This database contains a collection of historical upper-air intercomparisons and error assessments of radiosondes reaching back to the 1930s. Upper-air data form the backbone of weather analysis and reanalysis products, particularly in the pre-satellite era. However, they are particularly prone to errors and uncertainties, especially data from the early days of aerology. Information that allows to better characterize the errors of radiosonde data is thus important. We publish the digitzed data, that includes radiosondes ascents (e.g. pressure, temperature, humiditiy, wind), systematic differences between radiosondes and error assessments. In addition we make available the full information on the intercomparison campaigns, such as images, literature, and other metadata that may be relevant and can be used to inform homogenization approaches or reanalysis production.Either the full database can be downloaded, containing the digitized data and all metadata, or the digitized files of the intercomparison campaign can be downloaded as individual files.