We report the results of a recent study to examine possible causal factors that could explain the observed differences in PMSE intensity in the northern and southern hemispheres. A variety of satellite data, models and ground-based radar observations in the two hemispheres during local summer have been examined in this attempt. We will show results of inter-hemispheric comparisons of temperatures, winds, and water vapor. The primary conclusions that can be drawn from these studies are that, while water vapor differences in the two hemispheres are difficult to interpret at near-mesopause altitudes, the inter-hemispheric temperature comparisons show clearly that the southern hemispheric mesopause is indeed a few degrees warmer. Moreover, southern summer mesospheric mean winds are considerably weaker than they are in the north. These results provide support for earlier speculations that were made to explain the observed inter-hemispheric difference in PMSE occurrence. ¿ 1999 American Geophysical Union