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Atmospheric Chemistry

Does cloud seeding really work?

Researchers are combining studies of ice formation with remote-sensing techniques to give rainmaking scheme a firm scientific footing

by Janet Pelley, special to C&EN
May 30, 2016 | A version of this story appeared in Volume 94, Issue 22

Credit: Jim Brandenburg/Minden Pictures/Newscom
A plane launches a flare to seed clouds over North Dakota.

The pilots at Weather Modification Inc. were standing by on alert on Aug. 12, 2012, when they got word from their staff meteorologist to jump in their planes and head toward a budding thunderstorm just west of Calgary, Alberta. Their mission: to prevent the formation of crop-destroying, car-denting hail by shooting flares loaded with silver iodide into cumulus clouds.

Some of the pilots headed for the smooth, rain-free base of the clouds at 2,000 meters, where updrafts could pull the inorganic compound in. Other pilots flew to 5,500 meters, penetrating the tops of the billowy formations.

Once in position, the aviators ejected the flares mounted on their planes. Theoretically, the silver iodide particles that spewed forth would catalyze supercooled water droplets in the clouds to freeze at a warmer temperature and more abundantly than they might have otherwise. The pilots hoped that this maneuver would redistribute the water vapor in the clouds, releasing rain and small hailstones rather than the large golf-ball-sized ones that had been predicted.

Afterward, radar data revealed a storm nearly 27% less severe than what had been projected, says Terry Krauss, a meteorologist with the Alberta Severe Weather Management Society, a nonprofit agency funded by insurance firms. “Our data show that the seeding may have avoided up to C$100 million in damage to homes and cars,” he says. On a severe storm day, he adds, even a 1% reduction in hail intensity will more than pay for the annual C$4 million cost of Alberta’s hail suppression program.

Not only has cloud seeding been used to mitigate hailstorms for years, it has also been used to try to enhance rain- and snowfall for water storage in reservoirs and in the ground. These small-scale projects are not to be confused with geoengineering schemes that propose tinkering with the planet’s weather by modifying Earth’s ability to reflect solar energy. Currently, more than 50 countries worldwide participate in cloud-seeding operations.

And these operations are growing in popularity. Almost half of the world’s population will be living in water-stressed areas by 2030, according to estimates from the United Nations. This year, the United Arab Emirates (U.A.E.) awarded $5 million to rain enhancement researchers in Japan, Germany, and the U.A.E. to address the problem.

So it seems odd then that cloud seeding, so heavily touted, hasn’t actually been statistically proven to work. After the method was first tested 70 years ago, enthusiasm for cloud seeding led to experiments that claimed annual precipitation increases of 10% or more. But the studies lacked statistical rigor. And running control experiments in cloud-seeding studies is a challenge: Once a cloud is treated, you can’t measure how much it would have rained or snowed if left unseeded. Even the basic mechanics underlying the crystallization of water molecules on seeding agents remains mysterious.

After the 1980s, with few results to show for the millions of dollars invested in research, studies on weather modification dropped to a trickle. Yet over the past decade, advances in remote-sensing and modeling and new work on the physics of ice formation are reviving hopes for a more solid scientific footing for cloud seeding.

The puzzle of ice initiation

Modern-day cloud seeding was launched in the lab of noted surface scientist Irving Langmuir at General Electric in 1946. His colleagues Vincent Schaefer and Bernard Vonnegut, brother of author Kurt, discovered that silver iodide could transform supercooled water vapor into ice crystals at temperatures of –10 to –5 °C. In nature, clouds form when supercooled water vapor condenses and then freezes onto particles, called ice nuclei, made of dust and even bacteria. Droplets of pure water can’t form an ice crystal nucleus until the temperature drops to –40 °C. Yet if clouds contain aerosol particles, water molecules can use the solid surfaces of these “seeds” to organize themselves into a crystalline form at much warmer temperatures, from –20 to –5 °C.

Reasoning that precipitation must be limited by the scarcity of natural ice nuclei in the air, Schaefer and Vonnegut began atmospheric trials to inject artificial nuclei into clouds, and an industry was born. The researchers suggested that silver iodide was a good nucleating agent because its hexagonal crystalline lattice is nearly identical to the lattice that water molecules form in ice and snowflakes—one in which units of six water molecules assemble. Silver iodide is the seeding agent of choice for cold clouds, although firms also deploy potassium chloride and dry ice, says Bruce Boe, vice president of meteorology at Weather Modification Inc.

Credit: 图片由 Angelos Michaelides 提供
Credit: 图片由 Angelos Michaelides 提供
In this simulation, water molecules (red and white) form a pentagonal lattice when they stick to a copper surface.

