You may have heard of the Pineapple Express – not the 2008 buddy comedy about marijuana, hit men and corrupt cops – but the long, narrow “river in the sky” that brings tropical moisture from Hawaii to the West Coast.

Atmospheric rivers, which have been studied for nearly two decades, are flowing areas of water vapor in the upper atmosphere driven by areas of low pressure over an ocean. They are the source of most of the West Coast’s heaviest rains and floods.