The development of annually-resolved estimates of summer temperature patterns over the mid-to-high latitudes of the Northern Hemisphere land masses is described. The estimates were derived from a calibration of an extensive network of tree-ring density chronologies, with data extending back as far as 1400 (though with greatly reduced spatial coverage prior to 1600). The chronology data were first gridded onto a 5° longitude by 5° latitude grid, followed by a local calibration and verification of each grid box series against observed April-to-September mean temperatures. It was demonstrated that calibration by simple linear regression of individual grid boxes in isolation (i.e., without using any non-local proxy or temperature information) yields a spatial reconstruction with suppressed variance when averaged over large areas. The local calibration was adapted, therefore, to ensure that the magnitude of sub-continental, regional-scale variations is realistically maintained, while still achieving significant skill at the grid-box scale. A second version of the reconstructed grids was also generated, by incorporating additional variations on century time scales that may have been removed by the original standardisation of the tree-ring density chronologies. The spatial extent of both versions (with and without the additional low-frequency variability) was extended using principal component regression, for all locations where a verification correlation of 0.4 or above was achieved. These reconstructions of summer temperature have significant skill, with an overall space-time correlation of around 0.7 for both the calibration and verification against observed temperatures
