Amplitude of the (relative) temperature fluctuations, δT_rms/ ⟨ T ⟩  (top), and ratio of average line opacity to line opacity at mean temperature, of a weak (artificial) Fe ii line with excitation potential χ = 10 eV (bottom) as a function of the mean temperature ⟨ T ⟩ , where angle brackets indicate horizontal averaging at constant monochromatic optical depth. Solid and dashed curves show the results of averaging on surfaces of constant τ₈₅₀ and constant τ₁₆₀₀, respectively. Filled dots indicate the location of the center of the line formation regions at λ 8500 and 1600 nm.