Location of the homopause, where the diffusion coefficients caused by eddy mixing and gas-kinetic motions are equal (K=D_i). The colours show the hydrogen nuclei density, n_〈H〉, as a function of radius r and relative height z/r. The coloured contour lines show where the homopause is situated when K is calculated with different values of log₁₀ α_mix. The dashed lines show where the vertical optical extinction A_V,ver equals 100 (black), ten (dark grey) and one (light grey). The dust opacities correspond to our un-mixed model described in Sect. 3. The uneven A_V contours are caused by the uneven spatial distribution of the ices, which are taken into account when calculating the opacities.