Effective sky brightness, , (top panel) in a representative frequency window near a water line and several dry-atmospheric lines originating in the higher, warmer atmosphere, calculated with the AM model at a pressure altitude of 133 Torr and an ambient temperature of 200 K, for two values of pwv, 3 (red) and 30 μm (blue). The lower two panels show the dry atmospheric opacity (i.e., the c-coefficient; middle) and the wet atmospheric opacity at 10 μm pwv, i.e., b ×    10   μm; bottom), similar to Fig. 1 to identify the spectral features causing the excess effective brightness temperature.