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[1] Analysis of global meteorological assimilations between mid-July and late-August 2000 shows strong 5-day planetary waves in the middle atmosphere. Observations of temperature, zonal wind, noctilucent clouds and polar mesosphere summer echoes (PMSE) near Kiruna, Sweden, all at heights between 80 and 95 km, show variations correlating with the passage of the 5-day waves. Temperature variations correlated with the 5-day wave reach 15 K peak-to-peak and correspond to modulation of PMSE occurrence by up to 50%. These observations appear to be the first experimental evidence of amplification of 5-day waves at the summer mesosphere which was predicted theoretically in 1976. A close linear relation is found between mean daily temperature and mean daily occurrence of PMSE. This can be explained if temperature is the primary factor controlling PMSE occurrence and time and height variations within each day between 80 and 90 km altitude reach 30 – 50 K.