Variations in PMSE (Polar Mesosphere Summer Echo) occurrence with 4--6 day periods were observed by the ESRAD radar during the summer of 1997. These are compared with temperature fluctuations in 5-day planetary waves extracted from the UKMO assimilated global data analyses. At the beginning and end of the PMSE season, the PMSE variations are closely anti correlated with temperature variations associated with 5-day planetary waves at the 1 mb level. The planetary wave amplitudes expected at the mesopause are 1--2 K. This is found, by comparison with the seasonal decay of PMSE as the temperature rises at the end of the summer, to be sufficient to explain the observed 5--10% amplitude fluctuations in PMSE daily occurrence. ¿ 1998 American Geophysical Union