Cloud seeding
Based on Wikipedia: Cloud seeding
The Day a Scientist Accidentally Made It Snow
On July 14, 1946, Vincent Schaefer walked into his laboratory at General Electric in Schenectady, New York, annoyed. His deep freezer wasn't cold enough. He'd been trying for weeks to create artificial clouds inside the freezer chamber—breathing into it to produce water vapor, then testing various substances to see if any could trigger ice crystal formation. Table salt. Talcum powder. Soil samples. Nothing worked particularly well.
Frustrated, Schaefer grabbed a chunk of dry ice and tossed it into the chamber, just to lower the temperature. Then he leaned over and breathed into the box.
What happened next changed meteorology forever.
A bluish haze appeared instantly, followed by what Schaefer later described as "an eye-popping display of millions of microscopic ice crystals," glittering under the lamp light like a galaxy of tiny stars. He had discovered something remarkable: a reliable way to transform supercooled water—water that remains liquid even below freezing—into ice crystals on demand.
Four months later, Schaefer chartered a small plane, flew sixty miles from Schenectady to chase a promising cloud over western Massachusetts, and dumped six pounds of dry ice into it. Snow began falling near Mount Greylock. A human being had made it snow on purpose for the first time in history.
What Cloud Seeding Actually Does
Here's the counterintuitive thing about rain and snow: they're surprisingly hard to start. Clouds are full of water, but that water often exists in a suspended state, waiting for something to happen. The droplets are too small to fall. They need to grow.
There are two main ways water droplets grow large enough to become precipitation. They can collide and merge with other droplets—imagine tiny water balloons bumping into each other until they become one larger balloon. Or, in cold clouds, they can freeze onto microscopic particles and grow as ice crystals, eventually becoming heavy enough to fall. When those ice crystals pass through warmer air on the way down, they melt into raindrops. If the air stays cold all the way to the ground, you get snow.
The problem is that sometimes clouds lack enough of these microscopic particles—called nuclei—for precipitation to form efficiently. The water just hangs there, waiting.
Cloud seeding provides those missing nuclei.
The most common seeding agent is silver iodide, and there's a beautiful reason why it works. Silver iodide has a crystalline structure almost identical to ice. When supercooled water droplets encounter a silver iodide particle, they're essentially fooled into thinking they've found an ice crystal to freeze onto. The lattice spacing—the arrangement of atoms in the crystal—matches so closely that water molecules lock on and begin building ice.
This discovery came from Bernard Vonnegut, Schaefer's colleague at General Electric and brother of the novelist Kurt Vonnegut. (Kurt would later use ice crystallography as a central plot device in his novel "Cat's Cradle," imagining a form of ice that could freeze all the world's oceans.) Bernard found silver iodide by doing something almost quaintly old-fashioned: he sat at his desk, paged through a basic chemistry textbook looking for substances with crystal structures similar to ice, then started experimenting. Within a month of Schaefer's dry ice breakthrough, Vonnegut had his own method for making it rain.
The Two Schools of Thought
Cloud seeding comes in two main flavors, and understanding the difference reveals something fundamental about how clouds work.
Static seeding is the simpler approach. You introduce ice nuclei into clouds that contain supercooled water—liquid water at temperatures below freezing, which is more common than you might think. The nuclei trigger ice crystal formation, the crystals grow at the expense of surrounding water droplets, and eventually they fall as precipitation. This works best when cloud temperatures are between negative seven and negative twenty degrees Celsius (roughly twenty degrees to negative four Fahrenheit). Too warm, and there's no supercooled water to work with. Too cold, and ice crystals form naturally without help.
Dynamic seeding is more ambitious. Rather than just triggering precipitation from existing clouds, it attempts to supercharge the entire cloud system. The idea is that by releasing latent heat—the energy released when water vapor condenses or freezes—you can enhance the cloud's natural convection, causing it to grow larger and produce more rain than it otherwise would. Think of it as giving a cloud a growth hormone injection rather than just shaking loose what's already there.
The techniques for getting seeding agents into clouds are varied and sometimes inventive. Airplanes remain the most common delivery method, flying through or above target clouds and releasing silver iodide flares or dry ice pellets. Ground-based generators burn silver iodide in acetone, creating smoke that rises with natural air currents into the clouds above—a cheaper approach often used in mountainous terrain where the geography naturally funnels air upward.
