When Mark Twain said that everybody talks about the weather, but nobody does anything about it, he was wrong. In fact, the rain falling on his California roof at that moment might have been generated by secret chemicals being diffused into clouds by a hired rainmaker named Charles Hatfield. Hatfield got rich selling rain to farmers in the San Joaquin Valley until he was run out of the state of California by angry San Diegoans who accused him of triggering a flood.

Experiments in rainmaking flourished in the early 1900s in American farmlands where drought meant not only hunger but poverty. A little like snake-oil salesmen, early rainmakers sold their ability to make it rain, but it was always ambiguous: when it worked, they were paid. More often, it was hard to tell if they had performed their promised service. Lawsuits were abundant when rain intended...