Atmospheres

Abstract

In gaseous bodies like the outer planets the distinction between the interior and the atmosphere is not sharp; in the inner, solid planets the atmosphere terminates at the ground and is affected by complex interactions with it. In this and the following chapters we mainly deal with terrestrial atmospheres, taking the Earth as our focus of attention. As the density decreases with height, collision mean-free paths get larger than spatial scales and the assumption of local thermodynamical equilibrium ceases to hold; in this chapter we deal with the lower atmosphere, where the fluid description is appropriate. In the upper part, atmospheres gradually escape to interplanetary space, which makes them fragile structures; this issue, together with the geological history of atmospheres, is discussed in Ch. 8. The atmospheric energy balance is governed by how solar radiation is absorbed and re-emitted in the infrared spectral band, as well as by the ensuing greenhouse effect; radiative transfer models are discussed, to provide a tool for the understanding of these complex phenomena. Planetary atmospheres are exceedingly varied and complex according to their chemical composition, energy sources, phase changes, and so on. The rotation of a planet drastically affects their dynamics through Coriolis’ force and leads to an important flow regime (the geostrophic flow) and the generation of vorticity and turbulence on a global scale. The atmospheric dynamics of the Earth is, of course, known in great detail, but lies beyond our scope.