Comparison of the dry (top) and wet (bottom) opacity coefficients as defined in Eqs. (6), (A.29) and (A.27) for the frequency range of GREAT channels L1 (left) and L2 (right), calculated from the atmospheric models AM, ATRAN and MOLIERE (see text), showing the opacity for a pressure altitude of p_amb = 133   Torr and a pwv = 10   μm. The largest discrepancy is in the quasi-continuous dry-atmospheric opacity due to collision-induced absorption (CIA), which is completely neglected in ATRAN and differs by about a factor of 2 between MOLIERE and AM.