EISCAT VHF (224 MHz) radar observations of the vertical velocity under Polar Mesosphere Summer Echo (PMSE) conditions were performed with 2 s time and 300 m height resolution and velocity uncertainties better than 0.1 ms-1. We show that upward velocities that persist for ~10 min or more lead to greater variance in the motion field and tend to make the PMSE weaker. As smaller radar cross section also correlates with larger velocity uncertainty, any vertical mean velocity composed of threshold radar data tends to have a downward (negative) bias. We argue that this phenomenon can contribute to the discrepancy between the mean upwelling accounting for the summer mesopause thermal structure and radar observations of the mean vertical velocity. ¿ American Geophysical Union 1995