“Vonnegut’s proposal that silver iodide is an effective ice-nucleating agent because it provides a hexagonal crystalline template similar to that of ice is a compelling view that’s been widely accepted,” says Angelos Michaelides, a theoretical chemist at University College London. But there is no basis for this claim because scientists have not yet established the exact mechanism of the freezing process. “We still know rather little about the structures water forms as it transforms from the liquid to the solid state, particularly when this process happens at the surfaces of other materials such as silver iodide,” he says.

Ice nucleation is hard to probe experimentally because current imaging instruments don’t produce clear pictures of individual molecules as they freeze, Michaelides says. So he and his colleagues have created nanoscale computer simulations that interpret results from physical images. The simulations predict the interactions between molecules on the basis of the rules of quantum mechanics.

In 2009, for instance, Michaelides and his team collaborated with experimentalists at the University of Liverpool who used scanning tunneling microscopy to detect water freezing on a copper surface. The simulations run on the resulting imaging data provided strong evidence that the 1-nm-wide chains that formed on the copper surface were not built from water hexagons—the traditional ice lattice—but from groups of five water molecules bonded into pentagons (Nat. Mater. 2009, DOI: 10.1038/nmat2403).

These results suggested that perhaps Schaefer and Vonnegut’s hypothesis about what makes silver iodide such a good nucleating agent wasn’t correct. So more recently, Michaelides and his team computationally designed a theoretical set of surfaces, varying the extent to which their crystal structures matched ice. Allowing ice to nucleate on the surfaces via a computer simulation, the scientists found that there was no simple correlation between the similarity of a surface to ice and its ability to nucleate ice (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b08748).

Similarly, a recent investigation of ice-nucleating bacteria also suggests that surfaces don’t have to match the structure of ice crystals in order to nudge water into its solid phase. Ski resorts dose their snowmaking machines with the bacteria Pseudomonas syringae because proteins on its surface freeze water at temperatures around the melting point of ice (0 °C). “Yet no one understood the molecular mechanism by which the proteins trigger freezing of water,” says Tobias Weidner, a physicist at the Max Planck Institute for Polymer Research.

Weidner and his colleagues used sum frequency generation spectroscopy and computer simulations to demonstrate that the proteins on the outer membrane of the bacteria create alternating hydrophobic and hydrophilic sites (Sci. Adv. 2016, DOI: 10.1126/sciadv.1501630). This simple arrangement promotes ice crystal formation by manipulating water molecules into tight patterns of high and low density. If someone wanted to make a new cloud-seeding agent, maybe a polymer particle, “it might be possible to engineer this hydrophobic and hydrophilic pattern on a nanoscale,” Weidner says.

All these findings are getting scientists closer to identifying what makes a good ice-nucleating agent and why. “We hope this will lead to a general theory that will have predictive value used to design and identify new materials we can control ice formation with,” Michaelides says.

How cloud seeding is supposed to work
Weather modification firms launch silver iodide flares from planes or from the ground into cloud formations to try to increase rainfall or mitigate hailstorms.

1. Some planes drop flares from above the cloud formation. Some planes release silver iodide from flares into an updraft. Some operations shoot silver iodide flares into clouds from the ground.

2. Silver iodide helps form ice nuclei that can fall as rain or snow and steal moisture from larger hail particles.

3. Ice crystals fall, coming down as rain, snow, or small hail particles, depending on the conditions.

Credit: Yang H. Ku/C&EN/Shutterstock

Probing clouds

Last year marked the conclusion of a massive six-year study that has been the most comprehensive and rigorous to date to investigate whether cloud seeding actually increases precipitation. Called the Wyoming Weather Modification Pilot Project (WWMPP), the study was run by a team of researchers from government, academia, and private industry. In the end, WWMPP wasn’t able to provide a definitive answer. “But the results do provide a body of evidence that cloud seeding is working under certain conditions,” says Roelof Bruintjes, an atmospheric scientist at the National Center for Atmospheric Research (NCAR), who was not part of the project although his colleagues at NCAR were deeply involved.

Earlier studies would inject silver iodide into clouds, then compare precipitation gauges in areas inside and outside the seeding zone. But the studies weren’t repeatable, and they didn’t include enough trials to guarantee that observed increases in precipitation weren’t due to chance. The challenge with measuring the effect of weather modification is that natural rain- and snowfall variability is 10 to 100 times as large as the amount of precipitation augmented by seeding, Bruintjes says.

Still, the WWMPP researchers thought they could address the drawbacks of past studies. The researchers designed their $14 million project to run for six winter seasons in the mountains of Wyoming. They conducted more than 150 tests, randomly selecting clouds to seed and clouds to be their unseeded controls.

