The GSOD or Global Surface Summary of the Day (GSOD) data provided by the US National Climatic Data Center (NCDC).
For more information visit: GSODR package
Users of these data should take into account the following (from the NCDC website):
"The following data and products may have conditions placed on their international commercial use. They can be used within the U.S. or for non-commercial international activities without restriction. The non-U.S. data cannot be redistributed for commercial purposes. Re-distribution of these data by others must provide this same notification." WMO Resolution 40. NOAA Policy