Atmospheric weighted mean temperature (T_m) is a key parameter in precipitable water vapour (PWV) detection using GPS technique. This paper develops a new voxel-based empirical T_m model, which takes into consideration the lapse rate and diurnal variations of T_m. The theoretical RMS error of PWV resulting from the new model is generally less than 0.8 m over the globe. Therefore, it can be used as an alternative T_m determination method in the real-time GPS-based water vapour detection system.