Skin temperature (atmosphere)

The skin temperature of an atmosphere is the temperature of a hypothetical thin layer high in the atmosphere that is transparent to incident solar radiation and partially absorbing of infrared radiation from the planet.

It provides an approximation for the temperature of the tropopause on terrestrial planets with greenhouse gases present in their atmospheres.

The skin temperature of an atmosphere should not be confused with the surface skin temperature, which is more readily measured by satellites, and depends on the thermal emission at the surface of a planet.

Each layer is transparent to the visible radiation from the Sun but acts as a blackbody in the infrared, fully absorbing and fully re-emitting infrared radiation originating from the planet's surface and from other atmospheric layers.

The uppermost opaque layer (the emission level) will thus radiate as a blackbody at the planet's equilibrium temperature.

In other words, the skin layer acts as a graybody, because it is not a perfect absorber/emitter of infrared radiation.

Instead, most of the infrared radiation coming from below (i.e. from the emission level) will pass through the skin layer, with only a small fraction being absorbed, resulting in a cold skin layer.

[3][4][5][6][7] Consider a thin layer of gas high in the atmosphere with some absorptivity (i.e. the fraction of incoming energy that is absorbed), ε.

assuming the emission layer of the atmosphere radiates like a blackbody according to the Stefan-Boltzmann law.

where the factor of 2 comes from the fact that the skin layer radiates in both the upwards and downwards directions.

[8] A multi-layered model of a greenhouse atmosphere will produce predicted temperatures for the atmosphere that decrease with height, asymptotically approaching the skin temperature at high altitudes.

[3] The temperature profile of the Earth's atmosphere does not follow this type of trend at all altitudes, as it exhibits two temperature inversions, i.e. regions where the atmosphere gets warmer with increasing altitude.

These inversions take place in the stratosphere and the thermosphere, due to absorption of solar ultraviolet (UV) radiation by ozone and absorption of solar extreme ultraviolet (XUV) radiation respectively.

A multi-layered model of the atmosphere is shown, with the skin layer at the top. Arrows show energy fluxes exchanged between layers, and dotted lines indicate that the radiation passes through one or more layers. Note that the flux radiated upwards by the opaque layer at T 1 must be equal to the incident solar flux, σT eq 4 . Thus T 1 = T eq .
A theoretical temperature profile from a many-layered model (dotted) vs the measured temperature profile (solid) of the Earth's atmosphere. The named layers of the atmosphere apply only to the measured temperature profile, because their definition relies on the presence of inversions.