Comparison of ATLAS 12 (full line) and FASTWIND model atmospheres for an effective temperature of 33 000 K, surface gravity 4.0, microturbulent velocity 5 km s⁻¹, and solar metallicity. Left panel: temperature structure. Right panel: electron density structure as a function of Rosseland optical depth, τ_ross. The two FASTWIND models were computed for the parameters stellar radius R = 6.58R_⊙, wind terminal velocity υ_∞ = 1200 km s⁻¹, /β-velocity-field parameter β =1.0, and mass-loss rates log Ṁ (M_⊙yr⁻¹) = −9.5 (weak wind, dotted line) and log Ṁ (M_⊙yr⁻1) = −7.4l (wind mass-loss rate according to Vink et al. 2000, dashed line). Formation depths for the Balmer Ηα and Fly lines are indicated.