The formal solution of the radiative transfer equation and the Eddington-Barbier relation imply that each wavelength samples the temperature near its own photosphere. High opacity moves the emitting level upward. If temperature decreases upward, a molecular band samples cooler gas than the continuum and appears in absorption. If an atmospheric thermal inversion makes temperature increase upward, the same band can appear in emission.
In hot Jupiters, strong optical and ultraviolet absorption by TiO, VO, atomic metals, metal hydrides, or hydrogen-minus opacity can heat low-pressure gas; low infrared cooling efficiency and stellar activity also matter. Solar-System inversions arise from selective absorption, including ozone ultraviolet heating on Earth, methane and hydrocarbons on giant planets, and aerosols or dust. Around M dwarfs, water, methane, carbon dioxide, and hazes absorb the star's near-infrared radiation high in a sub-Neptune atmosphere.
On a temperate planet, a stratospheric inversion can provide a cold trap that limits water loss and can alter photochemistry and circulation. Excess upper-atmosphere heating can instead weaken the cold trap, promote escape, or create hostile surface conditions. Earth's ozone-heated stratosphere is the standard example of a habitable planet with an inversion.
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