Mean vertical velocities are of great interest in meteorology and vertically directed VHF radar have been used to measure directly. Recently it has been pointed out that in a gravity wave with upward energy propagation the radar reflectivity and the vertical velocity are negatively correlated, giving rise to a downward bias for the observed . In the present paper, it is shown that in regions of Kelvin-Helmholtz instability (KHI) the correlation between refractivity-surface tilting angles and the (apparent) vertical velocity observed with a vertically pointing VHF radar can lead to a bias with magnitudes of several tens of centimeters per second. This “KHI bias” is upward in the shear zone above the horizontal wind speed maximum of a jet stream and downward below. Theoretical vertical profiles of the predicted KHI bias exhibit considerable similarity with profiles observed during jet stream events by other authors, suggesting that a major part of the observed might be attributable to the presumed KHI bias.