Line formation in a flare ribbon (orange cross in Fig. 6). Top row: profiles of vertically emergent intensity, expressed as brightness temperature T_b, as function of frequency from line center (in Doppler shift units) for Ca II 8542 Å (orange), Ca II K (green), Mg II k (blue), and Hα (red). Bottom left: line source functions for Hα (red dashed) and Ca II 8542 Å (orange dashed), expressed as excitation temperature, and the gas temperature (solid black) as a function of column mass. Bottom middle: photon destruction probabilities for each of the lines. Bottom right: height of optical depth unity (solid colored) and the vertical velocity (solid black dots indicate the grid points, upflow is positive) as a function of frequency from line center in Doppler shift units.