Water up Above-anvil cirrus plumes are stratospheric cloud formations that form downwind from the tops of some very strong thunderstorms. Despite their common occurrence, an adequate physical model explaining many of their features and effects does not exist. O’Neill et al . show that the storm supercell that extends into the stratosphere functions like a physical barrier, deflecting wind streams there like a topographic obstacle and driving a hydraulic jump downstream at the tropopause (see the Perspective by Smith). This feature triggers the intense injection of water vapor deep into the stratosphere, leading to a substantial increase in stratospheric humidity. —HJS