ScienceDaily: If you want to assign blame on an overcast day, then cast your eyes on the tropics. Water vapor originating from the Earth's tropics is transported to midlatitudes on long filaments of flowing air that intermittently travel across the world's oceans. When these airy tendrils make landfall, they can cause severe floods and other extreme weather events. Yet despite the importance of these "atmospheric rivers" for the global water and heat cycles, the mechanism behind their formation is still a mystery....