The gridded data sets are based on the archive of UK weather observations held at the Met Office. The density of the station network used varies through time, and for different climate variables — for example, for the temperature variables the number of stations rises from about 270 in 1914 to 600 in the mid-1990s, before falling to 450 in 2006.

Regression and interpolation are used to generate values on a regular grid from the irregular station network, taking into account factors such as latitude and longitude, altitude and terrain shape, coastal influence, and urban land use. This alleviates the impact of station openings and closures on homogeneity, but the impacts of a changing station network cannot be removed entirely, especially in areas of complex topography or sparse station coverage. The methods used to generate the monthly and annual grids are described in more detail in a paper published in the International Journal of Climatology, vol. 25 (2005), pages 1,041-1,054, which can be downloaded here: Generation of monthly gridded data sets for a range of climatic variables over the UK (PDF, 590 kB). The methods used to generate the daily grids are described in more detail in the report 'The generation of the daily gridded data sets of temperature and rainfall for the UK' (PDF, 2.27 MB).

The 5 × 5 km grids of the 1961-1990 baseline climate average available on these webpages are simply calculated by averaging or summing the 30 monthly or annual gridded data sets for each variable. They are not the same as the 1 × 1 km data sets presented on our UK Climate pages. The methods used to generate the 1 × 1 km data sets for 30-year average periods, which were used to normalise the station data prior to the generation of monthly and annual grids, are described in a paper published in the International Journal of Climatology, vol. 25 (2005), pages 1,023-1,039, which can be downloaded here: Development of a new set of long-term averages for the UK
(PDF, 1.08 MB)

To help users combine the 5 × 5 km baseline data sets with the UK Climate Projections, values of the 1961-1990 baseline climate average have also been generated for the 25 × 25 km grid boxes of the HadRM3 regional climate model and for administrative regions and river basins.

Each 25 × 25 km grid box value is an average of the 5 × 5 km grid cell values that fall within it. Averages have been calculated for each month, season and the year as a whole (17 data sets). For the days of frost and days of rain variables the seasonal and annual averages are the total of the individual monthly averages. For the remaining variables the seasonal and annual averages are the mean of the monthly averages (allowing for differences in month length). To facilitate combining the baseline data with the UKCP09 climate projections, the 25 km baseline averages for rainfall have been expressed in units of millimetres per day (rather than total millimetres, as for the 5 km data sets).

Each regional value is also an average of the 5 × 5 km grid cell values that fall within it. Monthly averages (12 values) have been calculated for each monthly variable and an annual average has been calculated for each annual variable. As with the 25 km data, the averages for rainfall have been expressed in units of millimetres per day.