Data were taken from public-facing archives as well as by submission from co-authors. These came either as precalculated indices or as daily precipitation, maximum and minimum temperatures. Where necessary, the indices were calculated from the daily values using the Climpact2 code, or reformatted to standard outputs. We perform some quality control checks on the indices to identify erroneous values and remove these stations from further use.

In order to calculate the grid-box values, we use the Angular Distance Weighting scheme, which uses a search radius from the grid-box centre to identify stations that could contribute. This search radius is defined by the correlation structure of the station timeseries (a decorrelation length scale) and is determined within latitude bands. If at least three stations within this search radius have data values for a given year/month then the grid-box value is calculated.