The purpose of this International Standard is to provide a test method for the calibration of radiosonde humidity sensors with varying temperature and relative humidity using laboratory setups. The main scope of this International Standard is as follows: a) Technical requirements for laboratory setups needed for the calibration of radiosonde humidity sensors. b) Test procedure for calibrating radiosonde humidity sensors with varying temperature and relative humidity. c) Uncertainty evaluation method for the calibration of radiosonde humidity sensors. To be more specific, in part (a), the requirements for an absolute humidity generator, a test cell, and a precision hygrometer as essential laboratory setups are proposed. In part (b), the test procedure includes the installation of radiosondes in the test cell, the operation of laboratory setups, and the comparison between the reference relative humidity and the relative humidity measurement by radiosondes. In part (c), the method to evaluate uncertainties related with the reference relative humidity and the radiosonde humidity sensor will be proposed.