Vertical Stability

Abstract

In the previous chapter we have considered the atmosphere in hydrostatic equilibrium. It is obvious that this type of equilibrium does not imply a thermodynamic equilibrium; for instance, the vertical gradient of temperature will imply vertical heat conduction. In the real atmosphere heat will also be lost or gained by radiative processes. If vertical equilibrium is not prevalent, we should consider a third type of heat transport process: turbulent conduction. Conduction by molecular diffusion is a very slow process, negligible for all praticai purposes, so that it need not concern us. Conduction by radiation is more important by several orders of magnitude, and it is a legitimate question to consider if radiative processes play a major role in determining the vertical distribution of temperature. Calculations for an atmospheric column which would receive radiative energy through the base, while losing the same amount from the top (admittedly a gross over-simplification of actual radiative processes), lead to a steady distribution such as indicated by the dashed curve of Figure VIII-l. On the other hand, if we assume, in addition, a thorough vertical mixing, up to a level indicated by a discontinuity (tropopause), a distribution such as that of the full curve would be obtained. The real atmospheric stratifications are reasonably similar to this curve, especially for average conditions in temperate and tropical latitudes. The vertical turbulent transport of heat is thus a major factor determining the temperature distribution. In fact, when this mechanism is active, it may again be one or more orders of magnitude more efficient than the radiative transfer in determining local rates of change of temperature.