Often in the winter, you’ll hear that “atmospheric rivers” are causing big weather problems along the West Coast. But what are they?

Made visible by clouds, these ribbons of water vapor extend thousands of miles from the tropics to the western U.S. At 250 to 375 miles wide, they provide the fuel for massive rain and snowstorms that can cause flooding along the West Coast.

In general, atmospheric rivers pick up water vapor from the warm, moist air of tropical regions and then drop the water over land in cooler regions as rain or snow.