Machines that prod clouds to make snow may sound like something out of an old science fiction movie. But worsening water scarcity, combined with new evidence that “cloud seeding” can work, is spurring states, counties, water districts and power companies across the thirsty West to use the strategy. Last month, a study funded by the National Science Foundation tracked for the first time how the technology works in nature. The evidence for cloud seeding has been scarce, but recent research has encouraged officials and companies desperate to increase the amount of water in rivers and reservoirs.