Measurements from the high-resolution snow gauges on the ground indicated that seeding elevated snowfall by 5–15%. But this result was achieved only after the researchers threw out some of the tests where silver iodide drifted into control clouds or where not enough seeding material was released, so the final results weren’t statistically significant. “Nevertheless, all the results provided evidence for a positive trend,” Bruintjes says.

The scientists also took advantage of new developments in remote-sensing and atmospheric modeling to examine dynamics inside a small subset of seeded clouds.

Using cloud radar, a laser-based version of radar known as lidar, and other techniques, the team examined the chain of events, beginning with the distribution of the seeding material then moving to the conversion of supercooled liquid water into ice and finally to the deposition of snowfall. With the reflectance signal from lidar in particular, the researchers were able to monitor the real-time decline of supercooled liquid water as it condensed on the silver iodide particles. Cloud radar tracked the increase in the number of snow particles.

Remote-sensing observations are valuable because radar can describe growth of snow in a cloud in a much more immediate way than snow gauges can, says Bart Geerts, an atmospheric scientist at the University of Wyoming who was part of WWMPP. “Detailed remote-sensing measurements of cloud dynamics are cheaper and more doable than randomized statistical experiments that measure increases in snow on the ground,” Geerts says.

The remote-sensing equipment combined with modeling confirmed an increase in the size and number of snow particles within the cloud after seeding. “In the core of the silver iodide plumes, we may see the snowfall rate double or more, according to the model,” Geerts says. In the end, though, the researchers did not have enough remote-sensing data over a sufficiently long period of time to quantify the impacts they thought they saw.

Another climate modeling experiment conducted over eight winters in the WWMPP study area, however, estimates that about 30% of winter precipitation in the region comes from seedable clouds. Not every cloud in the mountains meets the criteria for seeding, says Jaclyn Ritzman, a meteorologist at the National Oceanic & Atmospheric Administration who ran the study. The temperature and the wind speed and direction have to be just right. Assuming that seeding promotes about a 10% rise in precipitation, Ritzman and her team suggest that seeding could augment the snowpack by a maximum of 3% over an entire season.

Nevertheless, not everyone is convinced of cloud seeding’s benefits. The WWMPP report’s findings are not too different from the conclusions drawn over a decade ago by the National Academy of Sciences (NAS) in a report on weather modification, says Rob Jackson, an ecologist at Stanford University. The NAS report concluded that it is difficult to show clearly that cloud seeding has a very large effect. “I think you can squeeze out a little more snow or rain in some places under some conditions, but that’s quite different from a program claiming to reliably increase precipitation,” he says.


Concerns remain

Even if cloud seeding does succeed at increasing precipitation, environmental activists are concerned about its impact. One scientist addressing those concerns is geochemist Shawn Benner at Boise State University. “The near impossibility of detecting a silver iodide signal in snowpack after seeding attests to its low environmental risk,” Benner says.

Natural background levels of silver iodide in snow are about 1 to 2 parts per trillion, and after seeding, researchers look for levels from 4 to 20 ppt. Although silver is toxic to aquatic organisms in large doses, the levels found in surface water after seeding are well below the toxic threshold of 50,000 ppt, Benner says. “Nevertheless, if the practice of cloud seeding intensifies at a larger scale, silver toxicity and other environmental issues could become a concern,” Jackson says.

Aside from the toxicity of silver, some cloud-seeding critics raise concerns about messing with the balance that Mother Nature holds on the atmosphere. The amount of moisture in the atmosphere is determined by the balance between evaporation and precipitation. If cloud seeding is done on a large scale, it might lead to increased evaporation from locations outside the seeding area, Jackson says. “It’s obvious that rain falling in one place would have fallen somewhere else, but the broader question is, who, if anyone, would have seen the rain and what else might it affect?” he adds.

Atmospheric budgets suggest that cloud seeding is unlikely to steal moisture from downwind sites, responds Weather Modification’s Boe. Because clouds represent a modest portion of the moisture in the atmosphere, a cloud-seeding effect of 15% would only remove about 1–2% of the total water vapor in the seeding area, he contends.

As chronic drought settles into parts of the Great Plains and western U.S., state and local water agencies don’t seem to be troubled by the uncertainties of cloud seeding, in part because the costs are so small compared with the potential benefits. A new study from the Texas A&M AgriLife Extension Service estimates that if cloud seeding augments annual rainfall by one inch, then for every dollar invested in weather modification, $19 is returned through more bountiful harvests and the use of less irrigation.

“But at present, making it rain is still more of an art than a science,” Jackson says. With countries increasingly spending hundreds of millions of dollars on weather modification, he argues, more research is needed to understand if the practice works and what its environmental, social, and governance impacts will be.  

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