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A warmer troposphere does not imply a warmer stratosphere: increased carbon dioxide can cool the middle and upper stratosphere by enhancing infrared emission. One direct kinetic effect is slower temperature-dependent destruction in the Chapman cycle, tending to increase upper-stratospheric ozone. Two indirect chemical effects can have the opposite sign. If the entry atmospheric cold trap warms, it can admit more water; reaction with an electronically excited oxygen atom then supplies more of the HOx family, enhancing catalytic ozone loss. In cold polar air, changes in water and temperature can favour polar stratospheric clouds, whose surfaces activate halogen reservoirs and can enhance ozone destruction when sunlight returns. These are conditional pathways, not a universal sign for total ozone: cold-trap temperature and circulation, halogen abundance and altitude all matter. Methane oxidation can also supply stratospheric water.^[ref]

An atmospheric thermal inversion is an altitude interval with $dT/dz>0$. Absorption of incoming stellar radiation above the usual thermal-emitting layers can heat the upper gas faster than it cools, producing an inversion. In a semi-grey irradiated atmosphere, a large shortwave-to-infrared opacity ratio favors such high-altitude energy deposition; local infrared emitters and the intrinsic flux also matter.

In the Solar system, Earth and all four giant planets have well-known stratospheric inversions. The ozone layer absorbs ultraviolet sunlight on Earth; methane and photochemical hydrocarbons absorb solar radiation in the giant planets, with aerosols contributing. The giant planets are Jupiter, Saturn, Uranus and Neptune. This refers to their stratospheric temperature rise, not to the gradient at every atmospheric level.

For hot Jupiters, influential factors include the stellar flux and spectrum; the abundances of high-altitude absorbers such as titanium monoxide and vanadium monoxide; atmospheric metallicity of a giant planet and atmospheric carbon-to-oxygen ratio; thermal dissociation, atmospheric photochemistry and condensation; a atmospheric cold trap or atmospheric condensate rainout that removes absorbers; replenishment by vertical mixing; exoplanet clouds and atmospheric hazes; and heat redistribution by circulation. The ratio of visible heating to infrared cooling, rather than a single chemical species in isolation, determines whether an inversion persists.

The pressure axis increases downward in the sketches. Solid red segments indicate efficient convection; blue segments indicate mainly radiative transfer. The boundaries are nominal examples, since radiative-convective boundary pressure depends on opacity, gravity and intrinsic flux.

For Earth, the convective troposphere lies beneath a radiative stratosphere, with the transition near $0.1$--$0.2$ bar. The larger solar-system planets likewise have deep convective regions beneath largely radiative upper atmospheres; their tropopause/upper radiative-convective boundary is commonly of order $0.1$--$1$ bar. Their temperatures differ greatly from Earth's, and detached radiative layers can occur deeper down.

For an irradiated hot Jupiter, absorbed stellar flux maintains a hot, extended, relatively shallow-gradient radiative atmosphere. In models with a weak old-planet intrinsic flux, the deep radiative-convective boundary can lie around $10^2$--$10^3$ bar, with a useful wider model-dependent range of tens to thousands of bars. A thermal inversion can appear at low pressures if visible/UV absorption heats the upper atmosphere. Below the deep boundary the profile joins a convective adiabat.

A young, directly imaged exoplanet on a distant orbit is primarily heated from within. Its photosphere commonly joins the convective interior at order $0.1$--$10$ bar, illustrated here at one bar. The temperature generally rises monotonically with pressure over the infrared-forming layers. Strong stellar heating is absent, so a broad stellar-heated isothermal layer is unnecessary.

Two differences in thermal inversions are their absorbers and their formation conditions. Earth's ozone layer and solar-system hydrocarbon absorption can heat upper layers; the proposed hot-Jupiter absorbers include refractory titanium monoxide/vanadium monoxide or other strong visible absorbers, since ozone and methane-rich cold-planet chemistry are unsuitable at very high temperatures. Hot-Jupiter inversions depend strongly on irradiation, atmospheric cold traps and atmospheric transport, and can occur at mbar-to-sub-bar pressures; Earth and the solar-system giants have cooler, established stratospheres above their shallow tropospheres.

Two differences between the hot Jupiter and distant young-giant profiles are external versus internal heating, and the depth/shape of the radiative zone. The former can have a broad warm radiative layer, a much deeper convective boundary and sometimes an inversion; the latter typically has a steeper outward decrease toward a photosphere, a shallower boundary and no irradiation-driven inversion. These are class trends, not a unique temperature profile for every planet.

A thermal inversion is outward-increasing temperature over part of an atmosphere. In an irradiated radiative model, an upper absorber that captures incoming stellar energy efficiently compared with its infrared cooling can create an inversion. Early hot-Jupiter models proposed titanium monoxide/vanadium monoxide as strong visible absorbers, and an irradiation-based division between hotter titanium monoxide/vanadium monoxide-rich inverted atmospheres and cooler non-inverted atmospheres followed.

The simple temperature classification needed refinement. Condensation and vertical or day-night atmospheric cold traps can remove Ti/V even when the observed dayside is hot; settling competes with vertical mixing. Bulk C/O and metallicity determine whether oxygen-bearing absorbers are available, so irradiation alone does not specify their abundance. Photochemistry, stellar UV activity and alternative absorbers were also proposed to affect inversion formation or destroy an absorber; such correlations were hypotheses rather than a unique established cause.

Observationally, molecular bands in emission rather than absorption can diagnose an inverted contribution region, but sparse broadband eclipse measurements are degenerate with chemistry. By 2016, spectroscopic constraints had weakened some early inversion claims and supported the need for planet-specific opacity, abundance and transport models. The key development is from a single irradiation threshold to a composition- and transport-dependent visible/infrared opacity balance, tested by spectra rather than assuming every very hot planet must have the same inversion.

The thermal optical depth increases inward, so an atmospheric thermal inversion requires $dT/d\tau <0$. Differentiating the semi-grey irradiated atmosphere profile givesTherefore the inversion criterion with intrinsic planetary flux isAn inverted layer exists at the top precisely whenFor $T_{\mathrm{int}}>0$, it extends overprovided the logarithm is positive. Otherwise the profile has no inverted interval. For a strongly irradiated hot Jupiter, $T_{\mathrm{int}}\ll T_{\mathrm{irr}}$ and the threshold is approximately $\gamma >1$; the intrinsic flux eventually restores an inward-increasing temperature at greater depth.

Large visible absorption opacity deposits stellar heat high in the exoplanet atmosphere. Gas-phase titanium monoxide and vanadium monoxide are candidate absorbers in sufficiently hot layers. An atmospheric cold trap or other condensate loss can remove them, while sufficiently high temperature and vigorous mixing can help keep them in the gas. The condition concerns absorbing opacity: highly reflective scattering alone does not deposit the required heat.

Four mechanisms that can produce disequilibrium chemistry in an exoplanet atmosphere are:

The exoplanet transport and photochemical examples follow kinetic atmosphere calculations and models including horizontal transport; the condensate example is examined in cold-trap calculations.