The High Resolution Doppler Interferometer (HRDI) measures winds in both the stratosphere and mesosphere. The tropical winds in the stratosphere undergo a slow two year variation called the quasibianunual oscillation. This oscillation controls mixing throughout the stratosphere and HRDI has given us much detail on wind changes associated with this oscillation. The animation indicates the line of zero wind speed in the zonal tropical winds, the height at which the winds change from eastward to westward. Note: Tropical Winds in the Stratosphere as measured by HRDI from 1991 to 1996. Animator: Greg Shirah (NASA/GSFC). Scientist: Mark Schoeberl (NASA/GSFC). Platforms/Sensors/Data Sets: UARS/HRDI.