Ionisation fractions of calcium (χ_Ca, thick solid, see Eq. (1)) and of hydrogen (dashed) as a function of column mass in the initial radiative equilibrium atmosphere. The height scale z₀ of the initial atmosphere is given as reference. The Ca ii-iii ionisation fraction rises from a minimum of 2 × 10^-4 at lgm_c = 0.0 (z ≈ 180 km) in the low photosphere to  ~ 5% at the base of the transition region. The grey lines represent χ_Ca in simulation snapshots at an interval of 10 s during the whole simulation sequence of 3600 s. The thin short-dashed lines are the 5% and 95% percentiles.