Slope of the opacity as derived from Eq. (25) for the following parameters: λ = 3 mm, β_s = 1.6, A₀ = 2/cm, λ₀ = 1 mm, ρ = 3.5 g cm⁻³, s_min = 0.5 μm, s_max = 100 m, Q_abs, o = 1, y = 0.75n², with (black) n = 8.5, (red) n = 3.4, and (blue) n = 1.14. In the range 3 <  p <  4, this order of colours reflects the vertical order of the corresponding lines, bottom to top. The dashed black and solid grey lines follow Eqs. (30) and (31), respectively.