Opacities of CH₄ (blue) and CH₃D (orange), shown at T = 300 K (upper panel) and T = 700 K (lower panel), at P = 0.1 bar, which is a representative pressure value probed by methane and CH₃D in the calculations considered here. The opacities have been scaled such that CH₄ and CH₃D constitute 98.8% and 8 × 10⁻⁵ of all CH₄ isotopologues, respectively. The telluric transmission is shown in black.