On the basis of rocket, nightglow and radiophysical measurements at heights of 25-110 km in middle latitudes the seasonal variations of the long-term temperature trends at various altitude levels are determined. At heights of 25-87 km the trends are mainly negative within all year. Only in winter period in middle and upper stratosphere the trend within several months becomes positive. At heights of 92-93 km the trend is very close to zero throughout the year. In summer period the altitude of zero trend extends downward to heights of 82-85 km, where the noctilucent clouds are usually observed. At heights of 95-110 km the trends are positive all year round. Mean annual height distribution of the temperature trend shows long-term cooling at atmospheric heights between 25 and 90 km, leading to a systematic subsidence of the upper atmospheric layers.