Solar heating of the atmosphere is responsible for most of the diurnal oscillations in the neutral wind velocities and temperatures of the mesosphere and lower thermosphere. When these oscillations are global-scale waves, they are called atmospheric tides. Excited in the lower atmosphere, tides can propagate up into the upper atmosphere where they can break and deposit considerable energy and momentum into the mean flow. Ground-based estimates of the diurnal winds over Christmas Island have been made using a narrow-beam meteor radar system. Previously, data collected using the three-beam antenna configuration of the meteor radar were processed under the assumption that the vertical wind component was at least 2 orders of magnitude smaller than the horizontal component for all temporal scales of motion. The addition of two oblique beams to the radar configuration in late 1993 made it possible to estimate the horizontal wind field without applying the negligible vertical wind assumption, by using a coplanar analysis technique. Not only did the diurnal fits of the horizontal coplanar winds agree better with the results of the collocated medium-frequency radar and model predictions, but also the Christmas Island meteor radar appears to be measuring a significant vertical velocity. This velocity has a diurnal amplitude of 10--15 m s-1 and maximizes at midnight across all heights. Under the assumption that this strong vertical motion is produced by geophysical phenomena, two hypotheses to explain this velocity are presented: the vertical motion associated with gravity wave breaking and the influence of strong electric fields on the ionized meteor trails. ¿ 1999 American Geophysical Union