Interest in the precise measurement of water vapour in the upper-air is growing along with the consciousness of climate change. Because better knowledge of high altitude humidity levels improves the accuracy of climate models and weather forecasts, the Global Climate Observing System (GCOS) is putting more effort into the quality of these measurements. The GCOS established the Reference Upper-Air Network (GRUAN) and set the requirements for its measurements accuracy and traceability to the International System of Units (SI). To fulfil these requirements, improved radiosondes and methods to calibrate them are being developed. This paper presents a new calibration system for radiosondes based on mixing air flows from two independent dew-point generators. The system operates in the air temperature range down to??-80 °C. Our results show that the new calibration set-up is able to provide calibrations for radionsondes at the uncertainty level of 2% in terms of the water vapour mixing ratio as required by GRUAN. A complete calibration covering the whole temperature and humidity range lasts less than three days. Additionally, the apparatus provides an option to characterize the behaviour of a radiosonde in changing temperatures and water vapour concentrations.