Top: contribution functions for HO transition at 557 GHz normalized to the maximum value to clearly indicate the peak altitude at each velocity/frequency point. These were calculated for an optically thick case of water column density 5 × 10¹⁶ cm⁻². The terminal velocity in the input atmosphere for these calculations is set to −0.75 km s⁻¹ and is reached at about 3.5 km from surface. The other color lines correspond to different velocity offsets for comparison, and correspond to the vertical lines in the bottom panel. The rest of the velocities are shown in gray lines. Bottom: synthetic spectra line are shown for reference. The vertical lines depict locations in velocity space corresponding to the contributionfunctions shown in the top panel.