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The free atmosphere is an essential part of the climate system, and the processes in the free atmosphere are strongly interrelated with the processes in other climate system components. Temperature in the free atmosphere is better studied than other parameters defining the upper-air climate, but nevertheless there exist numerous problems in the upper-air temperature trends. This paper gives the overview of the current status of radiosonde based empirical studies of the upper-air temperature climate changes. Problems such as data availability, data quality and completeness, inhomogeneity detection in the upper-air temperature time series, temperature trend estimates, overall patterns of upper-air trends are discussed in the paper. Special attention is given to the problem of whether the reanalysis outputs can reproduce trends over long periods in the upper-air temperature series. [ABSTRACT FROM AUTHOR]