Asymmetry of the spectra (color coded) as a function of outflow velocity, v_max, and covering factor, f_c, for a fixed total column density of N_HI,total = 4/3 × 10¹⁹ cm^-2. As tracer of the asymmetry we chose to display the ratio of the integrated flux on the blue side (x ≥ 0) of the line L_blue over the integrated red flux L_red minus one. This implies that a value of 0 (in white) corresponds to a symmetric spectrum, whereas −1 (in dark red) corresponds to a spectrum in which all flux is redward of line center. The contour lines highlight values of (−0.75, −0.5, −0.25). Also shown are the f_{c, crit} boundary (Eq. (13)) and the more precise 4/3τ_cl(x_next) = 1 deviation. The relatively low values of f_{c, crit} imply that large covering factors as predicted by McCourt et al. (2016) lead to asymmetric spectra.