The unequal heating of Earths surface due to its tilt, rotation, and differential in-solation, results in the wide distribution of pressure over Earths surface. Study figures 3-1-3A and 3-1-3B. Note that a low-pressure area lies along the intertropical convergence zone (ITCZ) in the equatorial region. This is due to the higher temperatures maintained throughout the year in this region. At the poles, permanent high-pressure areas remain near the surface because of the low temper-atures in this area throughout the entire year. The subtropical high-pressure areas at 30N and S latitudes are caused mainly by the "piling up" of air in these regions. Other areas are also dominated by relatively high or low pressures during certain seasons of the year.

Temperature differences cause pressure differences which in turn cause air move-ments.

The following sections show how air movements work and how they evolve into the various circulationsprimary, secondary, and tertiary.

To explain the observed wind circulation over Earth, three basic steps are used. The first step is to assume Earth does not rotate and is of uniform surface; that is, all land or all water. The second step is to rotate Earth, but still assume a uniform surface. The third step is to rotate Earth and assume a nonuniform surface. For now, we deal with the first two steps, a nonrotating Earth of uniform surface and a rotating Earth of uniform surface.

The circulation on a nonrotating Earth is referred to as the thermal circulation because it is caused by the difference in heating. The air over the equator is heated and rises (low pressure), while over the poles the air is cooled and sinks (high pressure). This simple circulation was shown in figure 3-1-1.