A four-panel Eddington–Barbier line formation diagram for the Ca II K from the FAL-C model atmosphere in CRD. The computations are performed with three different cases: without microturbulence (ξ(z) = 0 km s⁻¹; blue line), microturbulence (ξ(z) = 5 km s⁻¹; red line), and with a deeper temperature rise and microturbulence (green line). Panel a: the total opacity as function of wavelength at the formation height of the peaks; panel b: the optical depth as function of height at the wavelength of at K_2V (dashed) and K₃ (solid); panel c: the vertically emergent intensity for the K line, and panel d: the gas temperature as a function of height (solid and dashed black) with the line source function (solid colored). The cross marks the position of K_1V in panel c and the dot indicates the position of the K_2V in panel a and K_2V/K₃ in panel c.