One sweltering July day, when the freezer was struggling to keep cold, Schaefer popped over to the lab next door and borrowed a block of dry ice (frozen CO2) to tuck into one corner. It changed everything. The instant he lowered the cube into the freezer, millions of ice crystals began twinkling in the mist. They then wafted down onto the black velvet, glittering like microscopic diamonds. Schaefer thought at first that the dry ice had induced a chemical change in the mist, but further experiments ruled that out. Rather, the temperature of the dry ice seemed to be the key. Whereas the temperature in the GE freezer bottomed out at -9 degrees Fahrenheit, the frozen CO2 was below -100 degrees Fahrenheit. When exposed to such brutal, unnatural cold, even supercooled water said uncle and formed ice.

The discovery got Langmuir thinking. Scientists at the time knew that clouds in the sky were basically loose bags of supercooled water. They also knew that most rain actually begins falling from the sky as ice crystals, which melt on their way down. Langmuir reasoned that if he peppered clouds with dry ice, perhaps he could shock the supercooled water and create rain artificially. This led to his renting a plane that November and sending Schaefer up with six pounds of dry-ice pellets, just to see what happened. Twenty minutes later Langmuir was roaring about making history.

Langmuir’s team continued to toil in the lab and he soon sketched out an idea so revolutionary that he abandoned every other project on his slate to pursue it. It promised not only to improve rainmaking but to give Langmuir the superhuman power to control hurricanes.

The idea built on James Espy’s general theory of cloud formation. Espy said that clouds form when pockets of warm, less dense air rise into the sky. At some point the water vapor in them cools and condenses into droplets of liquid water. We on the ground see these collections of droplets as clouds, and for many years meteorologists assumed that rain followed automatically, whenever these droplets got around to falling. Turns out it’s not so simple. Most droplets that form within clouds don’t automatically sprinkle down as rain. They’re too small. As early balloonists knew, air provides a buoyant upward force on anything suspended within it, water droplets included. And when droplets form at high elevations, most of them are so tiny—one ten-millionth of a gram—that gravity can’t overcome the buoyant force and drag them down. Gravity keeps losing this battle, in fact, unless the droplets grow a million times larger, to a tenth of a gram. Clearly, then, for actual rain to fall, a million teeny droplets have to glom together into a larger unit. Otherwise they just sit there.

The obvious question, of course, is what makes the tiny droplets glom together. Intuitively, you might think that droplets simply collide at random and stick to one another. This process isn’t very efficient, though, and drops that form this way rarely grow large enough to precipitate out. A better way involves “seeds,” solid surfaces for water droplets to latch onto. For various reasons, once a few droplets latch onto a seed, many more follow in quick succession. As a result, droplets can finally grow heavy enough to fall out of clouds as precipitation. If you want to transform a cloud into rain, seeds are vital.