Absorption in the 220 GHz atmospheric window

The atmospheric absorption spectrum in the 220 GHz window region is controlled primarily by the water vapor lines centered at 183.3 and 323.8 GHz. There are however, numerous other lines in the submillimeter part of the spectrum which contribute significantly to the losses. At the longer millimeter wavelengths, it has been the practice to lump the contributions of the submillimeter lines using the Van Vleck low frequency approximations. This procedure is not permissible for the 220 GHz window region because of the promixity of some of the submillimeter lines. Experimental measurements substantiate this observation. The present study introduces an alternative approximation for calculating the residual effect of submillimeter lines. The results, larger by a factor of 2, are in closer agreement with experimental observations.