The HadISD station data were produced by the Met Office Hadley Centre. Individual station data within the ISD were selected to be merged to form composite stations using a hierarchical scoring system. Then stations were selected on the basis of their length of record and reporting frequency. A final set of 6103 stations were passed through a suite of automated quality control tests designed to remove bad data whilst keeping the extremes. None of the ISD flags were used in this process. The QC tests focussed on the temperature, dewpoint temperature and sea-level pressure variables, although some were applied to the wind speed and direction and cloud data. The data files also contain other variables which were pulled through from the raw ISD record, but have had no QC applied (e.g. cloud base and precipitation depth). Some final filtering was performed to select those stations which in our opinion are most useful for climate studies. Note: These data have not yet been homogenised and so trend fitting should be undertaken with caution. The homogeneity has been assessed and results are available from the Met Office Hadley Centre HadISD website (http://www.metoffice.gov.uk/hadobs/hadisd/). For further details see: Dunn, R. J. H., et al. (2012), HadISD: A Quality Controlled global synoptic report database for selected variables at long-term stations from 1973-2011, Climate of the Past and Dunn, R. J. H., et al. (2014), Pairwise Homogeneity Assessment of HadISD, Climate of the Past, 10, 1501-1522 (see Docs for links to publications).