The first studies of the mass balance of atmospheric trace constituents were focused on water vapor. The earliest of these studies were motivated by the fact that the release of latent heat of condensation in precipitation is an important heat source in the global energy budget, the subject of Chapter 5. These early studies also provided new insights into the hydrologic cycle, particularly over land, and were helpful in explaining the observed salinity distribution in the ocean.

Part I consists of two chapters. The first describes the observational basis for general circulation, documents its salient features, and introduces the reader to the kinds of models that are being used to simulate it.

When plotted as partial zonal averages in Fig. 16.1, the seasonality of the zonal mean circulation in the eastern and western hemispheres of the tropics is quite different. In the eastern hemisphere (from the Greenwich Meridian eastward to the Date Line), the zonal mean circulation is dominated by the seasonally reversing Australasian monsoon, which is strong and nearly synchronous with the annual cycle in the meridional profile of insolation. In contrast, in the western hemisphere, the seasonality is not as pronounced and the annual cycle is lagged by about two months relative to the solstices.

The tropical atmosphere encompasses the latitude belt equatorward of the subtropical anticyclones at the Earth’s surface and the tropospheric jet streams at the tropopause level. As shown in Section 2.6.1, the meridional extent of the tropics decreases with increasing rotation rate.

The balance requirement approach covered in Part II provides a reasonable explanation of how the atmosphere satisfies the various budget constraints imposed by the conservation of mass, momentum, total energy, and mechanical energy, but it does not go very far in addressing such questions as:

• Why is there a single pair of tropospheric jet streams located around 30∘N/S?

• Why do the eddies transport angular momentum poleward, across 30∘N/S, maintaining the trade‐wind and westerly wind belts?

• Why do the diagnoses based on the angular momentum balance in Chapter 3 and the energy balance in Chapter 5 yield the same configuration of mean meridional circulations?

The realization that correlation statistics could provide useful information on the three‐dimensional structure and evolution of the transients (i.e., variations about the seasonally varying climatological mean state) dates back al least 100 years, but at that time studies based on this methodology were largely restricted to the analysis of seasonal or annual mean time series at individual stations. Notable examples include studies of Exner, and Walker and Bliss.

The middle atmosphere encompasses the stratosphere and the mesosphere. Its geometric midpoint at ∼50 km corresponds roughly to the stratopause, the top of the stratosphere and the level of strongest heating (per unit mass) due to the absorption of solar ultraviolet radiation by ozone molecules.

In motion systems with timescales ranging from hours up to and including the diurnal cycle, gravity and inertio‐gravity waves are dominant. The influence of the Earth’s rotation is discernible, but geostrophic balance does not prevail and Rossby wave propagation and dispersion do not play a dominant role in the dynamics.

Tropical weather systems with timescales shorter than a few weeks can be divided into three broad categories: equatorially trapped waves, off‐equatorial waves, and tropical vortices.

Some of the most influential general circulation papers in the late 1940s, 1950s, and 1960s involved the formulation and diagnosis of balance requirements that can be applied to any scalar, conserved quantity.

The prominence of the stratosphere in general circulation research derives from

• its role as a reservoir for dangerous radioactive debris from nuclear explosions and reflective sulfate aerosols from volcanic eruptions,

• its role in mediating ozone photochemistry, which shields life on Earth from harmful effects of solar ultraviolet radiation,

• its distinctive low frequency variability, which is seen as a possible vehicle for extending the range of useful weather prediction,

• its relative simplicity as a dynamical system in which the interactions between waves and the zonally symmetric flow play a central role.