The occurrence of polar mesospheric summer echoes (PMSE) has been closely associated with the seasonal cooling of the high-latitude summer mesopause region wherein the echoes are embedded. Indeed, a threshold temperature has been suggested, with PMSE appearing as mesosphere temperatures fall below some critical value. A careful comparison between seasonally varying mean mesopause temperatures and PMSE occurrence statistics, however, suggests that the probability curve of PMSE occurrence is displaced from the seasonal curve of low summer mesopause temperatures by 1 to 2 weeks, with the temperature decrease leading the PMSE occurrence curve in springtime and the temperature increase leading the PMSE dropout in the fall. A similar displacement can also be seen in the occurrence statistics for polar mesospheric clouds (PMC), a closely related phenomenon observed by satellites. This lack of a direct correlation between mesopause temperature and PMSE occurrence suggests that PMSE occurrence is governed by more than just low mesospheric temperatures. The most likely possibility is that summertime mesospheric water vapor maximizes somewhat later than the minimum in mesospheric temperature, providing a more hospitable environment for PMSE generation. This possibility is supported by a model of the supersaturation region <Garcia, 1989>, which shows a comparable lag between the seasonal mesospheric temperature minimum and the maximum in water vapor mixing ratio.¿ 1997 American Geophysical Union