A1 atiiude-height schematic cross section of the upper troposphere and lower stratosphere is shown in Figure 5.34. The lowermost stratosphere is sometimes referred to as the "middleworld" and that above the 380 K potential temperature level as the "overworld." Tropospheric air enters the stratosphere principally in the tropics, as a result of deep cumulus convection, and then moves poleward in the stratosphere. By overall mass conservation, stratospheric air must return to the troposphere. This return occurs in midlati-tudes; the overall circulation is called the Brewer-Dobson circulation. The mitllatitude
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