This paper considers the use of upper-air data from radiosondes in long-term climate studies. The accuracy and precision of radiosonde humidity measurements, including temperature and pressure measurements used in calculating them, and their effects on the precision of reported and derived variables are estimated. Focusing on the U.S. radiosonde system, we outline the history of changes in instruments and reporting practices and attempt to assess the implications of such changes for studies of temporal variations in lower-tropospheric water vapor. Changes in biases in the data are highlighted, as these can lead to misinterpretation of climate change. We conclude that the upper-air data record for the United States is not homogeneous, especially before 1973. Because of problems with the humidity data in cold, dry conditions, the water vapor climatology in the upper troposphere, nominally above the 500-mb level, is not well known.

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