A water-vapor-weighted vertically averaged mean temperature of the atmosphere, Tm, is a key parameter in the retrieval of total water content from the measurements of Global Positioning System signal delays. The accuracy of precipitable water estimates is proportional to the accuracy of Tm. The geographic and seasonal variability of Tm based on 23 years of radiosonde soundings at 53 globally distributed stations is presented. Several methods for estimating Tm were evaluated by comparing the estimates against the actual Tm values. Site-specific climatology or site-specific linear regression was superior to the geographically and seasonally invariant regression relationship typically used to estimate Tm. Relative errors at most stations were less than 2%, which corresponds to absolute errors of precipitable water of 0.1--0.5 mm. The station-specific linear regression was superior to climatological means as an estimator except in the tropics, where correlations between Tm and Tsfc were not high. Also, a physical model was developed to indicate the relationship between Tm and other commonly used atmospheric variables.¿ 1997 American Geophysical Union