Velocity as a function of depth (solid line) for the metal-rich star ROA523 (top panel) and the three metal-poor stars (lower panels). A positive velocity implies outflow. No velocity fields were required for the two metal-rich stars WFI321293 and WFI140419, which are therefore not shown in this plot. The velocities were derived from the best match with the Ca K and Hα lines and are compared to the corresponding escape velocity (dotted line).