In planetary interiors, heat is generated by the decay of the ▶ radiogenic isotopes U, U, Th, and K. In addition, latent heat may be released and consumed through phase transitions. The dissipation of kinetic energy of ▶ planetesimals colliding with the protoplanets during planetary accretion has heated the planetary interiors, in some cases up to the melting temperatures of their outer layers. The formation of the cores in terrestrial planets through differentiation has further heated the deep interiors. Since planetary surfaces are colder – their temperature being determined by the rate of solar radiation and the atmosphere ▶ greenhouse effect – heat is transferred to these surfaces from the deep interior. The resulting flow of heat may be associated with the conversion of heat into mechanical work and the conversion of heat into ▶magnetic field energy. Moreover, the cooling may result in planetary contraction. The dominant heat transfer mechanisms in planetary interiors are heat conduction and convection (for a discussion of heat transfer in planetary interiors, see, e.g., Schubert et al. 2001). In a typical substantial planetary atmosphere, heat is transferred by convection or radiation in the lower atmosphere (more specifically, the ▶ troposphere and ▶ stratosphere) and heat conduction and radiation in the upper atmosphere (more specifically, the mesosphere and thermosphere). The pressure level at the radiative-convective boundary (the tropopause) in the lower atmosphere depends on the composition of the atmosphere and the ▶ opacity of the major atmospheric constituents. Above this level (i.e., at lower pressures), the outgoing energy is transported by radiation until heat conduction becomes important. The radiative-convective boundary occurs when the atmosphere becomes optically thin to thermal radiation. Usually, this translates into a column optical depth, t, approximately equal to unity. For t approximately equal to unity or larger, the atmosphere is opaque, and the radiative energy is reabsorbed by the surrounding media, hence convection is the dominant form of heat transfer. For t smaller than unity, the atmosphere is transparent and does not absorb efficiently (see Atmospheric Structure, Phase Change: Latent Heat, Thermal Inversion, Opacity). Heat Conduction. In gases, heat is conducted via the ▶ diffusion of molecules. Molecules with high kinetic energy diffuse from regions of high temperature to regions of lower temperature. Interaction