Effect of varying the ISM model parameters on the H I line profiles (top panels) and on the N_HI(v)−T_B relation (bottom panels) for the l = 70° sight-line. In all cases, the light-cone aperture is α = 0.6°. Panels a and d show the effect of varying the warm-to-total gas fraction f_w, panels b and e show the effect of varying the mean H I density in the cone n_HI, while panels c and f show the effect of varying the intra-cloud turbulence σ_turb. The grey squares in the top panels show the observed H I line profile from the LAB survey. In the bottom panels, the two dashed lines show the analytic solutions, given by Eq. (3), for two isothermal layers of H I at the spin temperature of 8000 K or 80 K.