Amplitude of the relative rms temperature fluctuations δT_rms/ ⟨ T ⟩ versus the average temperature ⟨ T ⟩, evaluated on different iso-optical depth surfaces: τ₈₅₀ (solid), τ₁₆₀₀ (dashed), τ_Ross (dotted). Filled circles indicate the values at the mean height of line formation at λ   850 nm and λ   1600 nm, respectively, as given in Table B.1.