We might see the objective of understanding meteorology, as getting a feel for why the planet has its observed mean temperature, which sets the field for water vapour transformations, clouds, latent heat, and life. A second objective might well be the assesment of fluctuations; the global and seasonal variation of winds and temperatures and humidities, which is what we are about to engage apon. Further objectives might well begin with persistent anomalies from the climate thereby established.

Definition of general circulation

Some set of mean overall properties of the atmosphere that change slowly with time are called the general circulation. Perhaps the most obvious of these sets, though not necessarily the easiest to explain, is the zonal mean of wind, temperature, humidity, etc. as functions of height. We find that some average variables are related to other average variables rather simply. For example it is difficult to justify a large imbalance between the zonal mean of the zonal component of the wind and the pole–equator temperature gradient, as constrained by the thermal wind. Other quantities depend almost entirely on the transport of various properties by eddy motion which may occur on a variety of scales. Thus the non-zero value of the zonally averaged surface wind depends almost entirely on the transport of momentum across latitude belts by eddies on the scale of weather systems.