Global Air Temperature: Regridded Monthly and Annual Climatologies

(Version 2.02)

reinterpolated and documented by

Cort J. Willmott, Kenji Matsuura and David R. Legates
(with support from NASA's Seasonal to Interannual ESIP)

For additional information concerning this archive, please contact us at:

Center for Climatic Research
Department of Geography
University of Delaware
Newark, DE 19716
(302) 831-2294


Archive (Version 2.02) released July 1, 2001


Legates and Willmott's (1990) station records of monthly and annual mean air temperature (T) were used to produce this archive. The number of stations (and oceanic grid nodes) used was 24,941.


Station averages of air temperature were interpolated (see below) to a 0.5 degree by 0.5 degree of latitude/longitude grid, where the grid nodes are centered on 0.25 degree. The number of nearby stations which influence a grid-node estimate was increased to an average of 20, from an average of 7 in earlier applications. This resulted in smaller cross-validation errors (see below) and visually more realistic air-temperature fields. A more robust neighbor finding algorithm, based on spherical distance, also was developed and used.

Digital-elevation-model- or DEM-assisted interpolation of air temperature was employed (Willmott and Matsuura, 1995). Station air temperature was first "brought down" to sea level at an average environmental lapse rate (6.0 deg C/Km). Traditional interpolation then was performed on the adjusted-to-sea-level station air temperatures. It was accomplished with the spherical version of Shepard's algorithm, which employs an enhanced distance-weighting method (Shepard, 1968; Willmott et al., 1985). Then, the gridded sea-level air temperatures were brought up to the DEM-grid height, again, at an average environmental lapse rate. This version (2.02) of our average air-temperature archive differs from the previous DEM-assisted version (2.01) in that we employed an alternate DEM and reduced the lapse rate by 0.5 degrees to 6.0 deg C/Km.


To indicate (roughly) the spatial interpolation errors, station-by-station cross validation was employed (Willmott and Matsuura, 1995). One station was removed at a time, and air temperature was then interpolated to the removed station location from the surrounding nearby stations. The difference between the real station value and the interpolated value is a local estimate of interpolation error. After each station cross validation was made, the removed station was put back into the network. To reduce network biases on cross-validation results, absolute values of the errors at the stations were interpolated to the same spatial resolution as the air temperature field.


Average monthly and annual air temperature interpolated to a 0.5 by 0.5 degree grid resolution (centered on 0.25 degree). The format of each record is
Field Columns Variable Fortran Format
1 1 - 8 Longitude (decimal degrees)
2 9 - 16 Latitude (decimal degrees)
3-14 17 - 112 Monthly Air Temperature (deg C, Jan - Dec)
15 113 - 120 Mean Annual Air Temperature

Cross-validation errors associated with air temperatures interpolated to a 0.5 by 0.5 degree grid resolution. The format of each record is
Field Columns Variable Fortran Format
1 1 - 8 Longitude (decimal degrees)
2 9 - 16 Latitude (decimal degrees)
3-14 17 - 112 Cross-validation errors for Monthly Air Temperature (deg C , Jan - Dec)
15 113 - 120 Cross-validation errors for Mean Annual Air Temperature


Legates, D. R. and C. J. Willmott (1990) Mean Seasonal and Spatial Variability Global Surface Air Temperature. Theoretical and Applied Climatology , 41, 11-21.

Shepard, D. (1968) A two-dimensional Interpolation function for irregularly-spaced Data. Proceedings, 1968 ACM National Conference, 517-523.

Willmott, C. J., C. M. Rowe and W. D. Philpot (1985) Small-Scale Climate Maps: A Sensitivity Analysis of Some Common Assumptions Associated with Grid-point Interpolation and Contouring. American Cartographer, 12, 5-16.

Willmott, C. J. and K. Matsuura (1995) Smart Interpolation of Annually Averaged Air Temperature in the United States. Journal of Applied Meteorology, 34, 2577-2586.