ABSOLUTE MEASUREMENTS OF THE ATMOSPHERIC TRANSPARENCY AT SHORT MILLIMETER WAVELENGTHS

Spectral measurements of the atmospheric opacity in the 3-13 cm-1 region have been made using a phase-modulated Michelson interferometer. The data are calibrated using a black body source to give absolute measurements. An independent measurement of the total atmospheric water vapour concentration is obtained from a near-infrared hygrometer which is itself calibrated against simultaneous radio sonde data. A theoretical atmospheric transmission model is used to compare the two sets of data. We have found a large discrepancy between our theory and the experimental data amounting to more absorption than predicted. This anomalous absorption shows no spectral features and scales with water vapour concentration. There is no indication of any water vapour dimer lines or of any other minor species features except those due to ozone. © 1979.