This results of this chapter are pertinent to a planet with a distinct surface, which may be defined as an interface across which the density increases substantially and discontinuously. The typical interface would be between a gaseous atmosphere and a solid or liquid surface. In the Solar System, there are only four examples of bodies having both a distinct surface and a thick enough atmosphere to significantly affect the surface temperature. These are Venus, Earth, Titan, and Mars; among these, the present Martian atmosphere is so thin that it only marginally affects the surface temperature, though this situation was probably different early in the planet's history when the atmosphere may have been thicker. Although thin atmospheres have little effect on the surface temperature, the atmosphere itself can still have interesting behavior, and the flux of energy from the surface to the atmosphere provides a crucial part of the forcing which drives the atmospheric circulation. This is the case, for example, for the thin nitrogen atmosphere of Neptune's moon Triton. Apart from the examples we know, it is worth thinking of the surface balance in general terms, because of the light it sheds on the possible nature of the climates of extrasolar planets already detected or awaiting discovery.
The exchange of energy between the surface and the overlying atmosphere determines the surface temperature relative to the air temperature.