Although their prime purpose is for weather analysis and forecasting, radiosonde observations have been used for monitoring climate variability and trends for over thirty years. Over the past decade, the availability of satellite datasets for climate monitoring and heightened interest in climate change have focused attention on the quality of the historical observations. In the past couple of years, several research groups and national and international assessments have advanced our understanding of both the value and the limitations of these historical observations for climate studies.

We will present a brief review of recent studies of upper-air temperature variability and trends based on in situ and satellite observations, with emphasis on data quality and continuity issues. Key areas of observational uncertainty will be highlighted. We will present a summary of recent recommendations for improving in situ upper-air observations for climate applications. The need for reference-quality observations will be motivated in terms of outstanding questions in climate research and monitoring.