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350
350
300
300
Surplus
250
250
200
200
Surplus heat energy transferred
by atmosphere and oceans
to higher latitudes
150
150
100
100
Net shortwave
Figure 9.2 Balance
between average net
shortwave and
longwave radiation
from 90° North to
90° South.
50
50
Net longwave
0
0
90
70
50
40
30
20
10
0
10
20
30
40
50
70
90
North
Latitude
South
of the Earth falls from the equator toward the poles but the latitudinal variation in
the amount of radiant energy leaving as longwave radiation is much less than
the latitudinal variation in the amount of energy received as shortwave radiation,
see Fig. 9.2.
On average across the surface of the globe, there is a near perfect balance
between incoming and outgoing radiant energy. At low latitudes, however, there is
more radiant energy incoming as shortwave radiation than is leaving as longwave
radiation; consequently surplus energy is available. At high latitudes the reverse is
true, when averaged over the year there is more outgoing radiant energy as long-
wave radiation than incoming radiant energy as shortwave radiation. This discrep-
ancy causes the atmospheric general circulation, because to support the persistent
latitudinal imbalance in radiant energy transfer, energy must be moved from low
latitudes to high latitudes in the atmosphere and oceans.
Lower atmosphere circulation
Latitudinal bands of pressure and wind
When mariners first began to travel the globe using sailing ships, they soon realized
that, on average, there are characteristic patterns of wind flow at different latitudes,
and they learned how to exploit these in their travels. Close to the equator they found
regions with little wind where progress was difficult under sail. In these regions
the  ensuing state of inactivity could make the travelers dull, listless and depressed.
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