Temperatures linger well above 100 – the outback bustles with scorpions, rattlesnakes and tarantula hawk wasps. Summer in West Texas is fierce. But, in this arid land, it also means the happiest of prospects: rain.

Between May and September, the North American monsoon brings the majority of our region's annual precipitation. For both wildlife and human communities, life here hinges on those summer rains.

And the forces that shape the monsoon are dauntingly complex. If, when and where it rains in West Texas depends on factors as distant as the South Pacific Ocean, as near at hand as mountains on the horizon.

It begins, in many years, on an afternoon in May or early June. The heat i...