technology costs very little. At best, nothing happens, but Breed said scientists still don’t know if seeding in the wrong conditions creates unintended effects.

Another unanswered question involves the downwind effects. “It’s extremely hard to look in a place where you’re affecting [the weather] and see it change,” Breed said. “If you’re looking downwind, it’s something that may be 10 times smaller.”

Areas downwind of generators typically fall within the mountain’s rainshadow, where weather patterns are naturally drier. That makes comparisons difficult, but studies that model cloud formation show that only a fraction of the moisture contained in a winter cloud actually falls out as precipitation. For this reason, researchers think cloud-seeding operations are unlikely to affect downwind regions, or will slightly increase precipitation.

Residents downwind of cloud-seeding projects also worry about floods, mudslides, and weather damage that could result from enhanced winter storms. Most programs have cut-off points where they cease operations if snowpack levels are high to prevent excessive flooding in the spring. Public perception remains a major obstacle to cloud-seeding programs, Breed said, because damage caused by weather cannot clearly be attributed to natural causes or human interference.

How much water?
Quantifying the amount of water created by cloud seeding remains “a tricky deal,” said Don Griffith, president of North American Weather Consultants and the Weather Modification Association. Studies suggest cloud seeding creates a 5–15 percent increase in precipitation, but those numbers fall well within the range of normal weather variability.

The Wyoming experiment may answer longstanding questions about how cloud seeding works. Water managers, however, rarely require the high level of certainty demanded by scientists. “When water managers have a chance to make a relatively small investment to get very inexpensive water compared to any other alternative approach, they’re willing to take that risk,” Griffith said.

 
As Breed explained, “They’re already believers. They don’t need to know the specific numbers.”

A 2006 study by Griffith estimates that new cloud-seeding programs could create 154,000 acre-feet annually for the Colorado River Basin, or slightly less than half the water used annually by Phoenix in 2007, according to the most recent data available. (An acre-foot is 325,851 gallons.) Griffith emphasizes that this water, while modest in amount, costs only a few dollars per acre-foot, compared to more than $1,000 for an expensive alternative like desalination.

Another alternative source of water is simply conserving more.  In the Colorado River Basin, Huggins said, “the water entities have instituted a lot of conservation measures that probably do as much or more as cloud seeding.” Yet cloud seeding remains on the table because water is so valued in the Southwest that even a small increase in precipitation seems worthwhile. Hydroelectric power companies and ski resorts commonly seed clouds, as do agencies charged with supplying water to growing cities and sprawling agricultural fields.

States rarely fund intensive research projects like the one in Wyoming, which require much more money than simply operating a cloud-seeding program on faith. That makes the final results from the project all the more valuable, because the data can either justify cloud seeding as a cost-effective way to increase water supply or point to other solutions as more viable.  

“I think it will have a fairly big impact on how western states proceed with cloud seeding,” Breed said. As desert states like Arizona, New Mexico, and Nevada plan for a future with growing populations and an uncertain climate, they look for new water supplies untouched by current claims. For now, cloud-seeding programs play a small but hopeful role in this search, and experiments like the one in Wyoming will help decide how the Southwest prepares to meet future demands.  

For more information on NCAR’s weather modification project visit http://www.ral.ucar.edu/projects/wyoming.

Melissa Lamberton is an MFA candidate at Iowa State University and a previous contributor to the Southwest Climate Outlook.