ositive and negative effects on human activities (Eagleson, 1982, p. vii). Such variability is controlled--in large part--by interaction between the atmosphere, the continents, and the oceans (Manabe, 1982, pp. 19-58). These different factors influence the globe's general circulation. Monin (cited in Grotjahn, 1993, p. 3) describes general circulation as "a statistical ensemble of large-scale components of state of the atmosphere". It essentially consists of both time and zonal (i.e., longitudinal zones) average circulation, and occurs as a result of inhomogenous radiative heating. Solar heating and terrestrial cooling create thermal sources and sinks. Typically, there is excess heating near the equator and excess cooling near the poles. The atmospheric and oceanic general circulations attempt to moderate such imbalances, and restore the globe's thermal equilibrium (Grotjahn, 1993, pp. 3-4).
General circulation is acted upon by several constraints. These may include radiative, hydrostatic, and--at least, outside the tropics--geostrophic alterations. The influence of radiative alteration on atmospheric circulation, for instance, is rather complex. Without atmospheric and oceanic currents, equator-to-pole temperature gradients would be much stronger than those observed. Heat transport from low to high latitudes weakens the meridonal gradient. This not only decreases elevation-associated temperature gradients, but also diminishes the energy source for currents which flow longitudinally. Other mechanisms affecting atmospheric circulation include pressure, zonal wind, surface torque, and mass. At high latitudes, temperatures in the lower atmosphere are colder. Hence, according to the hydrostatic and ideal gas laws, the thickness of pressure layers are greater in the tropics. However, surface pressure exhibits relatively little latitudinal variation as compared to constant pressure surfaces at elevation. Therefore, pres...