Controlling the weather seems like it should be well past the grasp of current technology; as long as meteorologists are still in demand, weather is more of a study in prediction than in manipulation. However, a tool called cloud seeding, used primarily to induce rainfall, already exists, and it's been tested and deployed at grand scales for nearly seventy years. (Seventy years!) As California withers under drought conditions, and as the incidence and severity of natural disasters climb worldwide, strategically regulating rainfall with cloud seeding could be invaluable. On the other hand, tampering with the weather over everyone's heads is ethically tricky: even if an existing weather event is dangerous, does that justify interventions of potentially global consequence? Just as puzzlingly, why are so many people in the dark about a technology that sounds so flashy and far-reaching?
First things first: Cloud seeding is fundamentally an attempt to stimulate raindrop formation inside clouds. This can be accomplished by cooling clouds with dry ice (solid carbon dioxide), which forces water inside the cloud to freeze into ice and potentially grow into a particle heavy enough to fall. Alternatively, and more commonly, small compounds like silver iodide are dispersed throughout clouds and serve as nucleation points for ice particles, in the same way that sand grains inside oysters can nucleate pearls. Once ice particles grow heavy enough, they fall as rain, hail, sleet, or snow. Of course, clouds usually release precipitation without aid, but the aim of cloud seeding is to “wring out” clouds, either to induce rain over a dry region, or to quickly coax rain out before the clouds pass over an area that should remain dry.
The first cloud seeding experiment was performed in 1946, accidentally and on a small scale by Vincent Schaefer, then a research assistant at General Electric. One afternoon, Schaefer added some dry ice to cool an experimental chamber. He then exhaled into the chamber; a short pause later, he was dazzled by innumerable brilliant ice nuclei suspended in the air. Seizing upon the phenomenon’s application, Schaefer used an airplane just a few months later in November to scatter six pounds of dry ice through clouds above eastern upstate New York. A few days later, snow fell in the Berkshire Mountains, just east of Schaefer’s experiment. During the same year, climate researcher Bernard Vonnegut (yep, Kurt’s brother) happened to discover cloud seeding via silver iodide particles.
How cloud seeding is hypothesized to work. Photo: Wikipedia
Since then, cloud seeding has been tested and executed by federal governments at large scales around the world – in the United States, India, China, Mexico, Germany, Russia, Australia, and elsewhere. Perhaps its most shocking non-experimental use was as a weapon of war, implemented by the US against North Vietnam from 1967 to 1972. For over five years, under the motto “Make mud, not war,” US military aircraft seeded clouds above Vietnam, Cambodia, and Laos with silver iodide and lead iodide particles in order to extend North Vietnam’s monsoon season so that supply trucks would face difficulty reaching troops across soaked, muddy ground. The mission was apparently successful: the monsoon season was 30 to 45 days longer on average during those years.
However, it’s important to note that the jury is still out on cloud seeding’s effectiveness. Only one major long-term seeding experiment, performed in Israel in the ‘60s and ‘70s, has been reproducible with high confidence. Moreover, attempts to use cloud seeding for other means – suppression of hurricane force and of hail storms – have largely failed. One obstacle to conclusive results is the difficulty of performing experiments in the first place: tests must occur on the scale of years, and it’s tricky to determine whether precipitation would have occurred on a given day without any cloud seeding. Sometimes small seeded ice particles remain suspended in a cloud without falling. Candidate clouds must be within a certain cool temperature range. It’s difficult to predict the time and spatial scales on which seeded locations will be affected, as well as the lifetimes of seeded clouds – an important consideration because clouds can trap heat from the sun and may contribute to global warming. Simply put, clouds, weather, and climate are still too complex for us to fully understand, let alone manipulate predictably.
Even so, instances of cloud seeding persist. Some areas in the US above ski-friendly regions are seeded in hopes of accumulating powder. Seeding particles were rocketed into the air above Beijing before the 2008 Summer Olympics opening ceremonies in order to empty clouds of rainfall and keep the celebrations dry, as well as to rid the air of smog. Although funding for cloud seeding research has diminished, some hope remains for the technology: concerns that increasing rainfall in one region will ‘steal’ neighboring regions’ rain have been debunked; in fact, the areas near seeded regions tend to see more precipitation as well. Additionally, although it might seem dangerous to shoot silver into the sky, studies of the soil and water in locations that have been heavily seeded do not reveal silver concentrations above normal background levels. And despite the paucity of funding for research in this area, some studies continue, like one German study that predicts that lasers, of all things, may help to seed clouds as well.
Perhaps during the coming decades, cloud seeding and more as-yet-undiscovered weather modification tools will emerge as effective technologies. They might ease conflict in regions governed by water supply, normalize extreme weather patterns, and extend crop-growing seasons. But the regulation of such technologies must contend with the fact that changing the weather may just as easily be invoked as a weapon of war. Either way, we may soon witness the results of the largest-scale scientific experiments ever performed on the earth.