More recently, the United Arab Emirates has experimented with drones that release electric charges into the air, essentially zapping cloud droplets to encourage them to merge and fall. In 2010, researchers from the University of Geneva tested infrared laser pulses over Berlin, hoping that the intense light would cause atmospheric sulfur dioxide and nitrogen dioxide to form particles that could act as seeds. These newer approaches remain experimental, but they suggest a future where cloud seeding might not require releasing any substances at all.
The Great Debate: Does It Actually Work?
This is where the story gets complicated.
Cloud seeding has been practiced for nearly eighty years. Dozens of countries have operational programs. Millions of dollars are spent annually on the technology. And yet, in December 2024, the United States Government Accountability Office released a report acknowledging that the scientific community remains genuinely divided on whether cloud seeding produces statistically significant increases in precipitation.
How can this be? How can we not know if something works after eight decades of trying?
The answer reveals the maddening difficulty of doing controlled experiments with weather. When you seed a cloud and it rains, how do you know the rain wouldn't have fallen anyway? Weather is chaotic, variable, and impossible to rerun. You can't seed half a cloud and leave the other half as a control group. You can't put the atmosphere in a test tube.
A study by the United States National Academy of Sciences failed to find statistically significant evidence that cloud seeding works. Stanford ecologist Jerry Bradley interpreted the data cautiously: "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."
Yet the same underlying data has been interpreted differently by others. The Wyoming Weather Modification Pilot Project, looking at similar evidence, concluded that seeding could increase seasonal snowpack by up to three percent. That's not nothing—three percent more water in a reservoir can matter enormously to a drought-stricken region—but it's also far from the dramatic rainmaking of science fiction.
A 2003 report from the National Research Council struck a notably humble tone: "Science is unable to say with assurance which, if any, seeding techniques produce positive effects. In the 55 years following the first cloud-seeding demonstrations, substantial progress has been made in understanding the natural processes that account for our daily weather. Yet scientifically acceptable proof for significant seeding effects has not been achieved."
Some researchers are more optimistic. In 2016, Jeff Tilley of the Desert Research Institute argued that new technology has finally made cloud seeding a "dependable and affordable water supply practice." The American Meteorological Society has stated that precipitation from certain mountain clouds can be increased by about ten percent through seeding. A 2010 Tel Aviv University study, however, concluded that cloud seeding with silver iodide and dry ice "seems to have little if any impact on the amount of precipitation."
The honest truth is that cloud seeding probably works sometimes, in some conditions, to some degree—but pinning down exactly when, where, and how much remains genuinely difficult.
When Nations Try to Control the Weather
Despite the scientific uncertainty, the appeal of weather modification has proven irresistible to governments facing drought, agricultural needs, or even diplomatic events.
China deployed cloud seeding extensively during the 2008 Summer Olympics in Beijing, attempting to squeeze rain out of approaching clouds before they reached the city—essentially trying to make it rain elsewhere so the opening and closing ceremonies would stay dry. Whether this actually worked remains disputed. Roelof Bruintjes, who leads the National Center for Atmospheric Research's weather modification group, noted dryly: "We cannot make clouds or chase clouds away."
The United Arab Emirates, facing a desert climate where freshwater is desperately scarce, has become perhaps the world's most enthusiastic adopter of cloud seeding technology. Their drone-based electric charge system made headlines in July 2021 when a significant rainstorm dropped nearly seven millimeters of rain on Al Ain—a modest amount by most standards but notable in a region where any rainfall is precious. Whether the drones actually caused that particular storm remains, as with so much in cloud seeding, uncertain.
The most dramatic military application came during the Vietnam War. From 1967 to 1972, the United States military ran Operation Popeye, seeding clouds with silver iodide over the Ho Chi Minh Trail to extend the monsoon season. The goal was grimly practical: make the roads so muddy that North Vietnamese supply trucks couldn't get through. The 54th Weather Reconnaissance Squadron's unofficial motto captured the strategy perfectly: "Make mud, not war."
According to military assessments, the operation extended the monsoon by thirty to forty-five days in targeted areas. Whether this significantly impacted the war's outcome is debatable, but the revelation of Operation Popeye contributed to international concerns about weaponized weather modification. Today, the Environmental Modification Convention prohibits military use of weather modification, though civilian and research applications remain legal.
