Layers of stratospheric wind move in varying, but predictable, directions. Ground engineers shift the balloon up or down through layers to steer it north, south, east, or west. To lower the balloon, they trigger an onboard pump to draw in air, which is seven times denser than helium. To raise it, they release air. Using algorithms based on National Oceanic and Atmospheric Administration wind-prediction data, they drive the balloon into the current that takes it where needed.