Today, at any moment, more water rushes through the atmosphere than flows through all the world's rivers combined.

Searching in an ancient rain-fed lake in northern India, paleoclimatologists using radiocarbon dating have discovered that 4,100 years ago, the summer monsoons began a rapid decline. They did not return to normal for two centuries. For an unimaginable two hundred years, the Harappan region saw hardly any rain. Around the same time in China, Egypt, and Mesopotamia, the three other earliest-known civilizations also were lost to the dry sands of history.

The water vapor accumulated in the upper atmosphere for so long that when the surface finally cooled enough for the rains to touch down, they poured in catastrophic torrents for thousands of years.

Not infrequently in the wide skies over Yuma and other parts of the arid Southwest, residents watch sheets of rain begin to unfurl from auspicious purple storm clouds, backlit by the sun. But the rain stops halfway, hanging mid-horizon like a magician's trick. Known as rain streamers or by their scientific name VIRGA, the half-sheets evaporate into the dry air before the rain can reach the ground.