In 1986, the Soviet minister of hydrometeorology, Yuri Izrael, had a regrettable decision to make. It was his job to track radioactivity blowing from the smoking Chernobyl reactor in the hours after the 26 April explosion and deal with it. Forty-eight hours after the accident, an assistant handed him a roughly drawn map. On it, an arrow shot north-east from the nuclear power plant, and broadened to become a river of air 10 miles wide that was surging across Belarus toward Russia. If the slow-moving mass of radioactive clouds reached Moscow, where a spring storm front was piling up, millions could be harmed. Izrael’s decision was easy. Make it rain.