The text file "Solar radiation.txt" contains hourly solar radiation data in Langleys and associated data-source flags from January 1, 1948, to September 30, 2021. The primary data for water year 2021 (a water year is the 12-month period, October 1 through September 30, designated by the calendar year in which it ends) were downloaded from the Argonne National Laboratory (ANL) (Argonne National Laboratory, 2022) and processed following the guidelines documented in Over and others (2010). The processed data were appended to ARGN20.WDM (Bera and Over, 2021) and renamed as ARGN21.WDM. Missing and apparently erroneous data values were replaced with adjusted values from nearby weather stations used as "backup." Temporal variations in the statistical properties of the data resulting from changes in measurement and data storage methodologies were adjusted to match the statistical properties resulting from the data collection procedures that have been in place since January 1, 1989 (Over and others, 2010). The adjustments were computed based on the regressions between the primary data series from ANL and the backup series using data obtained during common periods. The statistical properties of the regressions were used to assign estimated standard errors to values that were adjusted or filled from other series. The Illinois Climate Network (Water and Atmospheric Resources Monitoring Program, 2022) stations at St. Charles and DeKalb, Illinois, were used as "backup." Each data source flag is of the form "xyz", which allows the user to determine its source and the methods used to process the data (Over and others, 2010). References Cited: Argonne National Laboratory, 2022, Meteorological data, accessed on January 17, 2022, at https://www.atmos.anl.gov/ANLMET/numeric/. Bera, M., 2021, Meteorological Database, Argonne National Laboratory, Illinois, January 1, 1948 - September 30, 2020: U.S. Geological Survey data release, https://doi.org/10.5066/P9GP8COF. Over, T.M., Price, T.H., and Ishii, A.L., 2010, Development and analysis of a meteorological database, Argonne National Laboratory, Illinois: U.S. Geological Survey Open-File Report 2010-1220, 67 p., http://pubs.usgs.gov/of/2010/1220/. Water and Atmospheric Resources Monitoring Program. Illinois Climate Network, 2022. Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495. Data accessed on January 4, 2022, at http://dx.doi.org/10.13012/J8MW2F2Q.