The United States also attempted to weaken hurricanes through Project STORMFURY in the 1960s. Scientists seeded four hurricanes over eight days and observed wind speed decreases of ten to thirty percent on some of those days. Initially, this seemed promising. But subsequent analysis revealed a fundamental problem: they couldn't distinguish between changes caused by seeding and the natural fluctuations that hurricanes undergo anyway. The project was eventually abandoned.
Silver in the Sky: Environmental Concerns
If we're spraying silver compounds into clouds, shouldn't we worry about where all that silver ends up?
Silver iodide has an NFPA 704 health hazard rating of two, meaning intense or chronic exposure can cause temporary harm to humans and other mammals. That sounds concerning. But context matters enormously.
The quantities used in cloud seeding are tiny—about one percent of the silver that industry releases into the atmosphere through other processes. Multiple studies, including a 1995 environmental assessment in California's Sierra Nevada and a 2004 expert panel review in Australia, found that accumulations in soil, vegetation, and water runoff were too small to measure above natural background levels.
The comparison to dental fillings is illuminating: individual human exposure to silver from tooth fillings typically exceeds what anyone would receive from cloud seeding operations in their area.
Still, concerns persist in sensitive ecosystems. Cloud seeding over Australia's Kosciuszko National Park—a UNESCO biosphere reserve—sparked controversy because environmentalists worried about silver uptake affecting the endangered mountain pygmy possum and contributing to algal blooms in glacial lakes. The debate led to rapid changes in environmental legislation to permit seeding trials, which struck some observers as prioritizing water supply over ecological caution.
The International Weather Modification Association disputes claims of significant environmental damage, pointing to peer-reviewed research. But the fundamental tension remains: we're deliberately adding substances to the atmosphere over large areas, and our understanding of long-term ecosystem effects is incomplete.
The Unintentional Cloud Seeders
Here's a twist that scientists discovered in 2011: we may have been accidentally seeding clouds for decades without realizing it.
Airplanes, it turns out, can produce ice particles simply by flying through clouds. As air flows around propeller tips, wings, or jet engine housings, it expands and cools rapidly. If the cloud contains supercooled water droplets, those droplets can freeze on contact with the aircraft, creating ice particles that then act as seeds for further precipitation.
This unintentional seeding could have consequences for hailstone formation. When airplanes trigger ice crystal formation in certain types of clouds, they might inadvertently be contributing to hail development—adding an ironic wrinkle to weather modification research.
The Prehistory of Rainmaking
Humans have tried to summon rain for as long as humans have farmed. Rain dances, prayers, rituals—every agricultural civilization developed some approach to influencing precipitation. The scientific era of rainmaking arguably began in 1891, when Louis Gathmann proposed shooting liquid carbon dioxide into clouds to make them rain.
During the 1930s, the Swedish meteorologist Tor Bergeron and the German physicist Walter Findeisen developed what's now called the Bergeron-Findeisen process, explaining how ice crystals in mixed-phase clouds (clouds containing both ice and supercooled water) can grow rapidly and trigger precipitation. Their theoretical work laid the foundation for understanding why substances like silver iodide might work as seeding agents.
But it took Schaefer's serendipitous dry ice experiment and Vonnegut's systematic crystal structure analysis to turn theory into practice. The field they founded has grown into a global industry, with cloud seeding operations on every inhabited continent.
The Future of Weather Modification
Cloud seeding exists in an unusual space: too promising to abandon, too uncertain to fully embrace. Water managers in drought-prone regions are understandably eager for any tool that might increase water supply, even by a few percent. Skeptics point out that resources spent on cloud seeding might be better invested in water conservation, reservoir management, or desalination.
The technology continues to evolve. Electric charge methods avoid the environmental concerns associated with silver iodide. Improved computer modeling may eventually allow more precise targeting of seeding efforts, focusing on clouds most likely to respond. Better statistical methods might finally resolve the efficacy debate that has persisted for eight decades.
In the meantime, cloud seeding persists as one of our most ambitious and uncertain interventions in natural systems—a reminder that even after all our scientific progress, the weather remains humbling in its complexity. We can nudge, perhaps. Coax, maybe. But control? The clouds keep their own counsel.
Vincent Schaefer's accidental discovery in that General Electric freezer opened a door we're still learning how to walk through. Nearly eighty years later, we're still asking the same question he must have asked himself that July afternoon, watching ice crystals dance in the lamplight: What else is possible?