Ask people to name the world’s largest river, and most will probably guess that it’s the Amazon, the Nile or the Mississippi. In fact, some of Earth’s largest rivers are in the sky — and they can produce powerful storms, like the one now soaking California. Atmospheric rivers are long, narrow bands of moisture in the atmosphere that extend from the tropics to higher latitudes. These rivers in the sky can transport 15 times the volume of the Mississippi River. When that moisture reaches the coast and moves inland, it rises over the mountains, generating rain and snowfall and sometimes causing extreme flooding.