ABSORPTION BY H2O AND H2O-N-2 MIXTURES AT 153 GHZ

New experimental data on and a theoretical analysis of the absorption coefficient at 153 GHz are presented for pure water vapor and water va por-nitrogen mixtures. This frequency is 30 GHz lower than the resonan t frequency of the nearest strong water line (183 GHz) and complements our previous measurements at 213 GHz. The pressure dependence is obse rved to be quadratic in the case of pure water vapor, while in the cas e of mixtures there are both linear and quadratic density components. By fitting our experimental data taken at several temperatures we have obtained the temperature dependence of the absorption. Our experiment al data are compared to several theoretical models with and without a continuum contribution, and we find that none of the models is in very good agreement with the data; in the case of pure water vapor, the co ntinuum contribution calculated using the recent theoretical absorptio n gives the best results. In general, the agreement between the data a nd the various models is less satisfactory than found previously in th e high-frequency wing. The anisotropy in the observed absorption diffe rs from that currently used in atmospheric models.