Map of the ratio between H_τ=1, obtained from radiative transfer, tracing the upper emission surface and H_sim, representing the simulation scale height. The ratio H_τ=1/H_sim ≈ 2–3 across the disc. We consider H_τ=1 ≈ 2.5H_sim a conservative estimate of the location of the upper emission surface – ‘conservative’ since the higher in the disc atmosphere, the larger the vertical velocity.