The idea that humans might coax rain from the sky…

The idea that humans might coax rain from the sky is old, but the modern practice of cloud seeding dates from 1946, when Vincent Schaefer, a researcher at General Electric, dropped crushed dry ice into a cloud over Massachusetts and watched snow fall from it. His colleague Bernard Vonnegut soon showed that silver iodide, whose crystal structure closely resembles that of ice, could do the same job. Both substances work on clouds containing supercooled water, droplets that remain liquid below freezing because they lack a surface on which to crystallise. Seeding supplies that surface: ice crystals form, grow at the expense of the surrounding droplets, and eventually fall as snow or rain.

A second technique, hygroscopic seeding, targets warm clouds by dispersing salt particles that attract water and encourage small droplets to merge into larger ones. In either case, seeding cannot conjure precipitation from a clear sky; it can only nudge existing clouds that are already close to producing rain.

The central difficulty has always been proof. Clouds vary enormously, and no two can be compared directly, so operators cannot be sure that rain following a seeding flight would not have fallen anyway. Early statistical trials gave mixed results, and claims of increases of ten to fifteen per cent were widely disputed. Only in 2017 did a field campaign in Idaho use radar to track a seeded cloud in detail, observing ice forming along the aircraft's path and precipitation reaching the ground. Even so, the amount of extra water was modest, and scientists caution that results from one mountain range cannot be assumed to apply elsewhere. Nevertheless, programmes continue in the western United States, China, the United Arab Emirates and elsewhere, driven less by certainty than by the high value of even small gains in water supply.