Thermodynamic structures and Li 670.8 nm line formation properties in two model atmospheres of red giant stars (T_eff = 4480 K, log g = 2.5), at [ M / H ] = 0.0 (left) and [ M / H ] = −2.0 (right). From top to bottom: (1) temperature profiles in the 3D hydrodynamical (probability density, grayscale), average ⟨3D⟩ (dashed red line), and 1D LHD (solid red line) model atmospheres; (2) horizontal temperature fluctuations in the 3D model (ΔT_rms, solid black line) and differences between the temperature profiles of the ⟨3D⟩ and 1D LHD model atmospheres (blue dashed lines); (3–5) contribution functions of the Li 670.8 nm line (i.e., rates of the line equivalent width growth, dEW/ dlog τ_Ross, as a function of optical depth) in 3D NLTE/LTE (black), ⟨3D⟩ NLTE/LTE (red), and 1D NLTE/LTE (blue). For each metallicity, the lithium abundance is fixed such that W(1D LTE) = 0.5 pm.