Four decades ago, scientists discovered that the bacterium Pseudomonas syringae triggers the formation of frost on plants. Since then, some researchers have proposed the ice-making bug and others like it might be creating ice crystals in clouds that result in precipitation. It's not clear yet if airborne microbes really influence the weather, but that hasn't stopped some optimistic scientists from studying the bacteria as a tool to increase rain and snow. Harnessing bacteria to make precipitation could be big business. Dozens of states and countries run cloud-seeding programs using artificial ice-nucleating compounds. In California the effort is especially urgent. The Sierra Nevada snowpack, which provides about 65 percent of the state's water, has been declining since 1950, and a quarter of the snow is projected to disappear by 2050. Montana State University bacteriologist David Sands imagines using syringae to bring more rainfall to places like California. "I'm an agriculturalist,'1 he says. "I don't like droughts." Microbiologists have identified strains of syringae that form frost without damaging their hosts. It could be possible to plant large tracts of land with plants that harbor these strains. The microbes might then get into the air, form ice crystals in clouds overhead, and pull more rain from them.
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