In the last 10 years a total of 89 falling spheres (FS) have been launched at high latitudes (~70 ¿N) in the summer season between late April and late September. From this experimental technique, densities and temperatures in the mesosphere and upper stratosphere (~95--35 km) are deduced which represent nearly the entire data set regarding the thermal structure in the high-latitude summer mesosphere where optical methods have problems to give reliable results because of the large solar photon background. Some of the launches took place at times in the season where no measurements have been performed before. The seasonal variation of the mean temperatures and densities derived from the FS measurements deviates significantly from the latest empirical models, in particular, in the upper mesosphere during summer. For example, at the summer mesopause (88 km) the FS temperatures are lower by more than 10 K compared to CIRA-1986 in the time period from the beginning of June until the end of August. The thermal structure in the upper mesosphere is rather persistent throughout the core summer months and changes rapidly in the winter-summer transition at mid-May, and vice versa at mid-August. For example, at typical noctilucent cloud altitudes (82 km) the mean temperature is in the range 153¿3 K from the beginning of June until mid-August but changes by, typically, 5--10 K per week before and after this period. A comparison of the FS temperatures with the occurrence probability of noctilucent clouds and polar mesosphere summer echoes suggests that the thermal structure is the main controlling factor for these layers, whereas other ingredients required to form aerosol particles, such as water vapor or condensation nuclei, are of secondary importance. ¿ 1999 American Geophysical Union