The text file "Air temperature.txt" contains hourly air temperature data in degrees Fahrenheit and associated data-source flags from January 1, 1948, to September 30, 2019. The primary data for water year 2019 (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, 2019) and they were processed following the guidelines documented in Over and others (2010). The processed data were appended to ARGN18.WDM (Bera, 2019) and renamed as ARGN19.WDM. Missing and apparently erroneous data values were replaced with adjusted values from nearby weather stations used as “backup”. The Illinois Climate Network (Water and Atmospheric Resources Monitoring Program, 2019) station at St. Charles, Illinois, was 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, 2019, Meteorological data, accessed on November 6, 2019, at http://www.atmos.anl.gov/ANLMET/. Bera, M., 2019, Meteorological Database, Argonne National Laboratory, Illinois, January 1, 1948 - September 30, 2018: U.S. Geological Survey data release, ​https://doi.org/10.5066/P9H8P0F7. 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, 2019, Illinois Climate Network: Champaign, Ill., Illinois State Water Survey, accessed on November 6, 2019, at http://dx.doi.org/10.13012/J8MW2F2Q.