This past December, a mind-boggling 18 feet of snowfall fell in the California Sierra Nevada Mountains! How does so much snow fall in one place in such a short period of time? One of the primary phenomena responsible for such extreme rain and snowfall, particularly in regions like the western U.S., is the atmospheric river. Like their terrestrial counterparts, atmospheric rivers carry tremendous amounts of water over thousands of miles. These aerial versions, however, often bring both severe disruption and great benefit through the heavy rain and mountain snows that they produce.