This is the readme file for the self-calibrating Palmer Drought Severity Index (scPDSI) data for the global land surface (excluding Antarctica), spanning the period 1901-2015 with monthly resolution. The spatial resolution is 0.5 x 0.5 degrees. The information below relates to global scPDSI files. The format of the data is a self-describing netCDF file, with information about grid coordinates and time coordinates provided within the file. The grid size is 720 longitudes by 360 latitudes and the coordinates of the CENTRE of the grid cells end in 0.25 or 0.75 (i.e. the EDGES of the grid cells lie on either 0.00 or 0.50 degrees). The grid cells run from 89.75N, 179.75W (centre) to 89.75S, 179.75E (centre). The data run from January 1901 to December 2015, driven by a preliminary version of the CRU TS3.24 monthly climate dataset. CAVEATS and NOTES: 1. Values during 1901 may be unreliable because scPDSI was initialised in 1901 and no CRU TS climate data were available for 1900. 2. In CRU TS, grid cell climate values are relaxed towards the 1961-1990 mean climatology when there are no or few weather station observations within the Correlation Decay Distance (CDD). The CDD is 1200 km for monthly temperature but only 450 km for monthly precipitation. This means that the scPDSI of grid cells may be closer to zero (i.e. normal conditions), underestimating the occurrence of anomalously dry or wet spells, in regions and times with poor data coverage. 3. The underestimation of anomalously dry or wet spells (see note 2) will affect the early decades of the twentieth century the most, but it can also occur during recent years because not all observations are easily available in near-real time for updating CRU TS. A rough indication of the scale of this limitation is given by the number of precipitation observations incorporated into the preliminary version of CRU TS3.24. For the 1990s and 2000s, approximately 5,000 observations were available per month. For 2011-2015, the mean number of precipitation obserations per month was 2200, 2166, 2064, 1725 and 1676, respectively. 4. The scPDSI data values in 2015 are based on a preliminary version of CRU TS3.24. When using these data, please cite: van der Schrier G, Barichivich J, Briffa KR and Jones PD (2013) A scPDSI-based global data set of dry and wet spells for 1901-2009. J. Geophys. Res. Atmos. 118, 4025-4048 (10.1002/jgrd.50355). and Osborn TJ, Barichivich J, Harris I, van der Schrier G and Jones PD (2016) Monitoring global drought using the self-calibrating Palmer Drought Severity Index [in "State of the Climate in 2015"]. Bulletin of the American Meteorological Society 97, S32-S36. The publication which introduced the self-calibrating PDSI, and gives a large amount of detailed information about its calculation, is: Wells, N., Goddard, S. and Hayes, M. J. (2004) A Self-Calibrating Palmer Drought Severity Index, J. Climate 17, 2335-2351 Support for the 2015 update was provided by NERC through the SMURPHS project (NE/N006348/1) and for the 2015 update of CRU TS3.24 by the NERC National Centre for Atmospheric Science (NCAS).