A simple model for the radiometric determination of tropospheric transmittance is based on an isothermal troposphere. In this model the key parameter is the weighted mean tropospheric temperature Tm, which characterizes the radiation and temperature properties of the troposphere. Statistical approaches in modeling this parameter are presented here by using ground temperature, ground relative humidity, and radiometer data. In order to determine the statistical coefficients for Tm modeling and the parameters used in the transmittance retrieval algorithm, radiosonde data were used in a millimeter-wave propagation model for a site in the Swiss central plane and an Alpine site. Various observing geometries at different millimeter-wave frequencies were considered. A determination of Tm from ground temperature was achieved with a rms error between 4--5 K for the low-altitude site and 3--4 K for the high-altitude site. By incorporating relative humidity or radiometer data, an improvement of up to 25% relative to these values results, depending on frequency and site. The zenith transmittance estimations for the low-altitude site with our best model have a rms error of 0.5% at 38 GHz, 1% at 94, 110, and 142 GHz, 1.5% at 115 GHz, 2% at 204 GHz, and 3.5% at 279 GHz, whereas for the high-altitude site all rms errors are below 1%. The inclusion of radiometric information at 20 and 31 GHz did not provide any additional improvement, which was confirmed by actual measurements at 142 GHz. ¿ 1998 American Geophysical Union