At the equator, the local vertical direction (the "up" direction) is at a right angle
to the axis of rotation of the earth. This means that the so-called Coriolis effect i
s not important there, and not very important over much of the tropical
latitudes. On the other hand, the Coriolis effect is quite important to wind flow in the
middle and higher latitudes. This different degree on influence of the earth's rotation
on wind flow leads to quite different modes of circulation in the tropics as compared
with higher latitudes.
This sketch illustrates a strong and systematic band of convection in tropical
latitudes known as the Hadley Circulation. Not very strongly influenced by
earth's rotation, this circulation cell is pictured simply here as being a systematic
overturning of the tropical and subtropical atmosphere. At middle and higher
latitudes, the Coriolis effect becomes important, and the basic wind system
is predominantly westerly flow (from the west; toward the east; eastward; etc)
owing to a mean pressure gradient pointing toward the pole and away from the equator.
(To a simple approximation, this large-scale northward pressure gradient force
may be appreciated from the large amount of air mass which is pumped up to high
altitudes by the Hadley Circulation.)
In this sketchy representation of the atmosphere, the required transport of energy
out of the overly warm tropical region is apparent in the convective circulations
of the Hadley Circulation. But this sketch does not provide a clue as to how
energy can be transferred further poleward by mid-latitude westerly wind flow.
Perhaps a clue to this mystery will follow.
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TROPICAL AND MID-LATITUDE WIND
