Temperature data derived from the cryogenic limb array etalon spectrometer (CLAES) on board the Upper Atmosphere Research Satellite (UARS) are used to investigate planetary-scale temperature disturbances near the equatorial stratopause for January 1992 to May 1993. The disturbances are characterized by vertically stacked temperature extrema of alternating sign with a vertical scale of about 10 km and have a localized and stationary nature in the longitudinal direction with persistence of about 1 week. These are nearly identical to so-called pancake structures first identified by Hitchman et al. <1987> using data from the limb infrared monitor of the stratosphere (LIMS). Their analysis suggested that pancake structures are consistent with those predicted by inertial instability theory and that their appearance is synchronized with strong planetary waves in the winter midlatitude, though the LIMS observations were made only during the northern winter. Using the CLAES data for about 14 months, this study shows that pancake structures occur not only during the northern winter but also during the southern winter. In addition, it is found that an equatorial pancake structure has its counterpart with reversed phase in the winter midlatitude, suggesting clear evidence for inertial instability. Further analyses on the basis of Ertel's potential vorticity show that inertially unstable regions intrude locally far into the winter hemisphere around pancake structures that appear when planetary wave breaking is going on. This implies a mechanism of localized inertial instability and resulting pancake structures caused by midlatitude planetary waves in the winter hemisphere, as some numerical studies have inferred. ¿ 1998 American Geophysical Union