Rainfall averages for UK over the 30-year 1961-1990, interpolated to a 1 km grid. This 30-year period is a Primary Reference Period recommended by World Meteorological Organisation.
Publication date: 1993-12-15
Rainfall Averages for 1961/90 have been calculated from monthly values assembled from the Met Office Climatological Data Archive for stations open at some time during 1961-90. The monthly totals were compared automatically with nearby stations. Seriously discrepant values were inspected manually were either corrected or discarded. Missing and discarded observations were replaced by estimates calculated by regression against observations from up to 6 nearby long period stations. Average Annual Rainfall values were analysed by determining a fitting equation for each 10km square. The natural logarithm of the rainfall was related to 32 variables which included: relative geographical position (using a quadric representation in National Grid easting and northing), the ratio of land to sea within 3.5km, the local terrain height and a further 21heights at distances of 1km, 3.5km and 12km. The fitting method used an iterative regression on the residuals specially devised to avoid over-fitting. Rainfall values were weighted according to the number of years each station was open during 1961-90 and their distance from the square. The number of station values used for each square depended on the data density and ranged from about 20 within 75km in sparse data areas to about 70 within 15km where data were most plentiful. Final rainfall estimates were obtained bi-linearly interpolating the values calculated from the fitting functions for each of the 4 nearest squares. The resulting Standard Error of these values, worked out from those station averages based on more than 15 years of data, ranges from better than 2% in topographically simple areas to worse than 5% in some upland areas. Monthly values have been obtained by generating the percentage of annual average on a grid with a 5km spacing. These values were obtained as a weighted average of nearby station values where the weights depended on the distance of the station and the period for which data were available.
For further information see 
 Spackman, E. 1993. Calculation and Mapping of Rainfall Averages for 1961-90, University of Salford, Manchester, 15th Dec.