Climatic Research Unit (CRU) TS (time-series) datasets are month-by-month variations in climate over the last century or so. These are calculated on high-resolution (0.5x0.5 degree) grids, which are based on an archive of monthly average daily maximum and minimum temperatures provided by more than 4000 weather stations distributed around the world. They allow variations in climate to be studied. CRU TS 3.21 variables are cloud cover, diurnal temperature range, frost day frequency, PET, precipitation, daily mean temperature, monthly average daily maximum and minimum temperature, vapour pressure and wet day frequency for the period Jan. 1901 - Dec. 2012. In addition to updating the dataset with 2012 data, the v3.21 release corrects two errors in the v3.20 dataset.
University of East Anglia Climatic Research Unit (CRU). [Phil Jones, Ian Harris]. CRU TS3.21: Climatic Research Unit (CRU) Time-Series (TS) Version 3.21 of High Resolution Gridded Data of Month-by-month Variation in Climate (Jan. 1901 - Dec. 2012), [Internet]. NCAS British Atmospheric Data Centre.