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Then one day on a whim, he grabbed some muddy snow from under his kids' swing set. It was sloppy science: Researchers around the world were already testing powdered minerals for their ability to form ice, but they used minerals that were sterile and pure. Vali's quick and dirty experiment produced an unexpected result.
Water drops from the dirty snow froze at 23 degrees much more often than his cleaner samples had. When Vali added rotting leaves to the mix, some drops formed ice at 28 degrees, the highest temperature he had ever seen. The leaves' potency increased as they turned brown, suggesting that something growing on them was causing ice formation. By this time Vali had finished his degree and moved to the University of Wyoming. There, in 1972, a graduate student named Richard Fresh managed to identify Pseudomonas syringae, a bacterium from the rotting leaves that was triggering the ice.
The discovery had big implications for agriculture. Billions of dollars of crops worldwide are damaged by frost each year. Steven Lindow, a Ph.D. student at the University of Wisconsin who independently discovered P. syringae's ice-nucleating capability around the same time Vali and Fresh did, found that frost damage is not caused by temperature alone; syringae bacteria on leaves actually use ice crystals as crowbars to rip open plant cells and grab their nutrients. To prevent this, some growers of fruit trees and vegetables now routinely apply bactericides to crops before frosts.
During the 1980s Lindow managed further to isolate the gene that allows
syringae to form ice. That gene codes for proteins that spread out on the bacteria's surface in just the right arrangement to pull water molecules into their hexagonal crystal formation. Since then, four other bacteria have been found that nucleate ice at extremely warm temperatures. Although they are not all related, they share the same gene for ice nucleation.
The mystery of microbes
Even as the importance of biological ice nucleation was being recognized by agricultural scientists, it still wasn't embraced by atmospheric scientists, who stuck by the traditional view that soot, or sea salt, or some as-yet-unidentified mineral in dust was seeding ice in clouds. There was no real sense at the time of the number and diversity of microbes living in the atmosphere. Vali tried to collect ice-forming cells from the lowest few hundred feet of the atmosphere but failed to find anything. And no one was systematically looking thousands of feet up, where clouds actually form. People doubted that enough cells could get high in the atmosphere to make rain or snow.