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Fig. 6—-Ozone, Weisenau, Germany.
maximum is in the spring and minimum in autumn. Many meteorologists ascribe the winter
and spring maximum mainly to sinking motion. Thus ozone measurements can be interpreted
as follows: the stratosphere of the temperate and probably the polar latitudes contain an
ozone maximum in the winter or spring due to sinking motions in the winter; this ozone then
empties into the troposphere; therefore by the early autumn there is a minimum.
The final bit of evidence that provides a clue to air motions is the short- and long-wave
radiation balance in the stratosphere.’ It is well known that there is a net heating in the
troposphere in the equatorial regions, whereas the polar regions have a deficit. This drives
our atmospheric engine. The exact mechanism by which the exchange of heat occurs is not
completely known. A certain amount is exchanged by mixing processes, but some is probably
carried by direct circulation in which there is equatorward flow near the ground and poleward
flow aloft. Firm evidence for this cell is limited to the tropical trade winds. Since the same
areas are heated and cooled in both the troposphere and stratosphere, the stratosphere may
participate in this cell. In such a picture, all of the equatorward motion takes place in the
lower troposphere, but a small part of the circulation may move poleward in the lower stratosphere. If this is so, there should also be a seasonal variation in the poleward stratospheric
circulation since the radiation balance data indicate that the greatest net loss of heat occurs
during the polar winter.
The picture based on humidity, ozone, and heat budget is summarized by Fig. 7. This
particular version was taken from a paper by Dr. N. G. Stewart and his colleagues of the
U. K.® and shows, as Brewer argues, that the air leaves the stratosphere only in the temperate
or polar regions. (Reference 8 is included in Part 4 of this report as the second paper.) Those
of us who proposethis picture readily admit that the actual state of affairs is undoubtedly
much more complex than shown here and that some mixing is superimposed on the direct
circulation.
317