The internal data storage in Aiolos consists of files of proprietary formats specialized for optimal read/write performance and are vital to the rapid calculation speed. The data files are of three types, aio-files for equidistant collections of time series data, fc-files for blocks of forecast data and cl-files which is a special case of aio-file containing climate or normal weather data for a number of weather parameters. The file extensions for these files are .aioX, .fcX and .clX where X denotes a version number as an integer. The files may contain multiple time series or columns and the version number determines the number of columns and also how to interpret each column within the file. The version number refers to the same arrangement of columns across the different file types, so that an aio2-file contains the same columns as an fc2-file.