Evolution of the maximum velocity (black) and velocity at the spot boundary v_h(1.1 Mm) (red) of the flow field for all analysed active regions. If the maximum velocity is localised at the sunspot boundary, these values are equal to v_h(1.1 Mm) and are not visible in the diagram. The abscissa values are given in days with the loss of the penumbra being set as a common point in time.